Anticipating and preparing for life on a warming planet requires a predictive understanding of how increasing drought and heat stress will affect terrestrial plants and the many services they provide. The water potential of soils and plants – which can be imagined as the blood pressure of the natural world – is a fundamental driver of ecosystem water flows, and directly controls many aspects of plant functioning during drought. However, and especially when compared to the rich information contained in flux tower networks like AmeriFlux and FLUXNET, observations of water potential are relatively sparse, undiscoverable, and plagued by methodological disparities that constrain the synthetic research necessary to improve conceptual understanding and predictive models of plant drought responses. A new network – PSInet – will confront this water potential information gap by creating an open and accessible global water potential database that is harmonized with the structure of other established and related networks (e.g. flux tower networks, SAPFLUXNET). The database creation will be complemented by efforts to develop community-crafted protocols, best-practices, and analytical tools for soil and plant water potential observation and interpretation, with a particular emphasis on emerging techniques for continuous observation of water potential in-situ. Finally, the network will build a diverse “Community of Practice” to elevate the measurement, synthesis, and application of plant and soil water potential through early career training programs, community workshops, and novel teaching and outreach tools. In this talk, I’ll describe how these activities could provide the data, collaborative platforms, and training necessary to improve our predictive understanding of ecosystem water status and flows.

How to cite: Novick, K. and Guo, J.: PSInet – a global water potential network to improve understanding of ecosystems water status and flows, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10166, https://doi.org/10.5194/egusphere-egu24-10166, 2024.

Agriculture is the largest consumer of freshwater, accounting for approximately 70% of the total global usage. As the human population continues to grow, demand for water will be exacerbated by a changing climate and shifting temperature and precipitation regimes. Dynamically modelling crop physiological function will be crucial to optimising crop management strategies. In this study we synergise hyperspectral and thermal remotely-sensed data to model plant traits and water fluxes in spring wheat (Triticum aestivum) in growth chambers within a controlled environment experiment under water and/or nitrogen stress conditions. Results showed that plants which had first received nitrogen fertiliser and were subsequently droughted presented the lowest water fluxes, and the lowest leaf chlorophyll content and photosynthetic capacity (Vcmax) values. Partial least squares regression (PLSR) analysis of hyperspectral reflectance data revealed key wavelengths sensitive to six different plant traits and fluxes (including relative water content, leaf nitrogen, stomatal conductance), with strong correlations between measured and modelled values (R² = 0.84; p<0.001, 0.60; p<0.001, and 0.65; p<0.001, respectively). By incorporating optical reflectance data into a modified surface energy-balance model to incorporate the changing optical properties of the leaves under stress, we increased the accuracy of modelled water fluxes against leaf porometry measurements during abiotic stress (R² = 0.46; p<0.01 and R² = 0.61; p<0.001, for the original and improved transpiration model respectively). This work points to the importance of considering the influence of stressors on crop fluxes and traits both in isolation and combined. The novel integration of optical and thermal remote sensing techniques paves the way for the improved dynamic modelling of crop physiological function.

How to cite: Croft, H., Caine, R., and Khan, M.: Modelling crop traits and fluxes under multiple abiotic stressors, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19270, https://doi.org/10.5194/egusphere-egu24-19270, 2024.

Climate models project a further increase in the average global temperature for the following decades, with Alpine regions (and their ecosystems) expected to be over-proportionally more affected. Biogenic volatile organic compounds (BVOCs) comprise the largest, most highly complex, and diverse fraction of the volatile organic compounds (VOCs) emitted into the atmosphere (1). By emitting BVOCs, plants communicate, fight herbivores, and attract pollinators (2). It is well known that biotic stressors (e.g., insects feeding on plants) lead to changes in plants' BVOC emissions: certain compounds can be promoted, and others reduced. Atmospheric oxidation of BVOCs affects the concentration of methane, carbon monoxide, and tropospheric ozone, leading to the formation of Secondary Organic Aerosol (SOA). Atmospheric aerosol load is crucial in defining the radiative balance and negatively impacts air-quality standards (3). Stress-induced changes in plant emissions may thus lead to changes in atmospheric chemistry and SOA properties (e.g., ref. 4). The impact of prolonged changes in abiotic factors and abiotic stress (e.g., heat and drought) on plants' BVOC composition and emissions quantities, and how this may impact atmospheric chemistry and SOA properties, need to be better understood. 

Within the experimental project "Acclimation and environmental memory” (AccliMemo), we study BVOC composition and quantities at basal conditions and under prolonged heat and drought. To this purpose, Scots pine (Pinus Sylvestris) seedlings were grown from seeds collected from selected mother trees from the long-term irrigation experiment Pfynwald. Those mother trees experienced different long-term water availability. This also allows us to examine the consequence of transgenerational memory on BVOC emissions (5). 

