Displays

There remains large uncertainty about the global exchanges of carbon between the atmosphere and the terrestrial biosphere under different environmental change scenarios. Ecosystem and Earth system models rely on photosynthetic capacity (maximum rates of carboxylation (V_cmax) and electron transport (J_max)) to simulate carbon assimilation. Photosynthetic capacity has been related to environmental and climatic constraints, but also to leaf and soil nutrients. Views differ on which are more important.

We assembled and analysed a large dataset of global observations of photosynthetic and other leaf traits. Photosynthetic capacity was best predicted based on optimality hypotheses. V_cmax standardized to 25°C (V_cmax25) was proportional to light availability, and increased towards colder and drier environments – as expected due to the greater biochemical investment required at lower temperatures, or when stomata are more closed. The ratio J_max25/ V_cmax25 declined with growth temperature (also predicted). However, theoretical predictions slightly underestimated V_cmax at high growth temperatures, and overestimated it at low growth temperatures. This bias might be due to the difference between leaf and air temperatures.

Statistical models for photosynthetic capacity (all species, and site means) overestimated V_cmax in low-P leaves. Analysis of a subset of the data showed that leaf P tends to increase with measured soil P. A relationship of model bias to leaf N appears in the all-species analysis – perhaps reflecting a correlation of V_cmax, leaf N and light levels within communities. But site-mean analysis showed no such bias, and leaf N showed no relationship to the soil C:N ratio. These results support a previously noted dependency of V_cmax on P availability; but not the control of V_cmax by N availability that has been assumed in many models.

How to cite: Peng, Y., Bloomfield, K., Cernusak, L., Domingues, T., Lloyd, J., and Prentice, I. C.: Using plant trait data to extend a theory of global ecosystem function, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-794, https://doi.org/10.5194/egusphere-egu2020-794, 2020.

Carbon dioxide (CO₂) uptake by leaves and its conversion into sugar by photosynthesis – gross primary production (GPP) – is the basis for vegetation growth. GPP is important for the carbon cycle, and its interactions with climate are a subject of study in Earth System modelling. One assumption of many current ecosystem models is that key photosynthetic traits, such as the capacities for carboxylation (V_cmax) and electron transport (J_max­) for ribulose-1,5-bisphosphate (RuBP) regeneration, are constant in time for any given plant functional type. Optimality theory predicts they should vary systematically with growth conditions, both in space and in time, and are not necessarily depend on the plant functional type. Moreover, theory makes specific, quantitative predictions about their (acclimated) community-mean values, predictions well supported by evidence. Neglecting such acclimation could lead to incorrect model estimates of the responses of primary production to climate change.

We focus on a proof-of-concept based on a primary production model, the P-model – which combines the Farquhar-von Caemmerer-Berry model for C₃ photosynthesis with eco-evolutionary optimality principles for the co-optimization of carboxylation and water transport costs – to allow the model to reproduce short-term variations in photosynthesis and transpiration as well as longer-term, acclimated variations. Key to this effort is explicitly separating the instantaneous responses of photosynthetic rates, and the slower acclimation of photosynthetic traits. The model also includes a dynamic optimization of stomatal conductance via the c_i:c_a ratio, which separates the rapid response to vapour pressure deficit (VPD) from a slower, acclimated response of the single parameter (ξ) of the stomatal optimality model to growth temperature.

A day-by-day diagnostic investigation has been carried out in order to optimize the behaviour of the resulting model at a half-hourly timestep. The model reproduces well the daily variations of GPP evaluated against FLUXNET observations, when forced with site-specific, half-hourly meteorological data from flux towers, and satellite data on the slowly varying fractional absorbed photosynthetically active radiation (fAPAR). Our approach accounts for the memory effect of past environmental conditions on photosynthetic traits, by introducing a daily average computation of temperature, solar radiation, VPD, CO₂ concentration and elevation. The results show that plants coordinate their biochemical capacities to match the maximum level of light during a day, optimizing to conditions near midday, when light is greatest. This is consistent with an interpretation of the co-limitation theory, whereby the V_cmax and J_max of leaves at any canopy level acclimate to the prevailing incident light. One implication is that the canopy is light-limited for most of the time. This is strongly supported by the diurnal time-course of observed GPP.

How to cite: Mengoli, G., Prentice, I. C., and Harrison, S. P.: Adapting an optimality-based model to predict half-hourly carbon uptake by ecosystems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4154, https://doi.org/10.5194/egusphere-egu2020-4154, 2020.

The relationship of tree form and function has long fascinated humans, and now, much of our ability to improve maps and forecasts of the vital interactions of forests and global change hinges on our ability to understand this adaptive tree crown architecture.  To help address this challenge, I revisit Henry Horn’s classic 1971 monograph “The Adaptive Geometry of Trees”, and blend his theoretical framework with a contemporary ecological theory of species’ functional traits.  Then, I describe how this trait-based theory tree crown architecture can be robustly tested using state-of-the-art hyper-remote sensing techniques.  This suite of imaging techniques from hyper-spatial (e.g. UAV and satellite imagery), hyper-spectral (e.g. AVIRIS imagery), hyper-temporal (e.g. phenocams and tree- or tower-mounted time-lapse cameras), and hyper-dimensional (terrestrial and UAV LiDAR) sensors now enables us to visualize and measure the spectral and architectural properties of individual trees with unprecedented accuracy and precision.  Through analysis of hyper-remote sensing datasets collected in forests across eastern North America, I highlight how this testable trait-based theory of tree crown economics is already providing fresh insights into several important, but heretofore unresolved patterns of spatial and temporal variability in forest functioning.    

How to cite: McNeil, B.: Tree crown economics: testing and scaling a functional-trait based theory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5589, https://doi.org/10.5194/egusphere-egu2020-5589, 2020.

We argue that tree and crown structural diversity can and should be integrated in the whole-plant economics spectrum. Ecologists have found that certain functional trait combinations have been more viable than others during evolution, generating a trait trade-off continuum which can be summarized along a few axes of variation, such as the “worldwide leaf economics spectrum” and the “wood economics spectrum”. However, for woody plants the crown structural diversity should be included as well in the recently introduced “global spectrum of plant form and function”, which now merely focusses on plant height as structural factor. The recent revolution in terrestrial laser scanning (TLS) unlocks the possibility to describe the three dimensional structure of trees quantitatively with unprecedented detail. We demonstrate that based on TLS data, a multidimensional structural trait space can be constructed, which can be decomposed into a few descriptive axes or spectra. We conclude that the time has come to develop a “structural economics spectrum” for woody plants based on structural trait data across the globe. We make suggestions as to what structural features might lie on this spectrum and how these might help improve our understanding of tree form-function relationships.

How to cite: Verbeeck, H., Bauters, M., Toby, J., Schenkin, A., Disney, M., and Calders, K.: Time for a Plant Structural Economics Spectrum, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8670, https://doi.org/10.5194/egusphere-egu2020-8670, 2020.

Increasing frequencies and intensities of droughts are projected for many regions of the Earth. Water stress leads to a decline in the gross primary productivity (GPP) of plants. Plant responses to water stress vary with timescale, and plants adapted to different environments differ in their responses. Here, we present a unified theory of plant photosynthesis and plant hydraulics, which explains a wide range of observed plant responses to developing water stress.

Our theory is based on the least-cost hypothesis of Prentice et al. (2014). By integrating plant hydraulics into the least-cost framework, we attempt to improve upon the model of GPP by Wang et al. (2017), which accurately predicts the responses of global GPP to temperature, elevation, and vapour pressure deficit, but overestimates GPP under water-stressed conditions. Our model has three key ingredients. (1) The aforementioned least-cost framework, in which optimal stomatal conductance minimizes the summed costs of maintaining transpiration, the photosynthetic machinery, and the hydraulic pathways, including the potential costs of repairing embolized xylem. We also test a closely related maximum-benefit framework, in which optimal stomatal conductance maximizes the net benefit from assimilation while accounting for these summed costs, and obtain comparable results. (2) A trait-dependent model of water flow through the plant stem, in which water flow is limited by the conductivity (K_s) and embolism resistance (P₅₀) of the hydraulic pathway. At the shortest timescale, water stress causes stomatal closure to an extent that the transpiration demand determined by the vapour pressure deficit at the leaf surface is matched by the water supply through the stem. (3) A short-term response of photosynthetic capacity (V_cmax) to soil moisture, through which the potential V_cmax acclimates to prevailing daytime conditions to equalize carboxylation-limited and electron-transport-limited photosynthesis rates (A_c and A_j), while the realized values of V_cmax, A_c, and A_j are reduced from their potential values by a factor dependent on the leaf water potential and the leaf embolism resistance.

We estimate the parameters of our model using published data from short-term and long-term dry-down experiments. The key predictions of our model are as follows: (1) GPP declines with decreasing soil water potential and drops to zero soon after the soil water potential crosses P₅₀; (2) soil-to-leaf water potential difference remains relatively constant under developing water stress; (3) functional forms describing the declines in stomatal conductance, V_cmax, and GPP with soil water potential are consistent with observations; and (4) decreased photosynthetic capacity (V_cmax) recovers (in the long term) if the plant increases its Huber value (e.g., by shedding leaves), increases its conductivity (e.g., by growing wider new vessels), or decreases its height growth (e.g., by reducing allocation to growth). Our theory provides a potential way of integrating trait-based responses of plants to water stress into global vegetation models, and should therefore help to improve predictions of the global carbon and water cycles in a changing environment.

References: [1] Prentice IC, et al. Ecology letters 17.1 (2014): 82-91.  [2] Wang H, et al. Nature Plants 3.9 (2017): 734.

How to cite: Joshi, J., Dieckmann, U., and Prentice, I. C.: Towards a unified theory of plant photosynthesis and hydraulics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9687, https://doi.org/10.5194/egusphere-egu2020-9687, 2020.

