Vertical decoupling of soil nutrients and water under climate warming reduces plant cumulative nutrient uptake, water-use efficiency and productivity

Plant water source effects on plant-soil feedback for primary succession of terrestrial ecosystems in a glacier region in China

Despite the extensive research conducted on plant-soil-water interactions, the understanding of the role of plant water sources in different plant successional stages remains limited. In this study, we employed a combination of water isotopes (δ2H and δ18O) and leaf δ13C to investigate water use patterns and leaf water use efficiency (WUE) during the growing season (May to September 2021) in Hailuogou glacier forefronts in China. Our findings revealed that surface soil water and soil nutrient gradually increased during primary succession. Dominant plant species exhibited a preference for upper soil water uptake during the peak leaf out period (June to August), while they relied more on lower soil water sources during the post-leaf out period (May) or senescence (September to October). Furthermore, plants in late successional stages showed higher rates of water uptake from uppermost soil layers. Notably, there was a significant positive correlation between the percentage of water uptake by plants and available soil water content in middle and late stages. Additionally, our results indicated a gradual decrease in WUE with progression through succession, with shallow soil moisture utilization negatively impacting overall WUE across all succession stages. Path analysis further highlighted that surface soil moisture (0- 20 cm) and middle layer nutrient availability (20- 50 cm) played crucial roles in determining WUE. Overall, this research emphasizes the critical influence of water source selection on plant succession dynamics while elucidating underlying mechanisms linking succession with plant water consumption.

Intraspecific variation in fine root morphology of European beech: a root order-based analysis of phenotypic root morphospace

Fine roots are multifunctional organs that may change function with ageing or root branching events from primarily absorptive to resource transport and storage functions. It is not well understood, how fine root branching patterns and related root functional differentiation along the longitudinal root axis change with soil chemical and physical conditions. We examined the variation in fine root branching patterns (the relative frequency of 1st to 4th root orders) and root morphological and chemical traits of European beech trees with soil depth (topsoil vs. subsoil) and soil chemistry (five sites with acid to neutral/alkaline bedrock). Bedrock type and related soil chemistry had an only minor influence on branching patterns: base-poor, infertile sites showed no higher fine root branching than base-rich sites. The contribution of 1st-order root segments to total fine root length decreased at all sites from about 60% in the topsoil (including organic layer) to 45% in the lower subsoil. This change was associated with a decrease in specific root area and root N content and an increase in mean root diameter with soil depth, while root tissue density did not change consistently. We conclude that soil depth (which acts through soil physical and chemical drivers) influences the fine root branching patterns of beech much more than soil chemical variation across soil types. To examine whether changes in root function are indeed triggered by branching events or result from root ageing and diameter growth, spatially explicit root physiological and anatomical studies across root orders are needed.

Tight coupling between leaf δ13 C and N content along leaf ageing in the N2 -fixing legume tree black locust (Robinia pseudoacacia L.)

N2 -fixing legumes can strongly affect ecosystem functions by supplying nitrogen (N) and improving the carbon-fixing capacity of vegetation. Still, the question of how their leaf-level N status and carbon metabolism are coordinated along leaf ageing remains unexplored. Leaf tissue carbon isotopic composition (δ13 C) provides a useful indicator of time-integrated intrinsic water use efficiency (WUEi). Here, we quantified the seasonal changes of leaf δ13 C, N content on a mass and area basis (Nmass , Narea , respectively), Δ18 O (leaf 18 O enrichment above source water, a proxy of time-integrated stomatal conductance) and morphological traits in an emblematic N2 -fixing legume tree, the black locust (Robinia pseudoacacia L.), at a subtropical site in Southwest China. We also measured xylem, soil and rainwater isotopes (δ18 O, δ2 H) to characterize tree water uptake patterns. Xylem water isotopic data reveal that black locust primarily used shallow soil water in this humid habitat. Black locust exhibited a decreasing δ13 C along leaf ageing, which was largely driven by decreasing leaf Nmass , despite roughly constant Narea . In contrast, the decreasing δ13 C along leaf ageing was largely uncoupled from parallel increases in Δ18 O and leaf thickness. Leaf N content is used as a proxy of leaf photosynthetic capacity; thus, it plays a key role in determining the seasonality in δ13 C, whereas the roles of stomatal conductance and leaf morphology are minor. Black locust leaves can effectively adjust to changing environmental conditions along leaf ageing through LMA increases and moderate stomatal conductance reduction while maintaining constant Narea to optimize photosynthesis and carbon assimilation, despite declining leaf Nmass and δ13 C.

