Representations of interactions between the C and N cycles in terrestrial ecosystems are now implemented in a majority of state-of-the-art Dynamic Global Vegetation Models (C-N models). Standard models for simulating the response of individual processes to changes in N availability have not yet emerged and widely used models have not been tested against the full diversity of empirical data. Large remaining model structural uncertainty has important implications for projections and hindcasts of the land C uptake.

Here, we summarise the current state of global land C balance simulations by comparing C-N models to C-only models; summarise data from field surveys and experiments to elucidate the role of soil N in controlling photosynthesis and its acclimation, stoichiometry, allocation, and growth; and demonstrate how optimality principles can guide the representation of acclimation and allocation for simulating ecosystem responses to experimental treatments of CO2 and soil N – consistent with observations. Promising model results are achieved by assuming that the atmospheric environment, including CO2, is the principal driver for photosynthetic capacities and leaf N following optimality theory of photosynthetic acclimation (Prentice et al., 2014). In turn, the functional balance hypothesis (Bloom et al., 1985) yields accurate predictions for how soil N availability and CO2 influence allocation and growth in different tissues.

Our results show how confronting new theoretical approaches to simulating ecosystem C-N interactions against the collective constraints from diverse types of observations can guide model development and potentially reduce the large uncertainty in global carbon cycle projections.

References

Bloom, Arnold J, F Stuart Chapin, and Harold A Mooney. “Resource Limitation in Plants--An Economic Analogy” Annual Review of Ecology and Systematics, 16, no. 1 (1985): 363–92. https://doi.org/10.1146/annurev.es.16.110185.002051.

Prentice, I. Colin, Ning Dong, Sean M. Gleason, Vincent Maire, and Ian J. Wright. “Balancing the Costs of Carbon Gain and Water Transport: Testing a New Theoretical Framework for Plant Functional Ecology.” Ecology Letters 17, 1 (2014): 82–91. https://doi.org/10.1111/ele.12211.

How to cite: Stocker, B. D., de Boer, H., Dong, N., Harrison, S. P., Perkowski, E. A., Prentice, I. C., Rebel, K. T., Schneider, P., Smith, N. G., Van Sundert, K., Wang, H., and Xu, H.: Ecosystem C and N cycle interactions – diverse model representations and divergent model predictions versus collective empirical constraints, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9374, https://doi.org/10.5194/egusphere-egu23-9374, 2023.

The rate at which forests take up atmospheric CO₂ is critical because of their potential to mitigate climate change and their value for wood production. The allocation of carbon fixed through photosynthesis into biomass can be quantified through the tree carbon (C) use efficiency (CUE), which is determined by gross primary production (GPP) and plant respiration (Ra) via the relation CUE=(GPP-Ra)/GPP. The effect of future climate on CUE is unclear due to the highly uncertain response of plant respiration to the expected increases in temperature and possible changes in tissue nitrogen (N) concentrations that also affect GPP and Ra.

Within the project ”Improving tree carbon use efficiency for climate-adapted more productive forests” (iCUE-Forest), we aim to develop novel data-driven estimates of plant respiration, net primary production (NPP=GPP-Ra) and tree CUE covering the northern hemisphere boreal and temperate forests. These will be based on recent satellite-driven maps of tree living biomass, databases of N concentration measurements in tree compartments (leaves, branches, stems, roots) and the relationships between respiration rates and tissue N concentrations and temperature. Such estimates will enable the detection of spatial relationships between CUE and environmental conditions and facilitate the parameterization of dynamic global vegetation models to predict the change in CUE in response to future climate and forest management.

Here we compile an unprecedented database of N concentration measurements in tree stems, branches and roots covering all common boreal and temperate tree genera together with data available mainly for leaves from databases like TRY. We apply this database to test different hypotheses on the controls of tree tissue N concentration and allocation. We find that the variation in tree tissue N concentrations of boreal and temperate trees is controlled by their leaf type (broadleaf deciduous, needleleaf deciduous, needleleaf evergreen), growth rate (fast- vs. slow-growing), tree age/size and climate conditions. These relationships have important implications on the coupling of the C and N cycles in the vegetation, since tissue N concentrations determine photosynthesis, growth and plant respiration. Thus, by altering tissue N concentrations, changes in the distribution of tree species, in tree age/size or in climate, induced by climate change, forest management or disturbances, can affect the C sequestration potential of boreal and temperate forests.