Extreme temperature events are increasing in frequency and intensity across the globe. These extremes, rather than averages, drive species evolution and determine survival by profoundly changing the structure and fluidity of cell membranes, altering enzyme function, and denaturing proteins. Given not only our dependence on agricultural crops and natural vegetation, but also the role of photosynthetic processes within the carbon and hydrological cycles, it is imperative to assess the state of our understanding of the potential impacts of extreme events on plants. Scaling responses from the molecular and organ level to ecosystem function is not without challenge however. There is vast literature on plant thermal tolerance research, but the body of literature is so large, the approaches so disparate and often siloed among disciplines, that research in this field risks floundering at a critical time. We conducted a systematic review of more than 21,500 studies spanning over 100 years of research that yielded almost 1,700 included studies on the tolerance of cultivated and wild land plants to both heat and cold. Our review indicates that most studies on thermal tolerance focus on the cold tolerance of cultivated species (52%) and only a trivial percentage of studies have considered both heat and cold tolerance of any given species (~5%). Combined heat and cold tolerance are important in areas where plants are exposed to extremes of both or may be in the future. This review illustrates the global distribution and concentrations of thermal tolerance studies and the diversity of thermal tolerance methods, ranging from molecular to biochemical, physiological and physical examinations, from transgenic model plants to agricultural and horticultural crops, to natural forest trees, shrubs, and grassland herbs. Critically, it also demonstrates that methods and metrics for assessing thermal tolerance are far from standardised, such that our potential to achieve mechanistic insight and compare across species and biomes is compromised. Without reconciling these issues, the scope for incorporating this critical ecological information into vegetation elements of land surface models may be limited. To aid this, we identify priorities for achieving efficient, reliable, and repeatable research across the spectrum of plant thermal tolerance. These priorities, including meta-analytical approaches and comparative experimental work, will not only further fundamental plant science, but will prove essential next steps if we are to integrate such diverse data on a critical plant functional trait into a usable metric within biogeochemical models.

How to cite: Geange, S., Arnold, P., Catling, A., Coast, O., Cook, A., Gowland, K., Leigh, A., Notarnicola, R., Posch, B., Venn, S., Zhu, L., and Nicotra, A.: Plant thermal tolerance: a global synthesis for future research, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18977, https://doi.org/10.5194/egusphere-egu2020-18977, 2020.

We adopted the flexible trait Dynamic Global Vegetation Model LPJmL-FIT for European natural forests by eliminating bioclimatic limits of Plant Functional Types (PFTs) and replacing prescribed values of functional traits with flexible individual traits. Vegetation dynamics are simulated with permafrost and fire disturbance being considered in the simulation domain. Leaf and stem-economic traits are assigned to individual trees at establishment which then determine plant competition for light and water in a forest patch. We simulate vegetation dynamics in selected natural forests sites and at the Pan-European scale. We quantified functional richness (FR), functional divergence (FDv) and functional evenness (FE) from combinations of functional and structural traits of the simulated individual trees.

We find good agreement with observed productivity, biomass and tree height, and spatial PFT and local trait distributions. The latter is compared against TRY observations. We find site-specific trait distributions and spatial gradients of the simulated LES and SES traits to coincide with environmental and competitive filtering for light and water in environments with strong abiotic stress. Where deciduous and needle-leaved trees co-occur in a forest patch, functional richness (potential niche space) is high, and extreme ends of the niche space are occupied resulting in high FDv. Functional divergence declines where the performance of deciduous trees decreases due to increasing environmental stress as simulated along altitudinal and latitudinal gradients. When climate gets cooler, needle-leaved trees become dominant, occupying the extreme ends of the niche space. Under Mediterranean climate conditions, drought increasingly limits tree growth thus niche differentiation becomes more important.

Co-existence of functionally diverse trees within and across PFTs emerges from alternative life history strategies, disturbance and tree demography.

How to cite: Thonicke, K., Billing, M., von Bloh, W., Sakschewski, B., Niinemets, Ü., Penuelas, J., Cornelissen, J. H. C., van Bodegom, P., Shaepman, M. E., Schneider, F. D., and Walz, A.: Simulating co-existence of functionally diverse trees in European natural forests with LPJmL-FIT, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4100, https://doi.org/10.5194/egusphere-egu2020-4100, 2020.

Rooting depth governs plants' access to water during dry (rain-free) periods and is thus a key variable determining the sensitivity of transpiration and carbon assimilation to soil moisture drought and land-climate coupling during drought and heat events. Plant rooting depth variations of two orders of magnitude have been recorded across the globe and a substantial portion of this variation appears within species and functional groups. This suggests that plant rooting depth is a relatively plastic “trait” that may adjust to the local environment and soil hydrology and possibly also to temporally changing climatic conditions. Yet, plant rooting depth is commonly treated as a fixed parameter in global vegetation and climate models, specified for each plant functional type. How can respective trait flexibility be introduced in such models? 

Approaches to explain these large variations have focussed on the depth of the local water table as a constraint on the maximum rooting depth (Fan et al., 2017), on the mean seasonality and synchrony of precipitation and radiation (Gao et al., 2014; Schenk et al., 2005), or on optimality principles for balancing the trade-offs between the benefits of deep rooting and their associated costs (carbon used for root construction and respiration and maintaining water transport along the entire soil-root-leaf pathway) (Kleidon and Heimann, 1998; Schymanski et al., 2009).

Here, we follow the approach by Gao et al. (2014), assuming that plants’ rooting depth is adjusted to sustain cumulative water deficits (precipitation minus evapotranspiration: P - ET) of a magnitude, corresponding to an event with a return period of N years, where N is a global constant. The water deficit is determined using daily reanalysis data of precipitation, while ET is estimated from remotely sensed vegetation cover and observed radiation, and from simulated stomatal responses to observed vapour pressure deficit across the globe for the past 35 years. Cumulative water deficits are translated into a “soil drying depth” using information on soil texture and its estimated water holding capacity.

First results reveal a global pattern of plant rooting depth, adjusted to the apparent cumulative water deficit during rain-free periods, peaking at intermediate aridity, where dry periods are long, radiation is high, and vegetation cover is substantial. A comparison to biome-level distributions of observed rooting depths indicates that the control by the cumulative water deficit, embodied in our model, is a powerful driver of variations between biomes. Shallowest rooting depths are observed and modelled in boreal forests and alpine meadows, while deepest rooting is observed and modelled in xeric forests. Stomatal response to dry conditions appears to be an important mechanism that mitigates the necessity for deep rooting, particularly in broadleaved tropical and temperate forests. Our analysis also revealed challenges in bridging the scales of observations and our global-scale model, possibly due to small-scale heterogeneity in soil water availability driven by the topographic setting.

How to cite: Stocker, B. D., Tumber-Dávila, S. J., and Jackson, R. B.: Global climate controls on the plant rooting depth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7019, https://doi.org/10.5194/egusphere-egu2020-7019, 2020.

Global-change-type droughts, resulting from climate warming and changes in precipitation patterns are believed to be accelerating the rates of tree mortality globally. Simulation of these changes, for instance in global vegetation models, requires parameterisation of plant strategies with respect to drought. However, our understanding of realised combinations of drought-relevant physiological and morphological traits across the range of global forest types is still limited.

We constructed a dataset consisting of 12 functional traits related to resource acquisition, growth rate, plant defence, water conductivity and hydraulic vulnerability for 11,000 woody species globally. We used a novel envelope-based analysis to assess the functional space occupied by these species whilst circumventing the problem of sparse sampling for many traits. We then subdivided the space into a continuum of strategic “clusters”. These clusters only map partially onto groupings of traditional plant functional types based on leaf type and phenology.

We found that the functional spaces for the plant strategies are highly interrelated globally, showing that the traits related to resource acquisition are positively associated with growth rates and leaf water conductivity, which together are negatively associated with conservative traits of plant defence, although these associations differ greatly between needleleaf and broadleaf species. However, the trait related to hydraulic failure demonstrated positive associations with resource acquisition, but no relationship with woody defence. Furthermore, there are clear linkages between water flow and hydraulic vulnerability traits, and climatic drivers relating to aridity and plant distribution.

The clusters identified in this systemic work can form a basis for new plant functional type definitions, facilitating including plant hydraulics in global vegetation models and, taking a step towards making reliable large-scale simulations of drought-driven tree mortality.

How to cite: Liu, D., Esquivel-Muelbert, A., Sadler, J., Acil, N., Papastefanou, P., Rammig, A., and A. M. Pugh, T.: Incorporating hydraulic traits within the functional strategy spectrum of woody plants globally, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10214, https://doi.org/10.5194/egusphere-egu2020-10214, 2020.

The climate on Earth arises from multiple interactions between the different spheres, including the biosphere. Within the biosphere, the organisms composing the various types of ecosystem are characterized by a set of traits involved in biological processes that can influence the climate system. Identifying and integrating these traits into models such as earth system models (ESM) is thus crucial to predict the future of earth climate. While an important number of biological processes are similar, the amount and the type of traits considered to represent these processes can vary considerably among ecosystem types in current ESMs. Such inconsistencies could bias our perception of the global influence of biosphere on climate dynamics. Here we first list the biological traits that have been included in the terrestrial and oceanic modules of the CMIP5 Earth system models. By comparing the traits and associated processes, we reveal consistencies and inconsistencies in trait representation among both ecosystem types. Based on a critical evaluation we propose a new conceptual framework that allows to describe the climate relevant traits in a consistent way in terrestrial and oceanic modules of ESMs. This framework can also be used to identify new traits characterizing terrestrial and/or marine ecosystems, and to integrate them in ESMs.

How to cite: Pellerin, F., Porada, P., and Hense, I.: Towards a new trait-based framework bringing together terrestrial and marine ecosystems in Earth system models., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11248, https://doi.org/10.5194/egusphere-egu2020-11248, 2020.