Prediction of volatility and seasonality vegetation by using the GARCH and Holt-Winters models

Seasonality and volatility of vegetation in the ecosystem are associated with climatic sensitivity, which can have severe consequences for the environment as well as on the social and economic well-being of the nation. Monitoring and forecasting vegetation growth patterns in ecosystems significantly rely on remotely sensed vegetation indices, such as Normalized Difference Vegetation Index (NDVI). A novel integration of the Generalized Autoregressive Conditional Heteroskedasticity (GARCH) and the Holt-Winters (H-W) models was used to simulate the seasonality and volatility of the three different agro-climatic zones in Jharkhand, India: the central north-eastern, eastern, and south-eastern agro-climatic zones. MODIS Terra Vegetation Indices NDVI data MOD13Q1, from 2001 to 2021, was used to create NDVI time series volatility and seasonality modeled by the GARCH and the H-W models, respectively. GARCH-based Exponential GARCH (EGARCH) [1,1] and Standard GARCH (SGARCH) [1,1] models were used to check the volatility of vegetation growth in three different agro-climatic zones of Jharkhand. The SGARCH [1,1] and EGARCH [1,1] models for the western agro-climatic zone experienced the best indicator as it has maximum likelihood and minimal Schwarz-Bayesian criterion and Akaike information criterion. The seasonality results showed that the additive H-W model showed better results in the eastern agro-climatic zone with the optimized values of MAE (16.49), MAPE (0.49), NSE (0.86), RMSE (0.49), and R2 (0.82) followed by the south-eastern and central north-eastern agro-climatic zones. By utilizing the H-W and GARCH models, the finding demonstrates that vegetation orientation and monitoring seasonality can be predicted using NDVI.

Juvenile Plant-Microbe Interactions Modulate the Adaptation and Response of Forest Seedlings to Rapid Climate Change

The negative impacts of climate change on native forest ecosystems have created challenging conditions for the sustainability of natural forest regeneration. These challenges arise primarily from abiotic stresses that affect the early stages of forest tree development. While there is extensive evidence on the diversity of juvenile microbial symbioses in agricultural and fruit crops, there is a notable lack of reports on native forest plants. This review aims to summarize the critical studies conducted on the diversity of juvenile plant-microbe interactions in forest plants and to highlight the main benefits of beneficial microorganisms in overcoming environmental stresses such as drought, high and low temperatures, metal(loid) toxicity, nutrient deficiency, and salinity. The reviewed studies have consistently demonstrated the positive effects of juvenile plant-microbiota interactions and have highlighted the potential beneficial attributes to improve plantlet development. In addition, this review discusses the beneficial attributes of managing juvenile plant-microbiota symbiosis in the context of native forest restoration, including its impact on plant responses to phytopathogens, promotion of nutrient uptake, facilitation of seedling adaptation, resource exchange through shared hyphal networks, stimulation of native soil microbial communities, and modulation of gene and protein expression to enhance adaptation to adverse environmental conditions.

Nutritional changes in trees during drought-induced mortality: A comprehensive meta-analysis and a field study