Our conference contribution will give insight into our findings from plant chamber experiments and address i) gas-phase BVOC samples collected on sorbent tubes and analyzed by Thermal Desorption GC-MS and ii) gas-phase BVOC measurements collected in-situ using a PTR-ToF-MS. These data provide a well-resolved picture of terpene compositions and diurnal trends in emission levels. The BVOC analysis in the gas phase is complemented by a detailed analysis of the secondary metabolites in needle samples. Secondary metabolites are extracted in organic solvents and analyzed by liquid injection GC-FID/MS. 

Bibliography 

(1) Sindelarova, K., Granier, C., Bouarar, I., Guenther, A., Tilmes, S., Stavrakou, T., Müller, J.-F., Kuhn, U., Stefani, P., and Knorr, W.: Global data set of biogenic VOC emissions calculated by the MEGAN model over the last 30 years, Atmospheric Chem. Phys., 14, 9317–9341, https://doi.org/10.5194/acp-14-9317-2014, 2014.

(2) Niinemets, Ü. and Monson, R. K. (Eds.): Biology, Controls and Models of Tree Volatile Organic Compound Emissions, Springer Netherlands, Dordrecht, https://doi.org/10.1007/978-94-007-6606-8, 2013.

(3) Seinfeld, John H. and Pandis, Spyros N.: Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 3rd Edition., Wiley, 1152 pp., 2016.

(4) Smith, N. R., et al.: Viscosity and liquid–liquid phase separation in healthy and stressed plant SOA, Environ. Sci. Atmospheres, 1, 140–153, https://doi.org/10.1039/D0EA00020E, 2021.

(5) Bose, A. K., et al.: Memory of environmental conditions across generations affects the acclimation potential of scots pine, Plant Cell Environ., 43, 1288–1299, https://doi.org/10.1111/pce.13729, 2020.

Funding: Swiss National Science Foundation, Project Numbers 189109, 199317, and, 194390.

How to cite: Pieber, S. M., Molteni, U., Bose, A., Faiola, C., Gisler, J., Gu, S., Hunziker, S., Kalberer, M., Luo, N., Nazarova, T., and Gessler, A.: Scots pine (Pinus Sylvestris) seedlings BVOC emissions composition under basal,  heat and drought conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17558, https://doi.org/10.5194/egusphere-egu24-17558, 2024.

Slowing down climate change calls for a strengthening of natural carbon sinks. Estimating current carbon stocks and the carbon storage potential of natural ecosystems necessitates a good understanding of carbon and nitrogen cycles. As the increase of land carbon sink is likely to be nitrogen-limited in temperate and boreal ecosystems, it is important to constrain the uncertainties related to the carbon and nitrogen processes in the ecosystems. Leaf chlorophyll (chl_leaf) and leaf nitrogen allocated to photosynthetic fractions are closely related, as plants optimise their nitrogen resources between light harvesting and the reactions of the Calvin cycle. chl_leaf is consequently one of the key factors in determining leaf photosynthetic rates and a strong proxy for photosynthetic capacity. The recent advances in remote sensing (RS) provide a novel opportunity for benchmarking the modelled terrestrial nitrogen cycle through leaf chlorophyll content.

In this study, we utilize a terrestrial biosphere model, QUINCY, for simulating the chl_leaf content for different ecosystems in a global scale. QUINCY includes a comprehensive representation of coupled carbon and nitrogen cycles, and also diagnostics for chl_leaf. We use a satellite-based leaf chlorophyll RS product for evaluating how well QUINCY captures spatial and temporal patterns of chl_leaf. The evaluation is conducted for a selection of 400 locations distributed world-wide to represent all major global biomes. In addition, we analyse the accuracy of chlorophyll and productivity (GPP) simulation at 169 sites of the FLUXNET eddy covariance.

Our initial results reveal that on global scale, QUINCY chl_leaf matches well with the RS chl_leaf observations. However, the QUINCY chl_leaf values seem to be constrained to a more narrow numerical range than the RS observations, indicating that not all factors contributing to the observed variation are considered in the modeling framework. For instance, the modeled grassland chl_leaf shows much smaller variation between different locations when compared to RS observations at different sites. For the FLUXNET sites, the mean annual GPP values from QUINCY are slightly underestimated (on average, ~-260 gC m^-2 yr^-1) when compared to flux observations. Nevertheless, the QUINCY mean annual GPP for different sites correlates with the ground station data reasonably well (r=0.67). 

Our study paves way for more versatile use of satellite observations within terrestrial biosphere models. Harnessing satellite products to model evaluation helps to improve model parametrizations related to carbon and nitrogen cycles, which in turn would allow more precise modeling of the terrestrial carbon budget.

How to cite: Miinalainen, T., Ojasalo, A., Croft, H., Zaehle, S., Caldararu, S., and Thum, T.: Leaf chlorophyll: a global study employing a process-based model and remote sensing observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8382, https://doi.org/10.5194/egusphere-egu24-8382, 2024.