Subsequently, we combine the derived tree-level relationships between tissue N concentrations and underlying drivers, tree species distribution maps, and estimates of tree compartment biomass based on satellite remote sensing products. In this way, we derive novel estimates of the spatial distribution of N content in northern boreal and temperate forests that will in turn be used to assess CUE variations.

How to cite: Thurner, M., Yu, K., Manzoni, S., Prokushkin, A., Thurner, M. A., Wang, Z., and Hickler, T.: Tree tissue nitrogen concentration in boreal and temperate forests - Controls and implications for the vegetation carbon cycle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2955, https://doi.org/10.5194/egusphere-egu23-2955, 2023.

The ability to simulate vegetation dynamics and their feedback with nutrient cycling to affect ecosystem productivity underpins our prediction of the land carbon sink under climate change. Predictive models are now capable of simulating complex ecosystem processes, including the recent advancement in simulating vegetation dynamics and ecosystem phosphorus cycling, but there is a general lack of empirical evidence to form a systematic evaluation of the model predictions, especially how functional diversity affect ecosystem nutrient cycling and its consequence for productivity. Here, we developed a dataset based on 9 permanent plots (20 x 20 m) along an elevation gradient (300 – 1200m a.s.l.) in a subtropical forested mountain in eastern China. We measured vegetation growth, estimated forest structure and species composition, and compiled ecosystem-scale carbon (C), nitrogen (N) and phosphorus (P) budgets based on concentration, pool and flux data collected from dominant canopy trees, understorey herbaceous plants, and soil organic and inorganic components in these forested plots. Our aims are three-fold: 1) to understand how C, N and P are distributed along the plant-microbe-soil continuum; 2) to disentangle how different growth and nutrient use strategies of plant and soil microbes affect ecosystem productivity and regulate the rate nutrient cycling; and 3) to benchmark predictive models in simulating ecosystem vegetation dynamics and their interaction with C, N, and P cycle processes. Our research will contribute towards better understanding of the functional diversity and productivity relationship, and will contribute towards an improved predictive capacity to simulate vegetation dynamics and the land carbon sink under climate change.

How to cite: Jiang, M., Wang, Z., and Wang, Z.: Benchmarking models with data: ecosystem carbon and nutrient budget along an elevation gradient in a subtropical forest ecosystem, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13646, https://doi.org/10.5194/egusphere-egu23-13646, 2023.

Climate change is projected to cause not only higher mean temperatures but also higher climate variability. Although elevated CO2 concentrations can potentially increase the productivity of some ecosystems, higher temperatures and more frequent droughts may lead to increased respiration and mortality, possibly negating these productivity gains. The capacity of global forests to adjust to climate change depends on their functional diversity and the ecosystem’s adaptive capacity.

The Plant-FATE eco-evolutionary model describes vegetation responses to altered environmental conditions, including CO₂ concentrations, temperatures, and droughts. It represents functional diversity by modelling species as points in trait space and incorporates ecosystem adaptations at three levels: 1) to model acclimation of plastic traits of individual plants, we leverage the power of eco-evolutionary optimality principles, 2) to model shifts in species composition via demographic changes and species immigration, we implement a trait-size-structured demographic vegetation model, and 3) to model the long-term genetic evolution of species, we have developed new evolutionary theory for trait-size-structured communities.

First, we show that with just a few calibrated parameters, the Plant-FATE model accurately predicts the fluxes of CO2 and water, size distributions, and trait distributions for a tropical wet site in the Amazon Forest. Second, we show that under elevated CO2 conditions and in the absence of nutrient limitation, our model predictions are broadly consistent with observations, namely: an increase in leaf area, productivity and biomass, and a decrease in stomatal conductance and photosynthetic capacity. Third, we simulate the calibrated model with hypothetical future drought regimes to investigate three key features of ecosystem responses: 1) the change in species composition and ecosystem functioning in response to altered conditions, 2) the timescales of ecosystem response to new regimes, 3) the influence of functional diversity on the timescale of ecosystem adaptation and its consequences for ecosystem collapse.