Both macronutrients and micronutrients are essential for tree growth and development through participating in various ecophysiological processes. However, the impact of the nutritional status of trees on their ability to withstand drought-induced mortality remains inconclusive. We thus conducted a comprehensive meta-analysis, compiling data on 11 essential nutrients from 44 publications (493 independent observations). Additionally, a field study was conducted on Pinus sylvestris L. trees with varying drought-induced vitality loss in the "Visp" forest in southern Switzerland. No consistent decline in tree nutritional status was observed during tree mortality. The meta-analysis revealed significantly lower leaf potassium (K), iron (Fe), and copper (Cu) concentrations with tree mortality. However, the field study showed no causal relationships between nutritional levels and the vitality status of trees. This discrepancy is mainly attributed to the intrinsic differences in the two types of experimental designs and the ontogenetic stages of target trees. Nutrient reductions preceding tree mortality were predominantly observed in non-field conditions, where the study was conducted on seedlings and saplings with underdeveloped root systems. It limits the nutrient uptake capacity of these young trees during drought. Furthermore, tree nutritional responses are also influenced by many variables. Specifically, (a) leaf nutrients are more susceptible to drought stress than other organs; (b) reduced tree nutrient concentrations are more prevalent in evergreen species during drought-induced mortality; (c) of all biomes, Mediterranean forests are most vulnerable to drought-induced nutrient deficiencies; (d) soil types affect the direction and extent of tree nutritional responses. We identified factors that influence the relationship between tree nutritional status and drought survival, and proposed potential early-warning indicators of impending tree mortality, for example, decreased K concentrations with declining vitality. These findings contribute to our understanding of tree responses to drought and provide practical implications for forest management strategies in the context of global change.

Linking functional composition moments of the sub-Mediterranean ecotone with environmental drivers

Introduction

Functional trait-based approaches are extensively applied to the study of mechanisms governing community assembly along environmental gradients. These approaches have been classically based on studying differences in mean values among species, but there is increasing recognition that alternative metrics of trait distributions should be considered to decipher the mechanisms determining community assembly and species coexistence. Under this framework, the main aim of this study is to unravel the effects of environmental conditions as drivers of plant community assembly in sub-Mediterranean ecotones.

Methods

We set 60 plots in six plant communities of a sub-Mediterranean forest in Central Spain, and measured key above- and belowground functional traits in 411 individuals belonging to 19 species, along with abiotic variables. We calculated community-weighted mean (CWM), skewness (CWS) and kurtosis (CWK) of three plant dimensions, and used maximum likelihood techniques to analyze how variation in these functional community traits was driven by abiotic factors. Additionally, we estimated the relative contribution of intraspecific trait variability and species turnover to variation in CWM.

Results and discussion

The first three axes of variation of the principal component analyses were related to three main plant ecological dimensions: Leaf Economics Spectrum, Root Economics Spectrum and plant hydraulic architecture, respectively. Type of community was the most important factor determining differences in the functional structure among communities, as compared to the role of abiotic variables. We found strong differences among communities in their CWMs in line with their biogeographic origin (Eurosiberian vs Mediterranean), while differences in CWS and CWK indicate different trends in the functional structure among communities and the coexistence of different functional strategies, respectively. Moreover, changes in functional composition were primarily due to intraspecific variability.

Conclusion

We observed a high number of strategies in the forest with the different communities spreading along the acquisitive-conservative axis of resource-use, partly matching their Eurosiberian-Mediterranean nature, respectively. Intraspecific trait variability, rather than species turnover, stood as the most relevant factor when analyzing functional changes and assembly patterns among communities. Altogether, our data support the notion that ecotones are ecosystems where relatively minor environmental shifts may result in changes in plant and functional composition.

Plant water use theory should incorporate hypotheses about extreme environments, population ecology, and community ecology

Plant water use theory has largely been developed within a plant-performance paradigm that conceptualizes water use in terms of value for carbon gain and that sits within a neoclassical economic framework. This theory works very well in many contexts but does not consider other values of water to plants that could impact their fitness. Here, we survey a range of alternative hypotheses for drivers of water use and stomatal regulation. These hypotheses are organized around relevance to extreme environments, population ecology, and community ecology. Most of these hypotheses are not yet empirically tested and some are controversial (e.g. requiring more agency and behavior than is commonly believed possible for plants). Some hypotheses, especially those focused around using water to avoid thermal stress, using water to promote reproduction instead of growth, and using water to hoard it, may be useful to incorporate into theory or to implement in Earth System Models.