The Amazon Rainforest vegetation responses to climate change are still far from being fully understood, especially when the focus is not on the forest canopy stratum. Concerning the changes in water use related to rising atmospheric CO2 levels, the distinct responses between forest strata of the Amazon are still uncertain. The project presented here consists in exploring data from an experimental study coupled with a mathematical modeling approach, with the goal to elucidate, in relation to rising CO2 levels, the transpiration responses of Central Amazon understorey plants, which may influence the entire region due to forest-atmosphere water flux disturbances. More specifically, we artificially increase the CO2 atmospheric concentration inside open-top chambers installed in the understory, providing a way to evaluate the response of the plants inside the chamber to the increased CO2 levels. With the estimation of carbon assimilation and stomatal conductance processes parameters defined by mathematical models, it is possible to characterize physiologically the response of the individuals in the experiment and compare the differences between control and treatment groups. Then, the impact of these changes in the understory transpiration is estimated by measuring the leaf area index of the understory and combining the obtained stomatal conductance parameters with another mathematical model, which makes it possible to compare the impact of the experimental treatment in the amount of water that travels from the understorey plants to the atmosphere through stomata. The results and methodology from this study, in addition to the importance to accomplishing the main goal mentioned above, are also relevant for the use of Dynamic Global Vegetation Models (DGVM), as the mathematical equations employed are also frequently used in DGVM studies. Finally, the experimental site being studied is also where the first Free-Air CO2 Enrichment experiment in the Amazon, known as AmazonFACE, will be conducted, so the open-top chambers data can be considered a source of basal knowledge for the large scale project that is being implemented at the same location.

How to cite: Banstarck Marandola, G., De Kauwe, M. G., Garcia, S., Ferreira Domingues, T., and Montenegro Lapola, D.: Evaluation of leaf-atmosphere water fluxes and physiological characterization of plants subjected to an in situ elevated CO2 experiment in Central Amazon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22125, https://doi.org/10.5194/egusphere-egu24-22125, 2024.

Plants with the C₄ photosynthesis pathway typically respond to climate change differently than more common C₃-type plants, due to their distinct anatomical and biochemical characteristics. The different responses are expected to drive changes in global C₄ and C₃ distributions. However, current C₄ distribution models may not predict this response as they do not capture multiple interacting factors and in many cases lack observational constraints. Here, we used a global database of plant photosynthetic pathways, satellite observations, and photosynthetic optimality theory to produce a new observation-constrained global estimate of C₄ distribution. We found that global C₄ coverage decreased from 17.7% to 17.1% of the land surface during 2001 to 2019, as a net effect of C₄ natural grass decreases due to elevated CO₂ favoring C₃-type photosynthesis, and C₄ crop increases, mainly from corn (maize) expansion. Using an emergent constraint approach, we estimated that C₄ contributed 19.5% of global photosynthetic carbon assimilation, a value within the range of previous estimates (18-23%) but higher than the ensemble mean of dynamic global vegetation models (14 ± 13%). By improving the understanding of recent global C₄ cover and productivity dynamics, our study sheds insight on the critical and underappreciated role of C₄ plants in the contemporary global carbon cycle.

How to cite: Luo, X., Zhou, H., Satriawan, T., Tian, J., Zhao, R., Keenan, T., Griffith, D., Sitch, S., Smith, N., and Still, C.: Mapping the global distribution of C4 vegetation using observations and optimality theory, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2338, https://doi.org/10.5194/egusphere-egu24-2338, 2024.

The optimum air temperature for photosynthetic ecosystem productivity (T_opt) determines the annual maximum of terrestrial carbon uptake. Previous studies have quantified T_opt and explored its acclimation in response to climatic warming. However, there remains uncertainty regarding the extent to which T_opt has changed globally in recent decades and how it might evolve in the future. Our analysis, using both satellite- and ground-based datasets, reveals a slight increase in T_opt over the past two decades (+0.14°C, 2000-2019), contrasting with a significant 0.46°C rise in maximum global growing-season temperature (T_max). The lack of change in T_opt was attributed to an insignificant trend of T_max in cold areas and to drought inhibiting thermal acclimation of photosynthesis in temperate and arid regions. If global surface temperatures exceeded pre-industrial levels by 2°C, the mean T_max over the globe was predicted to increase slightly more than T_opt (1.5 vs. 1.2°C), with the gap widening under a 4°C temperature increase (4.1 vs. 3.4°C). In the +4°C scenario, soil drought dominated the widespread decline in photosynthetic acclimation across tropical and temperate vegetation, slowing the rate of T_opt increase at the end of the century (+0.04°C, 2080-2099). Conversely, increasing atmospheric CO₂ concentrations failed to have significant effects on T_opt and its acclimation. These findings imply that the absorption of atmospheric CO₂ by terrestrial vegetation, and subsequent carbon sequestration, may be further hampered by the limited acclimation capacity of T_opt to rising global temperatures.

How to cite: Xu, C., Liu, H., Yakir, D., Liang, B., Zhu, X., Feng, S., and Grünzweig, J.: Slowdown of the rise in the optimum temperature of photosynthetic productivity under future global warming, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5169, https://doi.org/10.5194/egusphere-egu24-5169, 2024.