We developed global maps of N, P, K concentrations and ratios in leaves for woody plants by modeling with Neural Networks at 1km resolution. We gathered georeferenciated data from published data bases (like TRY and ICP) and a total of 206 peer-published papers (ISI WEB) achieving 28736 records of N, P and K leaf concentrations that we split in 6 morphoclimatic groups (tropical evergreen, tropical deciduous broadleaves, temperate coniferous, temperate evergreen broadleaves, temperate deciduous broadleaves and boreal). We trained Neural Networks with climatic, soil and atmospheric deposition data and morphoclimatic groups to model the foliar elemental composition and project it at global scale according to a Land Cover map. The models provide maps with information of the foliar concentrations of each element at pixel level, their uncertainty and goodness of fit, and relative importance of the independent variables. Linear models were also created to show the relationship between dependent and independent variables. These maps and these relationships will improve the understanding of the biogeochemical processes and provide better input nutritional data for the global models of carbon cycle and climate change.

How to cite: Vallicrosa, H., Sardans, J., Zuccarini, P., Maspons, J., and Peñuelas, J.: Neural Networks to estimate world forest foliar elemental composition and stoichiometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8994, https://doi.org/10.5194/egusphere-egu2020-8994, 2020.

Humans have dramatically increased atmospheric CO₂ concentration as well as biologically available nitrogen (N). Nitrogen is an essential nutrient for vegetation growth and N availability represents a limiting factor on carbon (C) sequestration by the terrestrial ecosystems.  While there is a large infrastructure for measurements to constrain the C cycle, data to constrain the N cycle are less readily available. Using a combination of remote sensing products (MODIS), canopy N concentration data (ICP forest), plant functional type and environmental variables including soil, climate (WorldClim) and elevation (EU-DEM), we generated a canopy N map across European forests using a random forest statistical method (hereafter RF canopy N map).

Most current Global Vegetation Models (GVMs) have integrated C and N cycles, to account for the link between C and N for plant growth and respiration. Leaf N concentration is also important for other biomass compartments as N allocations are prescribed relative to leaf N.  The objective of this study is to compare canopy N of two GVMs, O-CN and LPJ-GUESS, and the RF canopy N map in European forests.

The obtained canopy N maps show contrasting spatial patterns. The RF canopy N map shows higher canopy N values, i.e. between 1.8 and 2.2 %N, in mid-western and eastern Europe, while showing lower values, i.e. 1.2 and 1.6 %N, around the Mediterranean region and in the south of Sweden. The canopy N map obtained from the O-CN simulation shows relatively lower canopy N values, ranging from 1.0 to 1.8 %N, in central and northern Europe, while in the Mediterranean region the values are higher, between 1.8 and 2.4 %N. Similar to the RF map, the LPJ-GUESS canopy N map shows relatively higher canopy N values in mid-western Europe compared to southern and northern Europe, however, the LPJ-GUESS canopy N values show little spatial variation in the Mediterranean region.  Also, the LPJ-GUESS values are higher, with canopy N values ranging between 2.0 and 2.8 %N in mid-western Europe, and canopy N values ranging between 1.6 and 1.8 %N in the Mediterranean region.

The analysis yields insight into spatial differences in RF canopy N and canopy N predicted by GVMs, with especially a mismatch in arid and warm regions.

How to cite: Loozen, Y., Karssenberg, D., de Jong, S., Lu, M., Olin, S., Wassen, M., Wårlind, D., Zaehle, S., and Rebel, K.: Canopy N across European forests: comparing spatial patterns of canopy N retrieved from remote sensing, environmental variables and global vegetation models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20651, https://doi.org/10.5194/egusphere-egu2020-20651, 2020.

Foliar nitrogen concentration (foliar N) and leaf mass per area (LMA) have been identified as key drivers of plant functional diversity and are strongly correlated with photosynthetic carbon assimilation in terrestrial ecosystems. However, these traits are not static between and among species, instead tradeoffs between light interception, photosynthetic capacity, and construction costs (e.g. leaf economics spectrum) lead to significant variation across landscapes. This diversity in leaf traits can lead to considerable differences in carbon assimilation rates at the leaf level, which is difficult to quantify  at ecosystem scales without advanced technologies. Much of our current understanding of landscape-scale heterogeneity in functional traits has come from airborne imaging spectroscopy, which can be linked with foliar trait data to map functional diversity across entire ecosystems. Yet, these remote sensing platforms primarily measure processes occurring in leaves at the top of the canopy, thus ignoring critical information about the three-dimensional structure of forest canopies. Moreover, there is a critical relationship between forest structure and function which drives ecological processes such as carbon assimilation, resource use and efficiency, and woody growth. With traditional remote sensing platforms assuming a 2D world, this leads to an important question in ecosystem functioning: Do total canopy foliar N patterns match top of canopy N concentrations, or are these patterns different? In the United States, the National Ecological Observatory Network’s Airborne Observation Platform (NEON AOP) provides a unique opportunity to address this question by collecting airborne lidar and hyperspectral data in unison across a variety of ecoregions. With a fusion of hyperspectral and lidar data from the NEON AOP and field collected foliar trait data, we show that top of canopy leaf-level and whole canopy foliar N represent fundamentally different measurements regardless of spatial resolution, which could have critical impacts when scaled to landscape, continental, and global models. In addition, we examine the influence of topography, geology, and management regimes on these two measurements of functional diversity at a NEON site consisting of patches of open longleaf pine and dense broadleaf deciduous forests. By understanding how these measurements are linked to abiotic gradients and management regimes, we show that top of canopy functional diversity is more closely related to environmental gradients, reflecting species differences, while whole canopy functional diversity is more evenly distributed, which is a reflection of N availability and utilization across this ecosystem.

How to cite: Kamoske, A., Dahlin, K., Serbin, S., and Stark, S.: Leaf functional diversity is not equivalent to canopy functional diversity: Mapping whole canopy traits with imaging spectroscopy and lidar fusion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11293, https://doi.org/10.5194/egusphere-egu2020-11293, 2020.

Understanding the coordination of ecosystem functions across biomes and climate is still a major challenge that hampers our ability to properly predict biosphere response to climate change. Theories such as the leaf economics spectrum and the least cost investment strategy postulate that plants optimize the rate of investment in transpiration, photosynthetic capacity, and nitrogen (N) allocation dependent on the ratio of their costs to gain given their resources and environment.

In this contribution we test whether theories about functional traits coordination at leaf and organs level are emerging at ecosystem scale. We further investigate the existence of a global spectrum of ecosystem functional properties, and analyze how state of the art terrestrial biosphere models reproduce the spectrum.

To do so we used data of CO₂, water and energy exchange for 164 sites (1237 site years) from the FLUXNET LaThuile and FLUXNET 2015 datasets with at least 3 years of data. For 61 sites, we were able to compile site information on canopy-scale measurements of foliar N concentration, maximum leaf area index , and stand age, from the literature.

We find evidence that a global spectrum of ecosystem functional properties exist, and that most of the variability (66.2%) is captured by three dimensions. The first dimension represents ecosystem productivity; the second the water availability gradient, and climate limitations to productivity; the third dimension reflects ecosystem respiration potential and carbon-use efficiency and is related to aridity and stand age and disturbance regimes. The first two dimensions of the spectrum are well captured by ecosystem models, while the third dimension is poorly reproduced. This might be related to the spin up of the models (steady-state condition) or to an incomplete representation of processes related to age that might limit the ability of models to accurately predict the dynamic carbon, water and nutrient cycling in ecosystems in disturbed areas.

Finally, we show across ecosystems globally that leaf level theories can be in some cases translated to the ecosystem scale. As a main example we found an inverse relationship between photosynthetic N and water use efficiency as postulated by the least cost investment theory across FLUXNET sites. However, this is possible only when the contribution of vegetation is properly accounted for, and evaporation from soil and wet surfaces is removed from the analysis. This highlights that emerging biological patterns at ecosystem scale might be masked by other factors related to physical rather than biological responses.

How to cite: Migliavacca, M., Musavi, T., Mahecha, M. D., Nelson, J. A., Knauer, J., Baldocchi, D. D., Perez-Priego, O., and Reichstein, M. and the Ecosystem Functional Properties group: The spectrum of ecosystem functional properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12843, https://doi.org/10.5194/egusphere-egu2020-12843, 2020.

Despite high importance of macrophytes in shallow thaw lakes for control of major and trace nutrients in lake water, the chemical composition of different aquatic plants and trace element (TE) partitioning between macrophytes and lake water and sediments in the permafrost regions remain totally unknown. Here we sampled dominant macrophytes of thermokarst (thaw) lakes of discontinuous and continuous permafrost zones in Western Siberia Lowland (WSL) and we measured major and trace elements in plant biomass, lake water, lake sediments and sediment porewater. All 6 studies plants (Hippuris vulgaris L., Glyceria maxima (Hartm.) Holmb., Comarum palustre L., Ranunculus spitzbergensis Hadac, Carex aquatilis Wahlenb s. str., Menyanthes trifoliata L.), sizably accumulate macronutrients (Na, Mg, Ca), micronutrients (B, Mo, Nu, Cu, Zn, Co) and toxicants (As, Cd) relative to lake sediments. The accumulation of other trace elements including rare earth elements (REE) in macrophytes relative to pore waters and sediments was strongly species-specific. Under climate warmings scenario and the propagation of southern species northward, the accumulation of trace metals in aquatic plants of thermokarst lakes will produce preferential uptake of Cd, Pb, Ba from thermokarst lake water and sediments by the biomass of aquatic macrophytes. This may eventually diminish the transport of metal micronutrients from lakes to rivers and further to the Arctic Ocean.

Support from the RSF (RNF) grant 19-77-00073 “Experimental modeling of the formation mechanisms for elemental composition of water in thermokarst lakes of Western Siberia: vegetation effect”.

How to cite: Manasypov, R., Pokrovsky, O., and Shirokova, L.: Biogeochemistry of macrophytes, sediments and porewaters in thermokarst lakes of western Siberia in the discontinuous and continuous permafrost zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1128, https://doi.org/10.5194/egusphere-egu2020-1128, 2020.