Animal conflicts escalate in a warmer world

The potential for climate change to affect animal behaviour is widely recognized, yet its possible consequences on aggressiveness are still unclear. If warming and drought limit the availability of food resources, climate change may elicit an increase of intraspecific conflicts stemming from resource competition. By measuring aggressivity indices in a group-living, herbivorous mammal (the Apennine chamois Rupicapra pyrenaica ornata) in two sites differing in habitat quality, and coupling them with estimates of plant productivity, we investigated whether harsh climatic conditions accumulated during the growing season influenced agonistic contests at feeding via vegetation-mediated effects, and their interaction with the site-specific habitat quality. We focused on females, which exhibit intra-group contest competition to access nutritious food patches. Accounting for confounding variables, we found that (1) the aggression rate between foraging individuals increased with the warming accumulated over previous weeks; (2) the probability to deliver more aggressive behaviour patterns toward contestants increased with decreasing rainfall recorded in previous weeks; (3) the effects of cumulative warming and drought on aggressivity indices occurred at time windows spanning 15-30 days, matching those found on vegetation productivity; (4) the effects of unfavourable climatic conditions via vegetation growth on aggressivity were independent of the site-specific habitat quality. Simulations conducted on our model species predict a ~50 % increase in aggression rate following the warming projected over the next 60 years. Where primary productivity will be impacted by warming and drought, our findings suggest that the anticipated climate change scenarios may trigger bottom-up consequences on intraspecific animal conflicts. This study opens the doors for a better understanding of the multifactorial origin of aggression in group-living foragers, emphasising how the escalation of agonistic contests could emerge as a novel response of animal societies to ongoing global warming.

Water availability and plant-herbivore interactions

Water is essential to plant growth and drives plant evolution and interactions with other organisms such as herbivores. However, water availability fluctuates, and these fluctuations are intensified by climate change. How plant water availability influences plant-herbivore interactions in the future is an important question in basic and applied ecology. Here we summarize and synthesize the recent discoveries on the impact of water availability on plant antiherbivore defense ecology and the underlying physiological processes. Water deficit tends to enhance plant resistance and escape traits (i.e. early phenology) against herbivory but negatively affects other defense strategies, including indirect defense and tolerance. However, exceptions are sometimes observed in specific plant-herbivore species pairs. We discuss the effect of water availability on species interactions associated with plants and herbivores from individual to community levels and how these interactions drive plant evolution. Although water stress and many other abiotic stresses are predicted to increase in intensity and frequency due to climate change, we identify a significant lack of study on the interactive impact of additional abiotic stressors on water-plant-herbivore interactions. This review summarizes critical knowledge gaps and informs possible future research directions in water-plant-herbivore interactions.

Effects of drought and climate factors on vegetation dynamics in Central Asia from 1982 to 2020

Ecological security and ecosystem stability in Central Asia depend heavily on the local vegetation. Vegetation dynamics and the response and hysteresis relationships to climate factors and drought on multiple scales over long time series in the region still need to be further explored. Using the net primary productivity (NPP) values as the vegetation change index of interest, in this study, we analyzed vegetation dynamics in Central Asia from 1982 to 2020 and assessed the responses and time lags of vegetation to climate factors and drought. The results showed that NPP gradually decreased from north to south and from east to west. Vegetation was distributed along both sides of the mountains. The temperatures rose from northeast to southwest, while precipitation gradually increased from southwest to northeast. The proportion of dry and wet years was as follows: normal (56.41%) > slightly dry (28.2%) > slightly humid (15.39%). Precipitation and drought conditions were positively correlated with NPP during the growing season, while temperature was negatively correlated with NPP. Increased spring temperature, precipitation, and drought conditions positively affected vegetation, while sustained summer temperature resulted in suppressed vegetation growth. Autumn vegetation was positively affected by temperature and drought, and precipitation was negatively correlated with autumn vegetation. Increasing winter temperatures promoted vegetation growth. The time lag between NPP and temperature gradually increased from northeast to southwest, and the time lag between NPP and precipitation gradually increased from south to north. Spring temperatures had the greatest beneficial impact on forestlands; summer climatic factors and drought had little effect on shrublands; the autumn climate exhibited small differences in its influence of each plant type; and winter temperatures had the greatest positive effect on grasslands. No time lag effect was found between any of the four vegetation types and precipitation. A one-month lag was found between cultivated lands and temperature; a two-month lag was found between forestlands and temperature; and a one-month lag was found between forestlands and drought and between shrublands and drought. The results can provide a scientific foundation for the sustainable development and management of ecosystems.