Increasing atmospheric CO₂ concentrations due to human activities, is projected to enhance photosynthesis and carbon storage of forest ecosystem. However, it is unclear how nutrient limitation will constrain the projected CO₂ fertilization effect. Therefore, it is essential to evaluate how nutrient limitation will affect the response of forests to rising CO₂ concentration and how it will feedback on nutrient availabilities and more especially nitrogen (N) which can become limiting with time.

The purpose of this research is to evaluate the response of N cycling processes to elevated CO₂ enrichment in a N-limited northern deciduous temperate forest in the UK and a phosphorus (P)-limited Eucalyptus dominated forest in Australia. The research was conducted in two Free Air Carbon Dioxide Enrichment (FACE) facilities: BIFOR FACE located near Birmingham, UK and EucFACE in New South Wales, Australia. Furthermore, EucFACE (P-limited forest) received partial phosphorus fertilization to study its effect on N cycling.

We employed a ¹⁵N pool dilution method to assess gross protein depolymerization and gross mineralization at both study sites, along with the measurement of nitrous oxide emissions, extracellular soil enzyme activities and nutrient pools.

Results from the N-limited forest (BIFOR FACE) indicate that elevated CO₂ increased belowground carbon allocation, resulting in higher root biomass, dissolved organic carbon, microbial biomass and soil respiration. The additional carbon belowground stimulated net mineralization (+ 30%) (p<0.05) over a year of monthly measurement. Gross mineralization and ammonium immobilization were only enhanced in summer (+ 47%), whilst gross nitrification was overall downregulated (- 47%) and N₂O production was un-affected by elevated CO₂. This suggests that root exudates selectively influence microbial communities promoting SOM decomposition to enhance ammonium availability (+15%) (p<0.05) for trees. In the phosphorus-limited forest, carbon pools, nitrogen depolymerization and mineralization were unaffected by elevated CO₂. But elevated CO₂ increased soil nitrate pool (+ 37%) (p<0.05) and decreased soil moisture (-13%) indicating a potential reduction in denitrification activity.

Taken together, results from this research outline how nutrient limitation drives the response of a forest ecosystem to elevated CO₂. Although at EucFACE, P limitation was alleviated in the initial phase of the experiment (Hasegawa et al., 2016), it quickly truncated plant growth and carbon feedback in soils. While, at BIFOR FACE, N limitation has been alleviated via enhanced soil N mineralization and N supply to sustain plant growth enhancement for seven years. However, how long the N supply will be maintained in the face of declining nitrogen deposition in future climates remains uncertain. Results from this research aim at improving our understanding of forest response to future climate by unveiling the role of nutrient limitation in future C uptake.

Hasegawa, S., Macdonald, C.A., Power, S.A., 2016. Elevated carbon dioxide increases soil nitrogen and phosphorus availability in a phosphorus-limited Eucalyptus woodland. Glob. Change Biol. 22, 1628–1643. https://doi.org/10.1111/gcb.13147

How to cite: Rumeau, M., Carrillo, Y., Sgouridis, F., Mackenzie, R., Reay, M., and Ullah, S.: Nutrient limitation in soils regulate the effects of elevated CO2 on soil N cycling at BIFoR-FACE (UK) and Euc-FACE (Australia), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3495, https://doi.org/10.5194/egusphere-egu24-3495, 2024.

The effect of CO₂ fertilization is listed as the main cause of the observed increase in net primary productivity and biomass stocks in “undisturbed” tropical forests. This is generally considered to provide greater resilience to tropical forests against disturbances that cause forest degradation such as extreme droughts and logging. Here, we discuss how this may be conceptually short-sighted, given that these forests are being pushed more quickly to their growth limits compared to a situation if atmospheric CO₂ concentration was not rising. Once this growth limit is reached, this shift — from a phase dominated by the CO₂ fertilization effect to a subsequent state marked by progressive saturation or a decrease in carbon sinks — will potentially render these forests more vulnerable to changing climate and other disturbances. In this presentation we will discuss that, in the long term, CO₂ fertilization is a disturbance that leads to forest degradation, falling within the very definition of degradation, as it is human-caused and leads to transient or long-term deleterious change in forest condition (e.g. carbon storage, forest composition). Such a recognition of the CO₂ fertilization affect as a disturbance causing degradation is politically and scientifically relevant in light of climate policies, such that the responsibility for protecting these forests from climate change and other human interventions is shared with countries other than just those that host these forests.

How to cite: Lapola, D., Blanco, C., Cardeli, B., Martinelli, J., Quesada, C., Rius, B., and Silva-Junior, C.:  Just semantics?: CO2 fertilization is a disturbance leading to degradation of tropical forests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12358, https://doi.org/10.5194/egusphere-egu24-12358, 2024.