Our eco-evolutionary vegetation modelling strategy presents a powerful approach to leverage the power of natural selection to simulate ecosystem dynamics under novel conditions that plants may have never experienced before.

How to cite: Joshi, J., Hofhansl, F., Singh, S., Stocker, B., Brännström, Å., Vignal, T., Casagrande Blanco, C., Aleixo, I., Lapola, D., Colin Prentice, I., and Dieckmann, U.: Roles of diversity and adaptation in the eco-evolutionary responses of biodiverse plant communities to climate change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9757, https://doi.org/10.5194/egusphere-egu23-9757, 2023.

Biodiversity positively affects vegetation productivity and the ability of ecosystems to withstand disturbance events and invasions of alien species (ecosystem stability). However, the relationship between biodiversity and ecosystem functioning remains understudied at the landscape or whole ecosystem scales.

In particular, biodiversity can not be easily monitored at large spatial scales or frequent intervals. Therefore, confronting measurements of biodiversity and ecosystem functioning at the same spatial and temporal scales remains challenging. In this work, we present new methods to systematically bridge these scale gaps. We focus on collecting ecosystem data and developing metrics able to decypher the effect of plant biodiversity on ecosystem functioning. Based on eddy covariance fluxes from 78 NEON and ICOS sites, we compute key ecosystem functional properties related to ecosystem productivity and stability. Moreover, we calculate biodiversity indices from field surveys of species abundances, functional traits, and structural properties at these sites. Finally, we compute remote sensing metrics of biodiversity based on Sentinel 2 measurements. These metrics exploit the fine scale multispectral information from different and complementary perspectives, and are adapted to match the footprint of typical eddy covariance sites.

We investigate the relationship between ground- and satellite-based biodiversity metrics to understand the capability of remote sensing to contribute to biodiveristy-ecosystem function analyses that may one day be scaled globally. Despite dominant environmental and climatic constraints, we hypothesize that ecosystem functional properties covary with biodiversity metrics. To elucidate this point, we analyze the multivariate relationship between the different biodiversity estimates, ecosystem functional properties related to water, carbon, and energy fluxes, structural variables of the vegetation, and climate.

Assessing whether biodiversity effects apply to the functioning and stability of ecosystems is pivotal to understanding ecosystem processes and developing appropriate forecast models and climate change mitigation strategies.

How to cite: Gomarasca, U., Duveiller, G., Ceccherini, G., Cescatti, A., Girardello, M., Pacheco-Labrador, J., Reichstein, M., Wirth, C., and Migliavacca, M.: A systematic exploration of biodiversity-ecosystem function relationship using remote sensing and eddy covariance networks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13232, https://doi.org/10.5194/egusphere-egu23-13232, 2023.

Interannual variability in climatic drivers can have a strong impact on dryland ecosystem functioning globally. While interannual variations in dryland ecosystem processes are mainly driven by rainfall, other global change drivers such as CO₂ fertilization and rising temperatures can play an increasingly important role for these ecosystems. Yet, the high complexity of dryland ecosystems makes it difficult to unravel the individual and compound impacts of these different drivers. In this work we study the impacts of interannual climatic variability on the dryland ecosystems of the Sudano-Sahel region for the period 1981–2019. By using a dynamic vegetation model (LPJ-GUESS v4.0), we show that the year-to-year variability in dryland ecosystems that originates from interannual variability in rainfall is modulated by effects of CO₂ fertilization, which can strongly impact woody encroachment and resource competition between vegetation types. We also show that this response varies with aridity subtype, depending on the amount and type of woody cover. By untangling the impacts of climatic drivers on dryland vegetation, this study helps us to understand the different sensitivities of dryland ecosystems to climatic variability.

How to cite: Verbruggen, W., Schurgers, G., Meunier, F., Verbeeck, H., and Horion, S.: Simulated tree-grass competition in drylands is modulated by CO2 fertilization, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14168, https://doi.org/10.5194/egusphere-egu23-14168, 2023.