Semiarid forests characterized by the presence of “trub” species, which have short heights but large canopy sizes, can maintain a high carbon sequestration rate. By integrating terrestrial laser scanning (TLS), we quantified drought-forced tree morphological variation along a precipitation gradient; annual precipitation (MAP) explained 70.3% of variation in tree height (Height) but did not explain the variation in canopy area (CA). Theoretical CA-Height relationships widely adopted by dynamic global vegetation models (DGVMs)  matched only the 5^th percentile of our results, which is problematic for simulating carbon sequestration of open forests in semiarid regions. The trend toward “trubs” under a drying climate implies two decoupled functions of stems, mechanical stability and hydraulic efficiency, and can be an important strategy for trees to balance water and carbon. Our results demonstrate the importance of tree morphological studies for both tree environment-acclimation strategies and the improvement of DGVMs.

How to cite: Dai, J., Liu, H., Wang, Y., and Guo, Q.: Drought-forced tree morphological changes facilitate trubs in a semiarid region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2346, https://doi.org/10.5194/egusphere-egu2020-2346, 2020.

Usually hydraulic conductance and vulnerability are measured under extreme conditions never experienced by living plants (e. g. centrifugation, bench dehydration, and large pressure gradients). A common factor that is known to inhibit the water transport in plants is cavitation, which is believed to occur either by air entry through the pit valves on the walls of the xylem, or by ex-solution of dissolved gases, or vaporization of water at very low pressures. Various physical characteristics of the xylem influence the efficiency of transport and the vulnerability to cavitation.

Here we explore possibilities to measure hydraulic conductance and induce cavitation under close to natural conditions. We designed a very simple “artificial plant” consisting of a root and a transpiring membrane, equipped with pressure and flow meters, where a twig can be inserted in the flow path to measure its hydraulic conductance. Attempts to induce cavitation resulted in surprising results, provoking new questions on the role of xylem structural traits and their relevance for water transport in plants.

How to cite: Krieger, L. and Schymanski, S.: Measuring plant hydraulic conductance and xylem vulnerability under close to natural conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2675, https://doi.org/10.5194/egusphere-egu2020-2675, 2020.

The higher plants species during introduction demonstrate their properties, sometimes going beyond their traits in the natural areas. The most striking example may be given in this case is Acer nugundo L. (ash-leaved maple from the North America, a common component of forests in river valleys). In the forest-steppe –steppe landscapes of the Middle Volga region, it became a tree weed that exhibits exceptional resistance to abiotic stress conditions, including droughts.

Being introduced to alien territories, tree species generate different sorts of  “distortions” into local biogeochemical cycles in natural ecosystems and anthropogenically transformed environment. We would like to list briefly some kinds of such influence expressed in the conditions of the forest-steppe-and steppe ecosystems of our region.

The direct or indirect effects on water cycle may be connected with:

- The changes in water balance due to additional transpiration during the overgrowth of previously treeless localities with the transition from grassy to pseudo-forest communities (Ulmus foliaceae L., Acer negundo L., Elaeagnus angustifolia L.).

- The emission of additional amount of terpenes and other aeroions into the air (various types of coniferous and deciduous trees and shrubs), which can act as centers of water vapor condensation.

The direct or indirect effects on carbon cycle (as well as nitrogen and phosphorus) may be connected with:

- The formation of leaf mass not eaten by local phytophages, replenishing the fund of leaf litter (Acer negundo L., Aesculus hyppocastanum L., species of Juglans genera.).

- The influence on the soil biological activity by stimulating or inhibiting the development of soil microbiota members (different tree species including Juglans cinerea L., J. mandshurica Maxim. , J. nigra L. and others).

- The changes in the soil nitrogen balance, especially pronounced for species with "symbiotic support" (Elaeagnus angustifolia L., Hyppophae ramnoides L.).

The above effects were detected by us for the few species including named above using various field and laboratory methods. Now we can consider them at the level of their identification as such Their scale assessment at the ecosystem level may become a next stage.

An analysis of the possibilities of identifying new pseudo-forest communities developing on the grassy deposits was carried out in local conditions by integrating ground-based survey data and remote sensing. This aspect seems to be valuable for our region with highly mosaic combination of natural, cultivated, anthropogenically transformed and other territories.

How to cite: Kavelenova, L., Prokhorova, N., Rozno, S., Pomogaybin, A., and Yankov, N.: Introduced species in new ecosystems: concerning possible distortions of local biogeochemical cycles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4827, https://doi.org/10.5194/egusphere-egu2020-4827, 2020.

The Sokol'i Mountains are a small continuation of the Zhiguli mountains on the Volga River left bank. We studied the quantitative characteristics of the mineral nitrogen forms content and the Azotobacter activity levels in soils along the ecological profile that crosses the Sokol'i Mountains massif from south to north from Samara city.

The plots in the profile were represented different types of plant communities: in the Samara city suburb adjacent to the Sokol'i Mountains - a part of the birch planting; on the southern, northern slopes and the watershed – the broad-leaved forest with a predominance of Tilia cordata Mill., Acer platanoides L. and Quercus robur L. with a small participation of Betula pendula Roth.; on the terraces and the bottom of the carbonate quarry at the northern slope of the mountains - sparse forests dominated by small-leaved species (Populus nigra L., P. tremula L.), and Pinus sylvestris L. with the addition of Salix sp.; near the quarry - rocky steppes  and steppe meadows.

The main nitrogen suppliers to the geochemical cycle in the study area are the organic residues (leaf litter in the forest, steppe felt in steppes) and the fixation of atmosphere nitrogen. Nitrogen-containing compounds from atmospheric precipitation due to transport emissions and other sorts of air pollutions can also make a certain contribution into nitrogen cycle.

The broad-leaved species prevail in the natural forests of the Sokol'i Mountains, and small-leaved species and pine dominate in the secondary forest stands, which are formed during the self-growth of the former quarry. It is known that the litter of broad-leaved species is much richer in nitrogen than the litter of small-leaved and coniferous species. The undisturbed soils of the forest trial plots in the Sokol'i Mountains contain significantly more mineral forms of nitrogen (nitrites, nitrates, ammonium) than in the fine-grained soil fractions of the former quarry with their low fertility, as well as in the soils of stony steppes and steppe meadows.

The content of mineral forms of nitrogen directly correlates with the content of humus (correlation coefficient from 0.77 to 0.92), which is significantly higher in soils under natural broad-leaved forests. A high and reliable positive correlation was found for all analyzed forms of nitrogen among themselves (correlation coefficient from 0.64 to 1.0), which proves the natural  pattern of nitrogen cycle in the study area.

A regular increase of the Azotobacter activity level in the humus-poor soils of the quarry and grassy ecosystems has been established. This activity was characterized by a negative correlative relationship with the content of humus and all mineral forms of nitrogen (correlation coefficient from 0.4 to 0.71).

The Azotobacter activity increases as soil alkalization rised, which is especially pronounced in the quarry.

In general, the nitrogen cycle occurring in the Sokol'i Mountains ecosystems demonstrates association with the type of plant organic matter, nitrogen fixation levels, with the influence of  the anthropogenic activity ( past open-cast mining of raw materials and  the self-growing of secondary forests in the former quarry).

How to cite: Prokhorova, N., Makarova, Y., and Kavelenova, L.: On the traits of the nitrogen cycle in natural and anthropogenically disturbed ecosystems of the Sokol'i Mountains (Samara Region, Russia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5029, https://doi.org/10.5194/egusphere-egu2020-5029, 2020.

We investigated the vertical and spatial distribution of chemical elements (ChEs) in four cross-sections within a catena formed in typical southern taiga on Retiosols , underlying loess  loams and carbonate moraine deposits. Catena located in the Tver' region (Russia). In plants (70 samples, 19 species) and soils (31 samples), the total content of the ChEs was determined by mass spectrometry. In soil samples, we measured pH, grain size and levels of ChE mobile fractions (exchangeable (F1), bound to organic complexes (F2) and bound to Fe and Mn hydroxides (F3).

In the A-horizons the average total concentration of Fe is 1,2%, Ti – 0,33%; Mn – 482 mg‧kg-1, Zr–292, Sr–90, Zn–39, Cr–21, Pb–21, Ni–9, Cu–8. The concentration of metal F1 diminishes in order: Fe>Mn>Sr>Zn, Pb>Ti, Cr, Ni, Cu, Co, Zr. The concentrations of F2 and F3 show the following order: Fe>Mn>>Ti, Zr, Pb>Co>Ni, Cu, Zn>Cr, Sr and Fe>Mn>Ti>Zn, Sr, Pb>Cr>Cu, Ni, Co>Zr, respectively.

In all studied Retisols, vertical distribution of the total Pb and Zr, F1 of Co, Fe, Mn, Pb and Zn, F2 of Cu, Fe, Pb and Zn, F3 of Pb accumulate in topsoil. For the total Co, Fe, Ni, Sr and Zn, F1 of Co, Cr, Cu, Mn, Pb, Zn and Zr, F2 of Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn and Zr and F3 of Co, Cr, Cu, Ti, Zn, Zr the loss from the albic horizons and/or the accumulation in the argic horizons were registered.

Spatial distribution of the total concentration of ChEs increases in the A-horizon in the upper part of the catena slope position. In the A-horizons at footslope and toeslope positions, the concentration of F1 Ni, Cu, Sr and Zr, F2 Ni, Cu and Zn increases, and the concentration F2 of Co, Cr, Pb, Ti and Zn, F2 of Cr, Ti and Co, F3 of Mn, Ni, Zn, Pb, Zr decreases.

Ratios calculated on the basis of the total and mobile element content were applied to evaluate biogenic migration of ChEs with different biophilicity in the "plant-soil" system. According to soil-to-plant transfer ratios, Mn, Zn and Cd are actively involved in biological accumulation. Participation in biological accumulation of Mn and Zn was noted in many works (Avessalomova, 2007; Isachenkova, Tarzayeva, 2006, Kadata-Pendias, Szteke, 2015)

Mn and Zn have important physiological significance in plants; they actively migrate in plant tissues. Cd is not a necessary ChEs for plants but is easily absorbed by the root system and leaves (Kabata-Pendias, 2011). Cationic elements (Cd and Zn) have high mobility in the soils (Jen-How Huang, 2011). Our results indicate that in the reference forest communities, tree species play the major role in the uptake and turnover of biophilic microelements (Mn, Zn, Co) while sphagnum moss and grassy covers mostly absorb the elements with low biophilicity (Fe, Ti, Cr, Zr, Pb). Metabolic pathways carry out the absorption of Fe and Cr (Kabata-Pendias, 2011).