 Methane (CH₄) is the second largest greenhouse gas and affects global climate change. In turn, global changes strongly affect CH₄ fluxes from the terrestrial biosphere to the atmosphere. However, it is unclear how CH₄ fluxes are affected by warming (W), precipitation patterns (P), elevated carbon dioxide (eCO₂), and nitrogen (N) addition. Here, we synthesized terrestrial CH₄ fluxes data from 1,165 observations performed under changes in W, P, eCO2, and N across different vegetation types over the globe. Results showed N addition significantly reduced CH₄ emission and uptake across upland ecosystems (-31% and -14%, P<0.05), but stimulated CH₄ emission in rice paddies (3%, P>0.05) and wetlands (24%, P<0.05). CH₄ emission and uptake significantly increased by 42% and 11% under W, respectively. An increase in CO₂ concentration did not affect CH₄  emission in wetlands while enhanced CH₄ emission in rice paddies (39%, P<0.05). Increased precipitation inhibited CH₄ uptake (-21%, P<0.05), whereas decreased precipitation had a significantly positive effect on CH₄ uptake (26%, P<0.05) in uplands. The overall effects of four global change drivers were -9% for CH₄ uptake and 13% for CH₄ emission averaged across different ecosystem types. The interactive effect of multiple factors on CH₄ fluxes generally was antagonistic. In addition, the responses of CH₄ emission to global change drivers significantly shifted from negative to positive with the increases in wetness indices, soil clay content, and effects of global change drivers on belowground biomass (BGB) and methanogenic bacteria (mcrA) (P<0.05). In contrast, the effects of global change drivers on CH₄ emission switched from positive to negative with the increases in the responses of grain yield and aboveground biomass, respectively (P<0.05). CH₄ uptake increased with the increases of BGB, inorganic nitrogen, the ratio of carbon to nitrogen, and mcrA induced by global change drivers but decreased with the increase of NO₃- (P<0.05). The responses of CH₄ fluxes to N addition, W, and precipitation changes exhibited considerable variations in sensitivities and magnitudes. This synthesis showed an urgent need to consider the effects of changing multiple global change drivers on CH₄ fluxes for better understanding the methane-climate feedback.

How to cite: Zhu, T., Zhou, Y., Ju, W., Yan, R., Xie, R., and Mao, Y.: Impacts of global change drivers on terrestrial methane emission and uptake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2528, https://doi.org/10.5194/egusphere-egu24-2528, 2024.

Tropical forests are increasingly subjected to land-use change with important implications for biodiversity conservation and global carbon-, energy-, and water cycling. Particularly in Central Africa, slash-and-burn agriculture is the main source of forest losses, and the projected increase in the African population is only to exacerbate these dynamics. The imminence of this social-ecological front is already reflected in the decreasing length of fallow periods, combined with an increase in the number of clearing cycles.

Studying regrowth forests in a thorough and comprehensive way entails including not only biodiversity- and carbon stock recovery, but also the recuperation of nutrient cycling. The current paradigm on tropical secondary forest succession states that these forests move from a nitrogen (N) to a phosphorus (P) limitation during ecosystem recovery. However, recent research has shown that cations become scarce and potentially limiting in later successional stages in Central African forests. Additionally, in a context of repeated forest clearing, soil total and available cation stocks have been shown to decrease each clearing cycle, potentially affecting the regrowth of secondary forests.

An essential phase in the recovery of forests on fallows in Central Africa is the establishment of the pioneer tree species Musanga cecropioides R.Br. ex Tedlie, often exhibiting a temporary monodominance on abandoned fallows and creating a microclimate that facilitates the establishment of other tree species. To study the effects of nutrient losses due to land-use intensification on forest regrowth, our team installed a pot experiment with M. cecropioides close to Kisangani in the Democratic Republic of the Congo, with the following nutrient treatments: control, nitrogen (225 kg ha^-1 yr^-1), phosphorus (75 kg ha^-1 yr^-1), and a combined cation treatment (calcium, magnesium, and potassium; each 75 kg ha^-1 yr^-1). These four treatments were then combined in a full factorial setup with 15 replicates. 

Plant diameters and -heights were measured biweekly for one year. Plants that received the combined cation treatment quickly overtook the ones that only received single treatments of N and P, as well as the controls, both in terms of measured diameter and height. The height measurements showed a steep increase when N was combined with cations, however, the addition of P did not show any additional effect. Plant diameter measurements showed a first increase when cations were added solely and a second increase when the cation addition was combined with nitrogen. Again, the additional provision with P did not add to the observed diameter increase, emphasizing the importance of cations as potentially limiting nutrients for forest regrowth.

A final complete harvest of the plants will allow for total biomass quantification per tissue, as well as destructive sampling for chemical analysis. Analysis of tissue concentrations and stoichiometries of C, N, P, and cations will importantly supplement the growth rates by providing more insight into the relative effects of the added nutrients and will help to disentangle the effects of the separate cations that were combined in the aggregated ‘cation’ treatment. 

How to cite: Van de Velde, V., Magala, A.-J., Mande, J., Hassan, A., Meunier, F., Bauters, M., and Boeckx, P.: Cations limit tree growth of a keystone pioneer species in Central Africa, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1532, https://doi.org/10.5194/egusphere-egu24-1532, 2024.