Multiple global change factors, such as elevated atmospheric CO2 concentrations, warming, and drought, are progressively affecting ecosystems worldwide. Future drought events will likely intensify, possibly leading to strongly non-linear threshold responses of ecosystem functioning. However, while of high ecological relevance, ecosystem responses to drought intensity have rarely been studied, and even less is known about whether future conditions involving a combination of elevated CO2 and warming can alter such responses. In an in-situ multifactor experiment in managed montane grassland, we studied the drought responses of ecosystem productivity, water use, and related canopy surface temperatures along with leaf-level stomatal conductance and photosystem II quantum yield. We analyzed whether and how resistance to and recovery from drought changed in response to drought intensity under current versus future (+300 ppm CO2, + 3° C) climate conditions. With increasing drought intensity, productivity and water use were increasingly reduced and were associated with increasing canopy temperatures, the effects being more pronounced in the future compared to current conditions. Our results suggest that the additional heat stress triggered by drought under future conditions can strongly reduce ecosystem resilience under scenarios of more extreme droughts.

How to cite: Capponi, L., Neuner, G., Still, C., Schaumberger, A., Herndl, M., and Bahn, M.: Resilience of productivity and water use of montane grassland in response to drought intensity under current and future climate conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15751, https://doi.org/10.5194/egusphere-egu23-15751, 2023.

Drought impacts on vegetation function have been widely assessed globally but our understanding of the global patterns of drought recovery and its mechanistic underpinnings is comparatively less understood. The quantification of vegetation resilience to drought has been mostly based on the analysis of time series of remotely-sensed vegetation indices, tree-ring data or ecosystem-level fluxes. While useful, these approaches have not provided a mechanistic link between resilience patterns and post-drought effects on plant hydraulics because they lack sufficient temporal and spatial resolution. Resilience quantified from tree-level sap flow can provide these mechanistic insights on post-drought effects but its estimation using classical resilience metrics requires defining a reference sap flow, unaffected by drought. Here, we compare two different approaches to estimate tree water use resilience to soil drought for >500 drought events using global, tree-level sap flow data in the SAPFLUXNET database (Poyatos et al. 2021). For both approaches, soil droughts were defined using the same criteria based on soil relative extractable water (REW), ensuring a minimum intensity and duration (10 days) and the presence of well-defined pre- and post-drought periods with REW values sustainedly above the threshold. In the first approach, we apply classical resilience metrics obtained from the comparison of pre- and post-drought sap flow. We show that water use resilience is related to soil drought characteristics such as intensity and duration and also to atmospheric vapour pressure deficit. In the second approach, we present a model-based resilience framework in which actual sap flow during and after a drought is compared against a reference sap flow modelled using a random forest regression with hydrometeorological drivers, but excluding potential drought legacy effects. This latter approach is tested in two Mediterranean forests in SAPFLUXNET where additional data on tree water status (leaf water potentials or tree water deficit derived from automatic dendrometry) are also available. We show that the model-based resilience framework applied to sap flow data is a promising avenue to better understand the global patterns of drought recovery and the underlying hydraulic mechanisms.

Poyatos, R. et al.: Global transpiration data from sap flow measurements: the SAPFLUXNET database, Earth System Science Data, 13, 2607–2649, https://doi.org/10.5194/essd-13-2607-2021, 2021.

How to cite: Poyatos, R., Fatecha, B., Nelson, J. A., Flo, V., Granda, V., De Cáceres, M., Anderegg, W. R. L., Bittencourt, P. R. L., Fisher, R. A., Junttila, S., Konings, A., Migliavacca, M., Miralles, D. G., Novick, K. A., Rowland, L., Zhang, W., Mencuccini, M., and Martínez-Vilalta, J.: Quantifying water use resilience from sap flow data to better understand post-drought effects on tree functioning, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9563, https://doi.org/10.5194/egusphere-egu23-9563, 2023.

Amazon rainforests host a unique biodiversity, store vast amounts of carbon, and are an essential component of the Earth System. Future water balance changes put the Amazon's carbon storage potential at risk. Evidence from grasslands indicates that diversity can mediate responses to drought; however, it remains unclear how tropical forests will respond. We show that functional diversity increases forest resistance to biomass loss during sudden catastrophic drought and chronic climate change-associated precipitation reductions by up to 25%. Using a model capable of simulating drought responses and functional diversity, we found that distinct strategies emerged along hydraulic and carbon allocation axes of trait variation. Climate change and elevated CO₂ caused the re-assembly of communities towards increased water-triggered phenological strategy dominance, whereas climate change alone negatively influenced biomass stored across all strategies. By removing water-triggered evergreen and deciduous strategies, we show that more biomass is lost in the absence of these strategies and thus clearly illustrate that higher diversity buffers the impacts of water balance changes. Our results demonstrate that a predictive understanding of trait diversity and plant hydraulic traits is essential to understand the complexity of diversity-biomass relations under future climate. 