How to cite: Enchilik, P., Aseeva, E., Semenkov, I., Samonova, O., Iovcheva, A., Terskaya, E., and Kasimov, N.: Microelement mobile forms in southern taiga landscapes of the Central Forest State Biosphere Nature Reserve (Russia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5810, https://doi.org/10.5194/egusphere-egu2020-5810, 2020.

The xylem specific hydraulic conductivity (k_s) is a key trait for the description of the plant’s ability to sustain the long-distance water transport required for transpiration. In this work, we systematically analyze xylem flow in several woody plants with contrasting anatomical traits combining flow experiments under different hydraulic pressure gradients. Results show a time and pressure dependence of k_s similar to observations made a century ago by Dixon (1914). We mainly attribute the persistent drop in k_s, accentuated with higher-pressure gradients, to a wounding response of the xylem tissues. Evidence suggests that wounded xylem tissue releases polysaccharides (prominently pectin) that gradually occlude xylem conduits. The macroscopic definition of K is further affected by complex microscopic xylem dynamics, with a key role of the xylem network topology, interconduit pit membrane flexibility, and redundancy of flow paths. These findings validate the picture of a complex and delicate conductive system whose hydraulic behavior goes beyond that of passive and inert deadwood. Notable implications for xylem conceptualization, measurements protocols, as well as ecosystem modeling applications are discussed.

How to cite: Bonetti, S., Breitenstein, D., Fatichi, S., Domec, J.-C., and Or, D.: The hydraulic conductivity of wounded xylem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5856, https://doi.org/10.5194/egusphere-egu2020-5856, 2020.

The spatial and temporal variation in plant respiration is one of the largest unknowns in the global land carbon budget. While respiration rates are directly related to temperature, plant respiration of trees is also determined by their stem sapwood proportion, tissue nitrogen (N) contents and other factors. The sapwood proportion is related to the biomass fraction of respiring living cells in the tree stem. The respiratory costs that plants have to invest to maintain basic functions (maintenance respiration) are related to the vegetation N content, since maintenance respiration supports protein repair and replacement, and most plant organic N is in proteins.

Here we explore the variation and underlying drivers in these two plant traits (stem sapwood proportion, tissue N contents) and derive novel estimates of their spatial distribution in northern hemisphere boreal and temperate forests. For the first task, we make use of measurements of sapwood and total cross-sectional area in tree stems and of N contents per dry matter in stems, roots and leaves. Such data are collected from plant trait databases like TRY, the biomass and allometry database (BAAD) and extensive literature reviews covering the most common boreal and temperate tree species. For the second task, we apply the derived tree level relationships between these traits and the underlying drivers (species, climate, soil variables) in combination with satellite radar remote sensing based products of compartment (stem, branch, root and leaf) biomass and tree species distribution maps covering the entire northern boreal and temperate forests.

We find that both the proportion of sapwood to total stem biomass and the response of the N content to environmental conditions are fundamentally different among tree genera. For instance, the sapwood proportions are spanning from 20–30% in larch to > 70% in pine and birch forests. These findings highlight the need to consider genera-specific differences when estimating the response of plant respiration to changes in climate and forest management.

How to cite: Thurner, M., Beer, C., and Hickler, T.: Sapwood proportion and nitrogen content in boreal and temperate tree species, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6133, https://doi.org/10.5194/egusphere-egu2020-6133, 2020.

Background: Optimal yield is dependent on the collocations between plant population and individual growth. High plant populations for direct sown winter oilseed rape would be a prevailing way to achieve high yield under intensive cropping systems.

Results: We investigated the oilseed rape yield response to planting density while considering the productivity environment, nitrogen (N) fertilizer, and sowing date. A synthesis-analysis was conducted by collecting the density-yield data in the field experiments of oilseed rape from 2000 to 2016 in China. The population yield response to different planting density levels could be described by a quadratic model, with threshold value of 45-60 plant m^-2, and excessive density may cause yield loss as the weak individual growth. High planting density has no remarkable influence on the attainable population yield due to the decreasing individual potential yield. The population yield increment capacity by the increasing planting density was higher in medium yield environment (i.e., average yield at 1500-2500 kg ha^-1). The planting density presented remarkably effect on population yield after the N limitation was relieved. Increasing planting density at 10⁴ plants per hectare was equivalent to apply 1.17 kg N fertilizer on population yield, ranging from 0.42 kg to 4.76 kg under different yield environment levels. Yield loss caused by unsuitable sowing date (especially for the late sowing) could be compensated by increasing planting density.

Conclusion: Planting density played a crucial role in cooperating the other management practices. Optimizing the allocation of plant population and individual growth, establishing target plant phenotype under high planting density would help to achieve high population yield.

How to cite: Cong, R., Zhang, Z., and Lu, J.: A synthesis-analysis of winter oilseed rape (Brassica napus L.) yield response to planting density under intensive cropping system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6703, https://doi.org/10.5194/egusphere-egu2020-6703, 2020.

Revealing the seasonal and interannual variations in leaf-level photosynthesis is a critical issue in understanding the ecological mechanisms underlying the dynamics of carbon dioxide exchange between the atmosphere and shrub ecosystem. Artemisia ordosica is a dominant shrub species in semi-arid area of northwest China. Photosynthetic gas exchange, leaf nitrogen content(LN), specific leaf area (SLA) and some environmental factors were measured simultaneously on clear days (rotated every 10 days) of the growing season from 2011 to 2018, to quantify the temporal variations and environmental controls of photosynthetic parameters. Our results demonstrated that mean value of light-response curve parameters, the maximum photosynthetic capacity (P_max), appear quality efficiency (AQE), respiration in dark (R_d), light saturated point (LSP) and light compensated point (LCP) had a gradual decline with the growth (spring> summer>autumn). Structural equation modeling (SEM) was used to elucidate the direct and indirect effects of biophysical factors on P_max. The driven factors of P_max in growing season changed, but stomatal conductance (g_s) was the dominant factor in all stages. The g_s was influenced by SLA and LN，and the soil water content at a depth of 10cm (SWC₁₀) affected the P_max in spring. In summer, P_max was significantly positively related with g_s and transpiration rate (T_r), and g_s was influenced by SLA, LN and soil water content at a depth of 30cm (SWC₃₀). In autumn, P_max was significantly positively correlated with g_s, while was significantly negatively correlated with air temperature (T_a). This simulation based on situ ecophysiological research suggest that P_max of A. ordosica responded to the environment factors of seasonal and interannual variations, which is not the inherent genetic characteristics. Soil water content is the major environmental factor influencing P_max in spring and summer, while T_a is the major one in autumn. Knowledge of how environmental change will affect the photosynthesis of A. ordosica in the future is essential for their protection, adaptation strategies and carbon fixation prediction in shrub ecosystems.

How to cite: Tian, Y., Zha, T., and Jia, X.: Multiple drivers of seasonal and interannual variation in Pmax: Implications for leaf photosynthesis of Artemisia ordosica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7079, https://doi.org/10.5194/egusphere-egu2020-7079, 2020.

Increasing evidence now exists for a tight connection between tree diversity and carbon storage capacity. As part of the Paris Agreement (COP21), forests play a critical and prominent role to reach the ambitious goal of net-zero emissions in the second half of this century. Besides reducing emissions from deforestation and forest degradation (also known as REDD), maintaining and enriching tree assemblages could thus help mitigating climate change via increased abundance and more efficient resource use.

However, recent evidence questions this widespread idea of positive diversity effects on forest carbon storage. Specifically, tree diversity may not always be a causal mechanism but rather a consequence of tree abundance and productivity (following the ‘more individuals hypothesis’). To test these contrasting hypotheses, this contribution analyses the most plausible causal pathways and their stability along global climatic gradients in the diversity-abundance relationship across the World’s main forest biomes, using a dataset comprising more than 2,500 forest plots and 83,800 trees sampled in pristine forest landscapes in all continents (except Antarctica).

We demonstrate that causal relations can be reconciled along global climate gradients, with diversity effects prevailing in the most productive environments, and abundance effects becoming dominant towards the most limiting conditions. These findings have major implications on climate change mitigation strategies aimed at carbon sequestration: we find that future nature-based mitigation solutions focused on fostering biodiversity will only be cost-effective in productive forest landscapes. In less productive environments, by contrast, mitigation measures should promote the abundance of locally adapted functional strategies. Conservation of species diversity in equatorial and tropical areas is thus a priority, not only to preserve the inherent value of biodiversity but also to achieve the global goals on atmospheric decarbonization. In less productive lands on Earth, the conservation of abundance through productivity should be posed, next to diversity, as a major element in environmental policies and land management.

How to cite: Madrigal-Gonzalez, J. and the World's natural forests: Causality in the diversity-abundance relationship across the main World’s forest biomes: insights for nature-based mitigation solutions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8259, https://doi.org/10.5194/egusphere-egu2020-8259, 2020.