The increase of nitrogen (N) deposition is a human-induced process associated with industry and agriculture that disrupts the nitrogen biogeochemical cycle. N deposition has been associated with environmental impacts such as land carbon sink increase, loss of biodiversity, and risk of eutrophication and acidity but further understanding of how N deposition affects trees of different environmental conditions, size, and leaf habit is missing. In this study, we use the ICP forest inventory data and the EMEP N deposition data to track how N deposition affects tree growth in interaction with temperature and precipitation, tree size classes, or leaf habits in Europe since 1990. We use linear mixed models to describe the interaction between mean annual temperature (MAT) mean annual precipitation (MAP) and N deposition in tree growth. In addition, we use gam models to track the different saturation points. We found contrasting interactions between N deposition and temperature in conifers and broadleaves. Conifers living in colder environments have a more positive response to N deposition than conifers living in warmer environments. On the other hand, broadleaves living in warmer environments had the most positive response to high N deposition levels. Interestingly, broadleaves showed lower saturation points than conifers, being around 25 kg ha yr and 30 kg ha yr respectively. Nonetheless, factors such as tree size and species can modulate such relations, being especially relevant for secondary forests or restoration processes. In conclusion, our findings point out climate, tree size, and leaf habit as strong modulators of N deposition impacts in tree growth that should be considered in future assessments and policy-making, especially in Europe. Further research is needed to certify these relations in other regions of the world.

How to cite: Vallicrosa Pou, H. and Grossiord, C.: Nitrogen deposition effect on tree growth depends on climate, tree size, and leaf habit, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4053, https://doi.org/10.5194/egusphere-egu24-4053, 2024.

Our current understanding of the tropical terrestrial nitrogen (N) cycle has been shaped by decades of field-based research, predominately at a small subset of sites across the globe. These field data inform hypotheses on how N cycling is mediated by biotic and abiotic drivers, and inform the paradigm that tropical wet forests are characterized by high rates of N inputs and outputs compared with other systems, driven by high inorganic N availability. However, recent findings that do not seem to conform to this paradigm call into question how well the bulk of our underlying data represents the diversity of the tropics as a whole. We propose that there may be blind spots in our understanding of N cycling created by a paucity of sampling from areas where environmental factor combinations differ from those often studied. Identifying these blind spots may help to resolve which drivers can be generalized across the tropics as a whole, versus which sustain system heterogeneity. We conducted a pan-tropical synthesis of field sampling sites for N fixation and denitrification (two key processes that influence system N inputs and outputs, as a proxy for general understanding of N cycling) and sampling intensity between 1950 and 2022. As a metric of geographic biases in general understanding, we tallied citations counts for each study over time. We also collated globally gridded data for a range of factors hypothesized to control N cycling rates, including soil and climatic variables, productivity, topography, vegetation type, biogeographic region, and disturbance. With these data, we: 1) mapped major axes of variation in tropical environmental conditions using principal components, 2) determined the distribution of environmental variables within sampled sites versus the tropics as a whole, and 3) identified regions where unique combinations of conditions are under sampled.

Preliminary results show a relative over-representation of evergreen broadleaf forests, and under-representation of grasslands and savannas. This corresponded to a proportional oversampling of sites with higher soil fertility (soil N and P), high net primary productivity, high rainfall, and low rainfall seasonality. To quantify system-level biases we also explored intra-biome sampling variability for factors such as fertility and elevation (e.g., tropical montane versus lowland forests). Denitrification and N fixation tended to follow similar patterns in site characteristics, suggesting that these metrics are a good proxy for overall N cycling understanding.

Overall, our study identifies regions of the global tropics where environmental drivers are similar to those dictating existing knowledge, as well as understudied regions that should be targeted to explore system heterogeneity. Future work can leverage this information to design cross-system comparisons to explicitly test current hypothesized mechanisms to advance our understanding of the N cycle. Constraining nutrient availability and cycling in tropical ecosystems is especially important in the context of global change, where shifting environmental conditions may alter how forests cycle and retain N and, therefore, how nutrient limitation will constrain productivity responses to rising carbon dioxide.

How to cite: Nelson, K. and the Powell Group: Blind spots hinder understanding of the tropical nitrogen cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4469, https://doi.org/10.5194/egusphere-egu24-4469, 2024.

Liebig’s Law of the Minimum underscores the need for balanced nutrition for optimal plant growth; any deficiency in essential nutrients can influence plant growth and subsequent biomass accumulation. Anthropogenic nitrogen (N) deposition can alleviate the prevailing N limitation and stimulate plant growth in many terrestrial ecosystems, thereby helping to mitigate climate change. However, the N stimulation effects may diminish under conditions where plant growth is limited by soil cation availability that is susceptible to N-induced soil acidification. How variation in soil cations influences N stimulation of plant growth is unresolved. Here, we synthesized data from 282 field experiments and found that in agreement with the optimal allocation theory, N addition asymmetrically increased plant biomass aboveground (42.5 ± 7.4%) and belowground (20.8 ± 10.1%). The increment in aboveground biomass was soil pH dependent, shifting from neutral in low pH (pH ≤ 4.5) to positive in medium (4.5 < pH ≤ 7.5) and high pH (pH > 7.5) soils. In contrast, changes in belowground biomass were independent of soil pH. The variations in biomass increments across different soil pH ranges were mediated by the levels of foliar magnesium (Mg) and calcium (Ca), with the responses exhibiting a shift from negative in low pH soils to neutral in medium and high pH soils. These findings suggest that reduction in foliar Mg and Ca levels diminishes the N stimulation of plant biomass despite the enhancement of root growth in low pH soils. Given the widespread stimulation of plant biomass through N addition, the extent of this effect is soil pH dependent and mediated by the foliar cation status of plants.