How to cite: Langan, L., Scheiter, S., Hickler, T., and Higgins, S.: Hydraulic-trait diversity increases tropical forest resistance to water deficits., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5972, https://doi.org/10.5194/egusphere-egu23-5972, 2023.

Global change-type droughts increasingly endanger sustainable forest functioning in semi-arid and also in temperate European forests. Generally, mixed forests comprising different tree species are considered more resistant towards droughts, however, little is known about changes in species interactions (i.e. facilitation and competition) under increasing drought severity. In particular, knowledge on the regulation of ecohydrological processes, such as tree water fluxes, is lacking. We investigated responses during both natural extreme drought in 2018 and 2022 in a pine forest and experimental drought and competition treatment in a cork-oak forest between 2017 and 2020.

The heavily impacted Scots pine (Pinus sylvestris L.) forest at the ICOS ecosystem site Hartheim in the upper Rhine valley, Germany, hit a tipping-point during the 2018 drought showing very negative leaf water potentials, and over 47 % tree mortality in 2019. Net carbon exchange indicated slow recovery of NEE and a vegetation shift to broadleaved understory trees.

The combined precipitation exclusion and shrub invasion (Cistus ladanifer L.) experiment in a Mediterranean cork oak (Quercus suber L.) ecosystem in Portugal showed that the combination of imposed drought and shrub invasion amplified stress effects during an extreme drought, with strongly reduced tree transpiration. Contrarily, the imposed drought reduced the competitiveness of the shrubs in the following recovery period, which buffered the negative effects of shrub invasion on Q. suber.

Further a literature review on the impact of species interactions on tree resilience underlined that interactions can shift with increasing drought severity: beneficial species interactions, i.e. improved water relations, were prevalent under mild droughts. However, with increasing drought, negative effects, such as interspecific competition increased. These prevailed under extreme droughts, where even trees with complementary resource use strategies competed for water resources.

Moreover, under extreme droughts, competition effects and reduced recovery of some species were observed, which can strongly compromise tree resilience. Our results demonstrate the highly dynamic and non-linear effects of interacting stressors on ecosystems and urges for further investigations on biotic interactions in a context of climate change induced alteration.

How to cite: Werner, C., Haberstroh, S., Seifert, T., Christen, A., and Caldeira, M.: Impacts of severe droughts on species interaction in forests, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9525, https://doi.org/10.5194/egusphere-egu23-9525, 2023.

Climate change will lead to more frequent, longer, and more severe drought and heat periods, with unforeseen consequences for ecosystems globally. In Central Europe, for instance, grasslands deteriorated immediately in response to unprecedented drought and heat in recent years with major impacts on vegetation productivity. However, drought impacts can vary considerably in space and time, suggesting a complex network of underlying drivers. Factors such as soil characteristics, topography, species composition, and land-use modify the severity and duration of vegetation drought in grasslands on local to regional scales, yet our understanding is still underdeveloped. To better understand the complex drivers of grassland response to drought, it is indispensable to characterize drought impacts covering large environmental gradients in a spatially explicit way. While challenging, this task can be addressed with dense satellite-borne multispectral time series. In this study, we investigated how grasslands respond to meteorological and soil moisture drought and how this relationship varies with environmental and land management gradients in Central Europe. We used four decades of remote sensing time series from Landsat/Sentinel-2 to quantify vegetation drought at 30m spatial resolution across all grasslands in Germany. We applied a modeling approach developed in previous studies (Kowalski et al., 2023, 2022) for estimating time series of green vegetation, dry vegetation and soil ground cover percentages. We then derived monthly time series of the Normalized Difference Fraction Index (NDFI), which contrasts dry vegetation and soil relative to green vegetation, thereby providing a physically grounded indicator tracking grass dieback over the growing season. We calculated mean NDFI anomalies from June to September for each growing season from 1984-2021 using the 1984-2021 average as a baseline. We assessed the relation of NDFI anomalies to vapor pressure deficit, climatic water balance, and soil moisture anomalies derived from monthly ERA-5 Land time series. Moreover, we investigated how these relations varied spatially by stratifying grasslands according to environmental (e.g., precipitation, temperature, topographic derivatives, soil available water capacity) and land management factors. For the 38-year timespan, we found several single- and multi-year vegetation drought events including the strongest events in 2003 and 2018. The 2018 event featured the most severe NDFI anomaly of +0.32, translating into 32% higher than average dry vegetation and soil cover across all grasslands in Germany. NDFI anomalies varied spatially with a tendency for highest anomalies in the central uplands and northern lowlands, while grasslands in the southern Alpine region were less affected. NDFI anomalies had consistent moderate to strong correlations with meteorological and soil moisture drought. The overall highest correlations occurred in July and August indicating short time lags of NDFI anomalies. Our results confirm strong and spatially heterogenous impacts of meteorological and soil moisture droughts on grasslands. Drought periods in the next decades will thus pose substantial challenges for grassland vitality and productivity in Central Europe. Our study further shows the value of remote sensing for analyzing vegetation dynamics across grassland ecosystems, thereby enhancing our knowledge on fundamental processes in these complex systems.