The resources (light, nitrogen and water) utilization efficiency of plant is a key indicator reflecting the adaptive ability of plant to environment. CO₂ enrichment would increase photosynthesis substrate supply and nutrient absorption in plants，and may also change the utilization efficiencies of light (LUE), nitrogen (NUE) and water (WUE) and their trade-offs relationship. However, the knowledge regarding how the LUE, NUE and WUE of woody plant change in the context of CO₂ enrichment is still weak. In order to understand the impacts of CO₂ enrichment on the LUE, NUE and WUE of Schima superba and their trade-offs, one-year-old container seedlings of S. superba were grown with ambient air (AA treatment), 550 ppm of CO₂ concentration (E1-CO₂ treatment), 750 ppm of CO₂ concentration ( E2-CO₂ treatment) and 1000ppm of CO₂ concentration (E3-CO₂ treatment) using open top chambers. In the growing season, we regularly examined the net photosynthetic rate, stomatal conductance, transpiration rate, nitrogen concentration and photosynthetic pigment concentration of S. superba leaves. In addition, the different organ biomass, leaf area, soil nitrate and ammonium nitrogen concentrations were also simultaneously examined. The results demonstrate that three CO₂ enrichment treatments significantly increased the LUE and NUE of S. superba leaves at the end of June, while the leaf nitrogen concentration and soil nitrate nitrogen significantly decreased under both the E2-CO₂ and E3-CO₂ treatments compared with those under the AA treatment. In contrast, only the E1-CO₂ treatment significantly increased the LUE and NUE of S. superba leaves at the end of August. The NUE of S. superba leaves under both the E2-CO₂ and E3-CO₂ treatments were significantly higher than that under the AA treatment at the end of October. With regard to the WUE of S. superba leaves, there were no significant differences between the four treatments. At the end of October, the total biomass of S. superba under the E1-CO₂ treatment was significantly higher than that under both the AA and E3-CO₂ treatments, while the total biomass of S. superba under the AA treatment was not significantly different from that under both the E2-CO₂ and E3-CO₂treatments. During the experiment, the LUE, NUE, stomatal conductance, and transpiration rate of S. superba leaves were significantly and positively related to each other. The LUE also had a significantly positive correlation with specific leaf weight. Furthermore, the NUE was significantly and positively correlated with the total biomass and the ratio of underground and aboveground biomass. Meanwhile, the NUE was significantly and negatively correlated with the chlorophyll a concentration, chlorophyll b concentration, carotenoid concentration, leaf nitrogen concentration, soil ammonia nitrogen and nitrate nitrogen concentration. The WUE was significantly and negatively related to the stomatal conductance, transpiration rate and total biomass. CO₂ enrichment may enhance both the LUE and NUE of S. superba seedlings, whereas the impacts of CO₂ enrichment on the LUE and NUE of S. superba seedlings varied with time. S. superba seedlings would appear photosynthesis acclimation with the persistently high CO₂ enrichment.

How to cite: Cao, J., Pan, H., Chen, Z., and Shang, H.: The impacts of CO2 enrichment on the plant resources utilization efficiency of Schima superba seedlings in subtropical China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8838, https://doi.org/10.5194/egusphere-egu2020-8838, 2020.

Future land carbon (C) uptake under climate changes and rising atmospheric CO₂ is influenced by nitrogen (N) and phosphorus (P) constraints. A few existing land surface models (LSMs) account for both N and P dynamics, but lack comprehensive evaluation. This will lead to large uncertainty in estimating the P effect on terrestrial C cycles. With the increasing number of measurements and data-driven products for N- and P- related variables, comprehensive model evaluations on large scale is becoming feasible.

In this study, we evaluated the performance of ORCHIDEE-CNP (v1.2) which explicitly simulates N and P cycles in plant and soil, in four aspects: 1) terrestrial C fluxes, 2) N and P fluxes and budget, 3) leaf and soil stoichiometry and 4) resource use efficiencies. We found that ORCHIDEE-CNP improves the simulation of the magnitude of gross primary productivity (GPP) due to more realistic strength of the CO₂ fertilization effect of GPP than the without-nutrient-version ORCHIDEE. However, ORCHIDEE-CNP cannot capture the positive and increasing C sink in North Hemisphere over past decades, which is mainly due to that a large fraction of N and P ‘locked’ in soil organic matter cannot be re-allocated into vegetation and leads to a strong N and P limitation on plant growth. ORCHIDEE-CNP generally simulates comparable global total N and P fluxes (e.g. N biofixation, P weathering, N and P uptake etc.) for both natural and agricultural biomes. Overall, ORCHIDEE-CNP doesn’t performance worse in C fluxes than ORCHIDEE, and gives reasonable N and P cycles, which is acceptable in simulating the coupling relationships between C, N and P cycles can be used to explore the nutrient limitations on land C sink from present to the future. 

How to cite: Sun, Y., S Goll, D., Chang, J., Ciais, P., Guenet, B., Helfenstein, J., Huang, Y., Lauerwald, R., Maignan, F., Naipal, V., Wang, Y., Yang, H., and Zhang, H.: Global evaluation of the nutrient enabled version of land surface model ORCHIDEE-CNP (v1.2) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9609, https://doi.org/10.5194/egusphere-egu2020-9609, 2020.

Shrub plants play important roles in both forest and shrubland ecosystems. Analyzing the differences of functional traits of shrubs grown in understory of forest communities and in various shrublands can explore the adaptation strategies of shrubs in different habitats. Nine functional traits of leaf and twig collected from 20 dominant shrub species in 24 plots distributed in three habitats: forest shrub layer, secondary shrubland and primary shrubland, in Beishan Mountain of Jinhua, Zhejiang Province, eastern China, were measured. The overall differences, inter- and intra-specific variations and the differences in various life forms of shrub traits in three different habitats were statistically analyzed. Results show that: 1) There are differences of nine plant traits for shrubs grown in three different habitats. The understory shrubs have larger leaf area (LA) and specific leaf area (SLA), smaller leaf dry matter content (LDMC), leaf tissue density (LTD) and twig tissue density (TTD), while shrubs in secondary shrubland have larger leaf thickness (LT) and LTD, smaller SLA and twig dry matter content (TDMC) compared with shrubs from the primary shrubland. 2) The intraspecific variation coefficients of SLA, twig diameter (TD), TTD, and TDMC in understory shrubs are the largest, while the interspecific variation coefficients of SLA, LDMC, TDMC, and TTD in ​​secondary shrubland are the smallest. 3) Among different life forms, the understory evergreen shrubs have significant higher LT, LTD, and LDMC than deciduous shrubs, while deciduous shrubs have significant higher SLA than evergreen shrubs. The differences of LT and SLA between evergreen and deciduous shrubs of primary shrubland are the same as those of understory shrubs, but the differences of LTD and LDMC between evergreen and deciduous shrubs have the opposite trend. 4) The main source affecting shrub traits is species, along with an explanation ratio from 38.01% to 78.92%. The second source is habitat. In short, compared to shrubs from shrublands, understory shrubs in forest communities form a series of trait combinations that are larger LA and SLA, smaller LTD, TTD, and LDMC to adapt to the understory environment with less light and stronger competition. Secondary shrubland, compared to the primary shrubland, has a series of shrub trait combinations that are larger LT, LTD and TD, smaller LA, SLA, TDMC and twig bark thickness (TBT) to store more nutrients.

How to cite: Ni, J., Cao, J., and Yuan, Q.: Shrub traits of forest and shrubland reveal different growth strategies , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12373, https://doi.org/10.5194/egusphere-egu2020-12373, 2020.

Purpose: This study was aimed to quantify the effect of different variety, planting density, training system and canopy position on tree water and nitrogen use efficiencies in relation to mango fruit yield and size as well as soil fertility in a 5-year-old mango plantation of tropical Australia.

Material and Methods: Soil (0-10 cm) and mango foliar samples were collected from a 5-year-old, factorial field experiment testing the effects of two mango varieties (Calypso vs Keitt), two planting densities (medium vs high), two training systems (single leader vs conventional) and two sampling canopy positions (north vs south) on foliar total carbon (TC, %), total nitrogen concentration (TN, %), and stable carbon (C) and nitrogen (N) isotope compositions (δ¹³C and δ¹⁵N) as well as the corresponding total C, total N and δ¹³C and δ¹⁵N in the surface soil of tropical Australia. In addition, mango fruit yields and sizes were determined. Soil and foliar total C and N as well as δ¹³C and δ¹⁵N were determined on mass spectrometers at Griffith University. Each of the above treatment was replicated 6 times for foliar samples and 3 times for soil samples.

Results: There were significant genetic effect on foliar total N concentration (TN, %), tree water use efficiency (WUE) as reflected by foliar δ¹³C, N use efficiency (NUE) as indicated by foliar TN and δ¹⁵N, mango fruit yield and sizes in the 5-year-old mango plantation of tropical Australia. Overall, mango variety of Keitt had higher tree WUE and NUE as well as higher mango yield and greater fruit size, compared with those of mango variety of Calypso. There were also significant environmental influences on mango tree WUE and NUE as well as mango yield and fruit size. In particular, high planting density had higher tree NUE, and lower WUE as well as higher N loss, compared with those of medium planting density. High planting density treatment also had higher soil total N, compared with that of medium planting density treatment. The convention training system also had higher tree NUE and WUE, compared with the single leader training system. The northern side of tree canopy (sunny side) had lower fruit number, compared with the southern side (shady side) of tree canopy.

Conclusion: There were significant genetic and environmental influences on tree WUE and NUE as well as mango fruit yield and sizes in the 5-year-old mango plantation, highlighting the significant and exciting opportunities to improve mango tree WUE and NUE as well as fruit yield and soil fertility with both genetic selection and site management regimes.

How to cite: Sun, W., Xu, Z., Ibell, P., and Bally, I.: Genetic and environmental controls of tree water and nitrogen use efficiency of 5-year-old mango plantation in relation to mango fruit yield and size as well as soil fertility in tropical Australia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12887, https://doi.org/10.5194/egusphere-egu2020-12887, 2020.

Flower-strips are increasingly recognized as mandatory elements in agricultural landscapes for pollinators to survive. In this study, we raised the hypothesis that flower-strip traits additionally affect biogeochemical cycling towards climate change mitigation. Therefore, we investigated soil carbon and nutrients stocks in paired comparison to adjacent land use and looked at water retention.