How to cite: Xu, X.: Nitrogen Stimulation of Plant Biomass Growth Mediated by Foliar Magnesium and Calcium Worldwide , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4852, https://doi.org/10.5194/egusphere-egu24-4852, 2024.

Wildfires are increasing across northern high latitudes. Besides the immediate carbon pool losses from directly disturbed areas, recent studies have reported high porewater nitrogen (N) and phosphorus (P) concentrations in burned areas and downstream waters for a few months to several years after fire occurrence. Increasing nutrient deposition and soil fertilizer use have been widely investigated for water quality and carbon loss in agricultural soils, but not for remote subarctic peatlands. In this study, we sampled soil cores (0-25 cm) from a bog and a fen peatland in the Scotty Creek watershed in the Northwest Territories and conducted an incubation experiment for the effects of added nutrients in porewater. Aliquots of the peatlands were divided into separate containers and artificial porewater was added, either amended with dissolved inorganic N (NH₄ + NO₃), P (PO₄), both N and P, or unamended. The production rates of gaseous CO₂, CH₄ and N₂O were measured at 1, 5, 15, and 25°C. We further analyzed the initial and final soil physical properties, porewater chemistry, and microbial biomass C:N:P ratios. The fen incubations yielded overall greater CO₂ and CH₄ production rates than the bog incubations, which we attributed to differences in soil properties and initial microbial biomass. The N addition to the bog samples increased CO₂ production, while the P addition to the fen samples increased CO₂ production. The addition of both N and P reduced CO₂ production but elevated that of CH₄ for both peatland soils. After a month, the pore water C, N, and P stochiometric ratios approached the initial soil microbial biomass ratios, suggesting microbial nutrient recycling in an inherently nutrient-poor soil environment. These preliminary results imply a complex response of carbon turnover in peatland soils to nutrient enrichment.

How to cite: Byun, E., Rezanezhad, F., Slowinski, S., Lam, C., Saraswati, S., Wright, S., Quinton, W. L., Webster, K., and Van Cappellen, P.: Complexity of nutrient enrichment on subarctic peatland soil CO2 and CH4 production , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7241, https://doi.org/10.5194/egusphere-egu24-7241, 2024.

Micrometeorological variability significantly influences the structures, functions, and dynamics of ecosystems. Despite this, there is a limited understanding of the feedback and causal relationships among micrometeorological drivers in various Himalayan ecosystems. The present study aims to investigate the meteorological controls that govern the variability in net ecosystem exchange (NEE) in Oak (Quercus leucotrichophora) and Pine (Pinus roxburghii)-dominated ecosystems of the Himalayas. We use half-hourly eddy covariance fluxes from Pine and Oak-dominated ecosystems located in Uttarakhand, India. We employ an information theory-based Temporal Information Partitioning Network (TIPNet) approach to generate weekly process networks with a 6-hour lag. The analysis conducted for the monsoon and post-monsoon seasons of 2016 and 2017 reveals that sub-daily scale variations in micrometeorological variables are responsible for fluctuations in NEE in both ecosystems. The Pine ecosystem exhibits greater sensitivity to air temperature, leading to increased carbon uptake compared to the Oak ecosystem throughout the study period. Causal connections indicate that the NEE of the Oak ecosystem is moisture-driven (influenced by precipitation and relative humidity), while that of the Pine ecosystem is heat-dominated (influenced by air temperature and net solar radiation). Precipitation effects on the Pine ecosystem are not immediate due to slower infiltration and lesser fine root production compared to Oak. However, the impact of moisture stress is evident in the network structure of both ecosystems, with more causal links occurring during dry periods compared to normal periods, indicating adaptive responses to resist moisture stress. This research enhances our understanding of micrometeorological influences on carbon dynamics in Himalayan ecosystems, providing valuable insights for ecosystem management and climate change mitigation strategies.

How to cite: Khadke, L., Mukherjee, S., and Ghosh, S.: Influence of Meteorological Variables on Carbon Uptake in Oak and Pine-dominated Ecosystems of Himalaya, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-615, https://doi.org/10.5194/egusphere-egu24-615, 2024.