Kowalski et al. 2022. https://doi.org/10.1016/j.rse.2021.112781

Kowalski et al. 2023. https://doi.org/10.1016/j.rse.2022.113449

How to cite: Kowalski, K., Senf, C., Okujeni, A., and Hostert, P.: Characterizing drought response patterns of Central European grasslands based on four decades of Landsat and Sentinel-2 data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13278, https://doi.org/10.5194/egusphere-egu23-13278, 2023.

Efficient heat dissipation under high radiation load is critical to plant functioning. It includes processes that are clearly observed in dry ecosystems but likely extend to other environments where it will be further enhanced by climate change. We observed that despite near-zero evaporation during the seasonal drought in a semi-arid pine forest, leaf temperatures were within the physiological range at about 35C. At the same time, exposed soil at the same site reached temperatures up to 70C. These leaf temperatures were also similar to that in an irrigated plot where evapotranspiration (ET) was enhanced by x10. A detailed energy budget demonstrates that heat dissipation under drought relies on a large sensible heat flux (H) that must depend, in turn, on reducing aerodynamic resistance to heat transfer. At the canopy scale, a “convector effect” of the high-roughness dry canopies generates a massive H that increases the depth of the planetary boundary layer and induces secondary circulations. Model simulation at larger scales indicated that such a process could modify local and regional climatic conditions. Assessing the global FLUXNET data from different environments further indicates that relying on H as a major heat dissipation process is not limited to dry ecosystems and is not dictated solely by the radiation load.  

How to cite: Yakir, D., Muller, J., and Rotenberg, E.: Ecosystem temperature management under water scarcity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8953, https://doi.org/10.5194/egusphere-egu23-8953, 2023.

Aridification threatens not only water availability but also adversely affects ecosystem health, and energy security. Using the (atmospheric) aridity index (AI) – defined as precipitation, (P) over potential evaporation (Ep) – several studies have shown that global drylands are either expanding or will expand in the future. Expansion is defined as the reduction of AI below 0.65, i.e., a change from a humid to a dry region may be owed to deficits in P and/or increases in Ep. However, the actual mechanisms and processes driving dryland expansion remain less explored. Here, we use an observationally-constrained Lagrangian transport model to test if expansion of drylands is self-fuelled: can reductions in moisture transport from existing drylands result in aridification of existing humid regions and thus lead to dryland expansion?

To estimate the spatial extent of drylands, we calculate AI using P from the Multi-Source Weighted-Ensemble Precipitation (MSWEP) (Beck et al 2019) and Ep from the hPET dataset (Singer et al. 2021). To quantify the changes in moisture and heat transport into newly expanded drylands, we use global simulations of the FLEXPART version 10.4, forced with the ERA-Interim reanalysis for a period of 38 years (1981–2018). The FLEXPART outputs include the properties of the air parcels at 3-hourly time steps, which are then post-processed using the Heat and Moisture Tracking Framework (HAMSTER v1.2.0) described by Keune et al. (2022) and bias-corrected using evaporation from the GLEAM-Hybrid dataset (Koppa et al. 2022).