Two study farms of 50 ha in southern Germany were sampled once in spring, summer and autumn. The examined flower-strips of both farms were sown in 2011 and are in use since then. Pairwise sampling reduces the influence of the expected high variation in soil parameters. For each pair we sampled 3 depths: topsoil (0-5 cm), plow horizon (20-25 cm) and subsoil (30-35 cm). Different parameters of soil carbon, nitrogen, nutrients and water will be presented with a focus on clay bonded carbon.

Preliminary results indicate that flower-strips significantly increased nitrogen availability, soil carbon stocks and accordingly showed a trend to improve the water storage capacity in the plow horizon. We did not observe a statistically significant effect on nutrient availability.

Provided that these results will be confirmed, flower-strips traits could go beyond the important trait of giving pollinators a home in vast agricultural landscapes. By slightly increasing the amount of flower-strips in these landscapes, a significant increase in carbon sequestration and water retention will be achievable adding to the 4 per mille goal of the UN.

How to cite: Vetter, V., Jungkunst, H., Schützenmeister, K., and Buhk, C.: Flower Power: flower-strips host bees & sequester carbon in soils, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12891, https://doi.org/10.5194/egusphere-egu2020-12891, 2020.

Semi-arid forests represent some of the most sensitive ecosystems to climate change. Identifying adjustments to extreme conditions can indicate their resilience, and that of forests undergoing increasing aridity trends. We used eddy covariance and close-range sensing measurements over four years in a semi-arid pine forest to identify canopy-scale adjustments to the short active season and long seasonal drought. Peaks in light use efficiency (LUE), leaf chlorophyll content (LCC), and increasing absorbed photosynthetic active radiation (APAR; based on canopy absorption coefficient in the green range), all converged to support an early peak (March) in gross primary productivity (GPP), exploiting the narrow optimum between PAR_in, temperature and the rapidly decreasing soil moisture in spring. In contrast, during the long dry period (>200 days), while PAR_in increased, LCC and LUE decreased, offering physiological photoprotection as GPP sharply declined under the stressful conditions. The strong negative correlation between ρ_NIR and PAR_in indicated canopy biophysical adjustments that enhance light absorption under low radiation and eliminate photodamage under excessive radiation.  The results provide clear indications of canopy-scale adjustments underlying the high productivity of the forest and its resistance to the harsh conditions, which may soon apply to forests in currently milder climatic regions.

How to cite: Wang, H., Gitelson, A., Sprintsin, M., Rotenberg, E., and Yakir, D.: Identifying canopy-scale adjustments to the extreme climate in a semi-arid pine forest using eddy covariance and close-range sensing data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13303, https://doi.org/10.5194/egusphere-egu2020-13303, 2020.

The functional diversity (FD) represented by plant traits is fundamentally linked to the ecosystem’s capacity to respond to changes in the environment. Thus, there is an ongoing debate about including FD considerations in management plans to safeguard forests and their services to the society under climate change. However, incomplete scientific knowledge and difficulties to understand the concept of FD hinder the implementation of FD-based management approaches. Our study fills these knowledge gaps by (i) mapping the current distribution, (ii) analyzing the drivers, and (iii) testing the sensitivity of FD to projected increases in temperature and precipitation in northeastern North America. Following the stress-dominance hypothesis, we expected the strongest effect on FD from environmental filtering (i.e., climatic conditions) within our study region.

We combined a literature and database review of 44 traits for 43 tree species with terrestrial inventory data of 48,426 plots spanning an environmental gradient from northern boreal to temperate biomes. Employing multiple non-parametric models, we evaluated the impacts of 25 covariates on FD. Subsequently, we conducted a climate sensitivity analysis. The effect of rarity and commonness were tested for all outcomes using Hill numbers with different abundance weightings.

We identified FD hotspots in temperate forests and the boreal-temperate ecotone east and northeast of the Great Lakes. Forest stand structure explained most of the variation in FD. Elevated temperature increased FD in boreal, but lowered FD in temperate forests. Different species abundance weightings affected trait diversity distributions and drivers only marginally.

As environmental filtering was of secondary importance behind forest structure in explaining the trait diversity distribution of tree species in northeastern North America, our study provides only partial support for the stress-dominance hypothesis. Forest management can increase FD by promoting structural complexity. In addition, mixing species from functionally different groups identified in this study can enhance the response diversity of forests to an uncertain future.

How to cite: Thom, D., Taylor, A., Seidl, R., Thuiller, W., Wang, J., Robideau, M., and Keeton, W.: Drivers, climate sensitivity, and management of functional diversity in northeastern North America, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18535, https://doi.org/10.5194/egusphere-egu2020-18535, 2020.

Spatial variability of ecosystem processes constitutes significant uncertainty source in greenhouse gas flux measurements and estimations. The major disadvantage of the chamber-based flux measurements is the poor spatial representativeness, but eddy-covariance measurements also have an uncertainty due to the unequal and not constant footprint area. One way to overcome these difficulties is the spatial sampling improving the field-scale data coverage.

The aim of this study was to describe the spatial variability of grassland soil CO₂ efflux under varying environmental conditions. For this reason, we conducted spatial measurements on a range of variables including soil respiration, above-ground biomass, greenness index of the vegetation, soil water content and soil temperature during a seven-year study in a dry grassland site in Hungary. Altitude and soil organic carbon (SOC) content of the measuring positions were also used as background factors. Measurements were repeated 19 times at 78 positions during the study, in the main phenological stages of the grassland vegetation: spring growth, summer drought, autumn regrowth. The sampling scheme was based on 80×60 m grid of 10 m resolution. SOC content was highly variable among the positions due to the exposure differences and their environmental constrains. We analyzed the effect of the drivers on soil respiration grouping the measuring positions by the SOC content of the soil.

How to cite: Balogh, J., Fóti, S., Gecse, B., Papp, M., Süle, G., de Luca, G., Pintér, K., Kardos, L., Mónok, D., and Nagy, Z.: Spatial soil respiration measurements under varying environmental conditions in a dry grassland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19701, https://doi.org/10.5194/egusphere-egu2020-19701, 2020.

Dynamic vegetation modelling is intensively used with plant functional types which limits the range of interest of obtained outputs for other fields of knowledge like conservation science. An alternative approach is to simulate plant species. This however requires additional data, i.e. morphological and physiological traits values characterizing the species and determining their functional properties. However, not only many traits vary among the species belonging to the same plant functional type but also the traits vary broadly according to climate factors.

Since most of the traits are functional, their values may be critical for dynamic vegetation model outputs. We measured several traits (specific leaf area, leaf and sapwood C:N) of Cedrus atlantica in its native range, the Rif and Middle Atlas Mountains of Morocco, as well as in some plantations in western Europe. Trait values exhibit significant variations between the sampled sites. It is possible to predict these trait values using multiple regression with climate factors as explanatory variables. Using regression equations, we produced spatial- and time-varying traits over the study area. We implemented these equations in the CARAIB dynamic vegetation model and tested whether they improve the simulation of C. atlantica in the Rif and Middle Atlas Mountains, by comparing the net primary productivities and biomasses computed with and without trait variation, with those retrieved from measurements on the sampled sites. We then performed simulations of the future using climate projections of the regional climate model RCA4 nested in HadGEM2 general circulation model under the RCP8.5 scenario, in order to test the influence of trait acclimation on the predicted future changes in the range and productivity of the species.

How to cite: François, L., Hambuckers, A., Henrot, A.-J., Trolliet, F., Pitance, J.-L., Cheddadi, R., and Dury, M.: Does acclimation of plant traits improve dynamic vegetation modelling of a tree species?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19768, https://doi.org/10.5194/egusphere-egu2020-19768, 2020.

Mediterranean ecosystems, such as the savannah-type cork oak (Quercus suber) woodlands, are hotspots for climate change, as the highest impacts are forecasted for the Mediterranean region, mainly by more frequent and intense severe droughts. These ecosystems are also threatened by shrub encroachment, which might further decrease tree water availability and affect ecosystem functioning and resilience. Nevertheless, the combined effects of drought and shrub encroachment on ecosystems have seldom been investigated. A precipitation manipulation and shrub removal experiment was established in a cork oak woodland located in SE Portugal and invaded by the native shrub gum rockrose (Cistus ladanifer). Here we present and discuss the combined effects of drought and shrub encroachment on litterfall production of cork oak trees, an evergreen species, over two contrasting years, a wet year (2018) and a dry year (2019) and assess the nitrogen and phosphorus resorption efficiencies from senescent to green leaves.

A previous study reported significant increases in cork oak’s nitrogen resorption efficiency in response to drought. Our preliminary results also indicate changes in nitrogen and phosphorus resorption efficiencies. An increase in nutrient resorption efficiency is likely to mitigate the limitation in nutrient uptake by the roots during drought, improving tree fitness in the short-term. However, it will probably exert a negative feedback on the nitrogen and phosphorus cycles in the long-term which might affect the ecosystem functioning under the forecasted droughts.

How to cite: Lobo-do-Vale, R., Rodrigues, J., Martins, J., Haberstroh, S., Alves, A., Werner, C., and Caldeira, M. C.: Nitrogen and phosphorus resorption efficiencies change under drought and shrub encroachment in a Mediterranean ecosystem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19834, https://doi.org/10.5194/egusphere-egu2020-19834, 2020.

Plant traits – the morphological, anatomical, physiological, biochemical and phenological characteristics of plants – determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystems properties and derived benefits and detriments to people. Plant trait data thus represent the essential basis for a vast area of research spanning evolutionary biology, community and functional ecology, biodiversity conservation, ecosystem and landscape management and restoration, biogeography to earth system modeling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community. Increasingly the TRY database also supports new frontiers of trait-based research, including identi:cation of data gaps and subsequent mobilization or measurement of new data. To support this development, in this article we take stock of trait data compiled in TRY and analyze emerging patterns of data coverage, representativeness, and gaps. Best species coverage is achieved for categorical traits (stable within species) relevant to determine plant functional types commonly used in global vegetation models. For the trait ‘plant growth form’ complete species coverage is within reach. However, most traits relevant for ecology and vegetation modeling are characterized by intraspecific variation and trait-environmental relationships. These traits have to be measured on individual plants in their respective environment: completeness at global scale is impossible and representativeness challenging. Due to the sheer amount of data in the TRY database, machine learning for trait prediction is promising - but does not add new data. We therefore conclude that reducing data gaps and biases by further and more systematic mobilization of trait data and new in-situ trait measurements must continue to be a high priority. This can only be achieved by a community effort in collaboration with other initiatives.