Vegetation optical depth (VOD), has been widely assessed for monitoring vegetation carbon and water status under different conditions. However, their abilities to reflect the integrated dynamics in vegetation status under a changing climate are rarely investigated, especially in China. To fill this gap, this study examines seven VOD products for their capabilities to monitor the changes in vegetation status under a varying climate from 2015 to 2021 in China, including X-, C- and L-band VOD products from AMSR-E, AMSR2, SMOS and SMAP. The results indicate that most VOD products generally show consistent responses to temperature (Ta), vapor pressure deficit (VPD) and soil moisture (SM) variations for the ecosystems with simple canopy structure, such as temperate grassland and shrublands, which are also water-limited ecosystems. Moreover, these VOD products also exhibit similar responses to a varying climate for Ta-constrained temperate forests, independent of retrieval frequencies and algorithms. For other ecosystems, however, the links between VOD and climate variables are sensitive to retrieval frequencies and algorithms. Specifically, due to the relatively high frequency, X-band VOD products can capture vegetation responses to Ta, VPD and SM stresses as the vegetation canopies respond rapidly to climate variations, especially for ecosystems located in the dry and warm regions. Furthermore, all seven VOD products, and especially X-band VOD, show high sensitivity to SM carry-over effects on vegetation dynamics, especially for temperate non-forests ecosystems. These findings help clarify the capability of different VOD products to mirror vegetation responses to a changing climate across different ecosystems in China, highlighting the importance of choosing the most appropriate VOD product as vegetation proxies in global and regional studies.

How to cite: He, M., Yi, Y., Li, X., Wigneron, J.-P., Kimball, J., Reichle, R., Fan, L., Chen, H., and Zhang, Q.: Divergent responses of vegetation dynamics to a changing climate through different VOD products in China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4516, https://doi.org/10.5194/egusphere-egu24-4516, 2024.

The springtime phenology in permafrost regions of the Northern Hemisphere is exhibiting an extensive advance in response to climate change that is considered to be primarily driven by the rising temperature that vastly exceeds the average rate of global warming. This phenological trend emerges as the result of spatial variability in temperature and the response of vegetation to temperature (referred to here as the temperature sensitivity (S_T) of phenology, change in phenological timing per unit change in temperature). In contrast to the more well-defined pattern of temperature variability, far less is known about the temperature sensitivity of vegetation phenology. Further, the above-average and highly heterogeneous warming in permafrost regions leads to changes in chilling exposure, frost risk, and other key controllers of temperature sensitivity, potentially allowing plants to adapt their phenological strategies. However, these phenological strategies  have not been investigated yet at the pan-Arctic scale in models or observations.

Here, we seek to combine remote sensing-based and process model-based approaches to reveal the spatiotemporal pattern and strategic mechanisms regarding the S_T of springtime phenology. To obtain generalisable dependencies, remote sensing retrieved phenology, climate data and soil physical property data are used to explore the linkages between S_T and temperature and permafrost drivers, providing insights on vegetation phenological strategies. The novel QUINCY model is then adopted at site levels to validate the plausibility of possible strategies. Sets of model experiments with respect to varying biological and environmental factors are applied to elucidate the major controllers. Results shed light on the importance of environmental variability, and provide a more elaborate explanation for the S_T variability of spring-time phenology.

How to cite: Zhu, Y., Lacroix, F., Liu, L., Zhao, D., and Zaehle, S.: Unveiling the springtime phenological strategies in permafrost vegetation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12018, https://doi.org/10.5194/egusphere-egu24-12018, 2024.

Arctic peatlands harbour enormous stocks of carbon (C) owing to the imbalance between photosynthesis and respiration rates. This carbon has been exposed to a changing climate, in particular warming, during the last century. The Arctic has warmed by on an average ~0.75°C (Post et al; 2019), which is almost double the rate of global average. There is a wide range of studies on Arctic peatland C cycle, but critical knowledge gaps remain. In particular, plant C traits such as allocation rates, residence time of C in foliage, structural, fine root and labile pools and their response to warming climate are rarely explored. In order to investigate these traits, we trained an intermediate complexity terrestrial ecosystem model (DALEC), which represents these key unknowns, with available in-situ data in order to generate a data constrained analysis of ecosystem function. DALEC is calibrated with a Bayesian model-data fusion framework (CARDAMOM) which retrieves a probabilistic estimate of DALEC’s parameters based on the combination of observations and their uncertainties. CARDAMOM’s analysis directly provides an estimate of our uncertainty. We used 7 years (2014–2020) of data from the Bonanza Creek rich fen peatland in Alaska using a weekly timestep. CARDAMOM used eddy covariance information, earth observation, and in-situ biophysical observations to calibrate DALEC. We found a switch from a C source to sink, which is forced by increase in photosynthesis and leaf area index . Gross and net primary production (GPP and NPP) almost doubled from 2014 to 2017, transforming the peatland from a C source to a sink. Our analysis also suggests that NPP allocation is directed primarily towards foliage over the fine root and structural C pools.

How to cite: Thayamkottu, S., Smallman, T., Williams, M., Pärn, J., and Mander, Ü.:  Carbon sink strength and allocation dynamics of a rich fen peatland in the warming Arctic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12300, https://doi.org/10.5194/egusphere-egu24-12300, 2024.