Preliminary results show that between 1981 and 2018, ~5.5 million km2 of the terrestrial land surface underwent aridification (humid to dryland transition). Further, our results indicate that, on an average, ~45% of the reduction in AI can be attributed to reduction in P, out of which ~32% can be traced to reduction in moisture transport from existing drylands. Preliminary findings support our hypothesis that drylands are indeed self-expanding. 

References:

Beck, H. E., Wood, E. F., Pan, M., Fisher, C. K., Miralles, D. G., van Dijk, A. I. J. M., McVicar, T. R., & Adler, R. F. (2019). MSWEP V2 Global 3-Hourly 0.1° Precipitation: Methodology and Quantitative Assessment, Bulletin of the American Meteorological Society, 100(3), 473-500.

Keune, J., Schumacher, D. L., & Miralles, D. G. (2022). A unified framework to estimate the origins of atmospheric moisture and heat using Lagrangian models. Geoscientific Model Development, 15(5), 1875–1898. doi:10.5194/gmd-15-1875-2022.

Koppa, A., Rains, D., Hulsman, P., Poyatos, R., & Miralles, D. G. (2022). A deep learning-based hybrid model of global terrestrial evaporation. Nature Communications, 13(1), 1912. doi:10.1038/s41467-022-29543-7.

Singer, M. B., Asfaw, D. T., Rosolem, R., Cuthbert, M. O., Miralles, D. G., MacLeod, D., … Michaelides, K. (2021). Hourly potential evapotranspiration at 0.1° resolution for the global land surface from 1981-present. Scientific Data, 8(1), 224. doi:10.1038/s41597-021-01003-9

How to cite: Koppa, A., Keune, J., and Miralles, D. G.: Are Global Drylands Self-Expanding?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2320, https://doi.org/10.5194/egusphere-egu23-2320, 2023.

Ecosystem response to drier climates is likely to employ stress-relaxation mechanisms operating at different levels of ecological organization. At the individual level, a plant can change its phenotype, e.g., from a shallow root plant to a deep root plant to reach a moister soil layer. At the population level, plants can self-organize in spatial patterns, a process that involves partial plant mortality and increased water availability to remaining plants. At the community level, shifts from fast-growing species to stress-tolerant species can occur. These mechanisms are naturally coupled, but their interplay has hardly been studied. In this talk, I will present model studies of the interplay between phenotypic changes and vegetation patterning and between vegetation patterning and community re-assembly.  I will show that phenotypic transitions from shallow-roots to deep-roots plants can result in multiscale vegetation patterns and increased resilience to drier climates. I will further show that spatial patterning can result in a homeostatic plant community that keeps its composition and diversity unchanged, despite the development of a drier climate, because of spatial re-patterning. Understanding pathways of ecosystem response, where mechanisms operating at different organization levels act in concert, is essential for assessing the actual resilience of ecosystems at risk and devising management practices to evade tipping points.

How to cite: Meron, E.: Multi-Level Ecosystem Response to Climate Change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-980, https://doi.org/10.5194/egusphere-egu23-980, 2023.

Many dryland soils absorb atmospheric CO₂ at night. Despite the relatively small annual carbon (C) uptake, ranging locally from 1 to 10 g C m^-2, it may have large-scale effects, as drylands cover almost 45% of the Earth’s land surface. This process might contribute to the global missing C sink of 3.1 ± 0.9 Pg C year^-1. As dryland soils have high inorganic C contents compared to organic C, mechanisms of the nocturnal CO₂ uptake likely involve both biotic and abiotic processes that are tightly coupled to water availability. Biological soil crusts (hereafter, biocrusts) cover about 30% of global drylands and provide favourable physico-chemical hotspots for this C exchange mechanism. In this study, we present a mechanistic model of inorganic C binding that is enhanced by the activity of biocrust communities. The model results show good agreement with field measurements of the soil-atmosphere CO₂ exchange dynamics under contrasting conditions of water availability and temperature in the Tabernas Desert (Spain). We further show that inorganic C sequestration rates at night vary, depending on the successional stages of biocrusts. Our findings have the potential to substantially improve the often-overlooked processes of dryland systems in the global C cycle. By unravelling the mechanisms occurring along the succession of biocrusts, this study highlights the roles of soil biological agents in mitigating CO₂ emissions in a drier future.