How to cite: Kattge, J., Boenisch, G., Diaz, S., Lavorel, S., Prentice, C., Leadley, P., Wirth, C., and Consortium, T. T.: The TRY Plant Trait Database - enhanced coverage and open access , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20191, https://doi.org/10.5194/egusphere-egu2020-20191, 2020.

Increased nutrient inputs and climate change are affecting ecosystems worldwide. However, there is a dearth of knowledge on how the interacting effects of multiple nutrient inputs and climatic variability may affect ecosystem functioning including grassland species and functional diversity, productivity or resilience to disturbances. This is particularly important in the Mediterranean Basin, a hotspot of climate change, where the frequency of autumn and spring droughts is projected to increase.

We conducted a 6-year nutrient addition experiment in an annual grassland site, in Portugal, that is part of a globally distributed experiment called the Nutrient Network (http://www.nutnet.org/). We added high rates of nitrogen, phosphorus and potassium to 5 × 5 m plots, following a full factorial combination in a complete randomized three block design. We established three treatments of one, two and three added nutrients and maintained control plots without addition of nutrients. We examined how a decrease in nutrient limitation and inter-annual climatic variability affected grassland productivity and diversity. We determined the community functional structure (e.g., Community Weighted Mean) and functional diversity (e.g., Function Dispersion) of key morphological and physiological leaf traits associated with the leaf economics spectrum, resource acquisition and water use strategies.

Our 6-year study period was characterized by contrasting climatological years, including two dry years (2017 and 2019). We found that grassland productivity was co-limited by multiple nutrients and that species richness decreased with nutrient enrichment. Dry years reduced productivity and species richness and were a critical factor reducing functional diversity of most of the studied traits. Species with competitive characteristics dominated nutrient enriched communities and were related to ecosystem stability by increasing mean biomass production relative to the standard deviation of biomass over time. Contrary to expectations species richness was not related to stability.

This study shows that mechanisms underlying ecological functioning of Mediterranean grasslands depend on interactions of multiple nutrient addition and precipitation variability. Understanding these mechanisms is crucial to anticipate potential effects of global changes on Mediterranean grasslands.

How to cite: Caldeira, M. and Nogueira, C.: Nutrient addition in a Mediterranean grassland decreases species diversity, increases productivity but does not affect stability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20933, https://doi.org/10.5194/egusphere-egu2020-20933, 2020.

Soil organic carbon (SOC) stocks play a significant role in global climate regulation. CO₂ fluxes between soils and atmosphere partly depend on soil organic matter (SOM) biogeochemical stability. Cold ecosystems are generally characterized by a high SOC stock, a large part of it being stabilized by environmental conditions (e.g. low pH and temperature). SOC stocks of cold ecosystems are also supposed to be highly vulnerable to climate change that is cancelling the stabilizing effect of low temperature on SOM.

The aim of this study was to investigate the biogeochemical characteristics of SOM in mountain meadows at the European scale. Our goal was also to determine how environmental factors, including climate, elevation and plant functional traits could drive SOM stability and chemistry. To do so, we used the soil sample set of the ODYSSEE project (), collected in 65 sites located in the main European’s mountains range (Alps, Pyrenees, Carpathians, Balkans). Topsoils (0–10 cm) from two plant communities (when both were present) were sampled in acidic meadows: Nardetum strictae and Caricetum curvulae. To assess SOM chemistry and biogeochemical stability, we used several indices based on Rock-Eval® 6 thermal analysis.

The topsoil samples showed a high concentration of organic carbon (114 ± 54 gC/kg of soil), and a weakly decomposed SOM as indicated by a relatively high C:N ratio (15 ± 2.5), hydrogen content (Rock-Eval® hydrogen index = 358 ± 44 mgHC/gC) and a relatively low oxygen content (Rock-Eval® OI_RE6 = 151 ± 10 mgO₂/gC). The decomposition state of SOM increased with mean air temperature in winter. The size of the thermally labile SOC pool was high for all samples (pyrolysable SOC = 27 to 44% of total SOC), and it strongly increased with elevation. The size of the labile SOC pool (pyrolysable SOC) was also negatively correlated to a plant functional trait: the mean height of the plant community.

The topsoils of European mountains meadows have a high SOC content characterized by a globally high lability that further increases with elevation. The high lability of SOM revealed by Rock-Eval® 6 thermal analysis indicates a generally high vulnerability of SOC to climate change throughout European mountain meadows ecosystems.

The grass adaptative strategy developed under a cold climate induces lower plant height and higher carbon allocation to the root system. Higher carbon input to soil and/or allelopathic mechanisms protecting SOM from decomposition could possibly explain that lower plant communities of European acidic alpine meadows are characterized by a more labile SOM.

How to cite: Raguet, P., Barré, P., Baudin, F., Khedim, N., Poulenard, J., Saillard, A., Choler, P., and Cécillon, L.: Soil organic carbon stability in European mountain meadows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21796, https://doi.org/10.5194/egusphere-egu2020-21796, 2020.

Global models of photosynthesis commonly use photosynthetic capacity parameters (V_cmax, J_max) that are estimated based on C_i, the intercellular CO₂ concentration. The underlying assumption of these models is that mesophyll conductance (gm) is infinite and therefore the CO₂ concentration at the site of carboxylation (C_c) is equal to C_i, despite a growing body of literature acknowledging that these assumptions are incorrect. Because relatively few studies on gm have been conducted under natural conditions and with high enough resolution, it is currently unclear how significant the assumption of infinite C_c is for the accuracy of long-term predictions by large-scale photosynthesis models. In this study we investigated this question with data collected in a mature Scots pine stand, one of the dominant species of the boreal region. We conducted high-resolution gas-exchange and online ¹³C discrimination measurements over a whole growing season (May-October), and analysed the relative contribution of diffusional and biochemical limitations to photosynthesis. We hypothesised that diffusional limitation will be significant in this species, as conifers typically have low stomatal and mesophyll conductance (g_s and g_m respectively). Accordingly, we found that diffusional limitations were similar or stronger than biochemical limitation during May-July, and that all limitations reached minima around the end of June when A_net values were highest. However, during August-October biochemical limitation became increasingly dominant, as the diffusional limitations were relatively small and stable. Over the whole period, both g_m and the relative mesophyll limitation were similar in magnitude to g_s and the stomatal limitation, respectively, resulting in a 40-100 ppm reduction in CO2 concentration between C_i and C_c. This meant that V_cmax was under-estimated by 20-40% when calculated from C_icompared to C_c, highlighting the importance of accounting for the finite g_m when determining photosynthetic capacity and modelling photosynthesis under natural conditions.

How to cite: Stangl, Z. R., Tarvainen, L., Räntfors, M., Wallin, G., and Marshall, J. D.: Seasonal analysis of photosynthetic limitations in mature Scots pine trees reveals that models based on intracellular CO2 concentration grossly underestimate photosynthetic capacity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15489, https://doi.org/10.5194/egusphere-egu2020-15489, 2020.

Changes in the availability of nitrogen (N) and phosphorus (P) can modify plant species composition, traits, and functional diversity with potential consequences on ecosystem functioning. Previous research on the effect of nutrient imbalances has mostly focused on the response of aboveground traits, while the response of belowground traits and their relationship with carbon fluxes remains relatively underexplored. We measured gross primary production (GPP) using canopy chambers in a Mediterranean tree-grass ecosystem with different levels of N and P availability: i) N addition (N+), ii) N and P addition (NP) and iii) control. In the same locations, we sampled five functional traits (specific leaf area (SLA, cm² g^-1), leaf nitrogen content (g g^-1), ¹³C isotopic composition (δ¹³C, ‰), specific root length (SRL, cm g^-1) and root tissue density (RTD, g cm^-3) in one individual for each species. We also measured above- and belowground biomass and plant species composition. For each functional trait, we computed community weighted mean values (CWM) to characterize dominant trait values in the community. We used linear mixed models to assess overall differences among treatments and structural equation models to assess indirect effect of plant traits through GPP on biomass. We fit a multigroup structural equation models to evaluate if relationships between traits and fluxes varied among nutrient availability treatments. Our results showed that species composition and below- and aboveground biomass were similar among treatments. We observed lower SLA CWM and higher RTD CWM values in the N+ treatment as compared to control and the NP treatment. The theoretic causal model fit the data well (R² = 0.47, P = 0.156), confirming the indirect effect of traits on biomass. ¹³C isotopic composition was positively related to GPP (P = 0.036), suggesting a positive relationship between water use efficiency and carbon assimilation. There were distinct relationships between root traits and GPP depending on the treatment (P = 0.035 and P = 0.027 for SRL and RTD, respectively). SRL was negatively related to GPP but the magnitude of the relationship varied among treatments, showing a stronger relationship in the N+ (-0.65) and NP (-0.84) treatment than in the control (-0.08). RTD also showed a negative relationship with GPP in the NP treatment (-0.66) and in the control (-0.18), but not in the N+ treatment (0.07) that showed a positive relationship. GPP showed a positive effect of aboveground biomass (0.16, P = 0.044), but no effect on belowground. Overall, our results demonstrate the potential effect of nutrient imbalances on carbon fluxes. Despite being SLA similarly sensitive to nutrient imbalances than root traits, both SRL and RTD showed a significant relationship with GPP, suggesting that root traits could be better indicators of ecosystem functioning.

How to cite: Rolo, V., Nair, R. K. F., Hammer, T. W., Migliavacca, M., and Moreno, G.: Nutrient availability imbalance modify the relationship between root traits and carbon assimilation at the community level, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20687, https://doi.org/10.5194/egusphere-egu2020-20687, 2020.