How to cite: Kim, M., Lopez-Canfin, C., Lázaro, R., Sánchez-Cañete, E. P., and Weber, B.: Mechanisms of nocturnal soil CO2 uptake influenced by a succession of biological soil crusts in drylands, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6616, https://doi.org/10.5194/egusphere-egu23-6616, 2023.

Decomposition of plant litter is a key process in the carbon cycle, controlled mainly by environmental factors, litter quality and the decomposer community, at least in mesic and humid regions. In drylands representing ~40% of the global terrestrial area, water as a crucial environmental factor is scarce, thus limiting the classic pathway of microbial degradation of plant litter. In the past two decades, there has been an effort to study litter decompositions under dry conditions, focusing on different decay mechanisms that operate without rain or snow as water sources. These mechanisms include abiotic processes, mainly photodegradation driven by solar radiation and thermal degradation driven by heat, and humidity-enhanced microbial degradation enabled by non-rainfall water sources, such as fog, dew and atmospheric water vapor. However, the involvement of these dryland decay mechanisms in litter decomposition is not well constrained. The objective of this study was to quantify the relative contribution of dryland decay mechanisms to mass loss and CO₂ flux under rainless conditions. In a full-factorial semi-controlled study with quartz tubes, we exposed six litter species from tropical, temperate, and Mediterranean regions to all combinations of high and low radiation, heat and humidity for 90 days. Our results suggest that in the early stage of decomposition, CO₂ fluxes are more affected by climate factors than by litter traits, with the combination of high solar radiation, low heat and high air humidity resulting in the highest fluxes for all species. Interactions between two or three decay drivers contribute to additional mass loss compared to the effect of each separate decay mechanism. The results from this experiment indicate the importance of the interaction between decay mechanisms to achieve significant mass loss and CO₂ flux. To assess the contribution of different dryland decay mechanisms on litter decomposition under field conditions, we conducted a litterbag experiment in 12 sites and two microsites per site (under and between shrubs), which widely diverged in microclimatic conditions during the rainless summer season. Decay of a more recalcitrant litter (oak leaves) was primarily related to solar radiation, while decay of a more labile litter (wheat straw) was related to both humidity and solar radiation. The shrub microsite was characterized by less heat and lower solar radiation, higher humidity, and generally lower mass loss than the intershrub microsite. The results from both experiments indicate that a combination of solar radiation and humidity is essential for litter decomposition in rainless periods, and that heat alone is insufficient to induce significant litter decay. Moreover, the more extreme the abiotic climate factors (warmer and drier) in our experiments, the slower the decay process, suggesting that under climate change, we should expect a lower litter decomposition rate. Hence, carbon and nutrient cycles might slow down ecosystem productivity in a future climate.

How to cite: Schechter, S., Angert, A., and Gruenzweig, J.: Abiotic and biotic dryland mechanisms controlling litter decomposition in rainless periods, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12168, https://doi.org/10.5194/egusphere-egu23-12168, 2023.

Among living tree species, Acacia raddiana (Savi) and Acacia tortilis (Forssk), species of the legume family, populate some of the hottest and driest places on Earth. Our research investigates the physiological processes underlying the unique survival of trees in extreme environmental conditions. We measured Acacia trees in their natural habitat together with a controlled experiment under scenarios of drought and low N on a lysimeters system to unravel their water use strategies and growth dynamics. In the field, temperature positively influenced the growth rate of the trees, daily and annual gas-exchange curves showed higher gas exchange during noon and in summer, when temperature and radiation are maximal (44°C, 2000 µmol m^-2 s^-1), and the air is dry (21% RH). Furthermore, in the controlled experiment, Acacia saplings keep transpiring water (180 g per day), especially at noontime (0.08 g_water/g_plant/ min), and therefore continue growing in low soil water content of 5%. These findings suggest a strong potential for acacia trees to contribute to ecosystem carbon sequestration in warming and drying climates.

How to cite: Uni, D., Klein, T., Winters, G., and Sheffer, E.: I'm a Survivor : Acacia trees ability to cope with extremely hot and dry environments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1595, https://doi.org/10.5194/egusphere-egu23-1595, 2023.