Ecophysiological responses of Nothofagus obliqua forests to recent climate drying across the Mediterranean-Temperate biome transition in south-central Chile

The forests of south-central Chile are facing a drying climate and a megadrought that started in 2010. This study addressed the physiological responses of five Nothofagus obliqua stands across the Mediterranean-Temperate gradient (35.9 ° -40.3° S) using carbon isotope discrimination (Δ¹³ C) and intrinsic water use efficiency (iWUE) in tree rings during 1967-2017. Moreover, δ¹⁸O was evaluated in the northernmost site to better understand the effects of the megadrought in this drier location. These forests have become more efficient in their use of water. However, trees from the densest stand are discriminating more against ¹³C, probably due to reduced photosynthetic rates associated with increasing competition. The strongest associations between climate and Δ¹³C were found in the northernmost stand, suggesting that warmer and drier conditions could have reduced ¹³C discrimination. Tree growth in this site has not decreased, and δ¹⁸O was negatively related to annual rainfall. However, a shift in this relationship was found since 2007, when both precipitation and δ¹⁸O decreased, while correlations between δ¹⁸O and growth increased. This implies that tree growth and δ¹⁸O are coupled in recent years, but precipitation is not the cause, suggesting that trees probably changed their water source to deeper and more depleted pools. Our research demonstrates that forests are not reducing their growth in central Chile, mainly due to a shift towards the use of deeper water sources. Despite a common climate trend across the gradient, there is a non-uniform response of N. obliqua forests to climate drying, being their response site specific. Keywords: Tree rings, stable isotopes, tree physiology, climate gradient, megadrought, climate change.

Mineral Nutrition of Naturally Growing Scots Pine and Norway Spruce under Limited Water Supply

The deterioration of plant mineral nutrition during drought is a significant factor in the negative influence of drought on plant performance. We aimed to study the effects of seasonal and multiyear water shortages on nutrient supply and demand in Scots pine and Norway spruce. We studied pine and spruce trees naturally grown in the Bryansk region (Russia). The dynamics of several nutrients (K, Ca, Mg, P, Fe, Mn, Zn, and Ca) in wood, needles, and bark of current-year twigs and the dynamics of the available pools of these elements at different soil depths were analysed. To assess the physiological consequences of changes in element concentrations, lipid peroxidation products and photosynthetic pigments were measured in the needles. Water shortage increased the wood concentrations of all elements except for Mn. In pine, this increase was mainly due to seasonal water deficit, whereas in spruce, multiyear differences in water supply were more important. This increased availability of nutrients was not observed in soil-based analyses. In needles, quite similar patterns of changes were found between species, with Mg increasing almost twofold and Fe and Mn decreasing under water shortage, whereas the remainder of the elements did not change much under differing water supplies. Neither the concentrations of photosynthetic pigments nor the contents of lipid peroxidation products correlated with element dynamics in needles. In summary, water shortage increased the availability of all elements except Mn for the plant; however, needle element contents were regulated independently of element availability for plants.

Coordination of leaf functional traits under climatic warming in an arid ecosystem

BACKGROUND

Climatic warming is increasing regionally and globally, and results concerning warming and its consequent drought impacts have been reported extensively. However, due to a lack of quantitative analysis of warming severities, it is still unclear how warming and warming-induced drought influence leaf functional traits, particularly how the traits coordinate with each other to cope with climatic change. To address these uncertainties, we performed a field experiment with ambient, moderate and severe warming regimes in an arid ecosystem over 4 years.

RESULTS

Severe warming significantly reduced the specific leaf area and net photosynthetic rate with a relatively stable change and even enhancement under moderate warming, especially showing species-specific performance. The current results largely indicate that a coordinated trade-off can exist between plant functional traits in plant communities in a dryland ecosystem under ambient temperature conditions, which is strongly amplified by moderate warming but diminished or even eliminated by severe warming. Based on the present findings and recent results in the relevant literature, we advance the ecological conceptual models (e.g., LES and CSR) in the response to climatic warming in arid grassland communities, where the few key species play a crucial role by balancing their functional performances to cope with environmental change.

CONCLUSION

Our results highlight the importance of coordination and/or trade-off between leaf functional traits for understanding patterns of climatic change-induced vegetation degradation and suggest that the plant community composition in these drylands could be shifted under future climate change.

Growth, Nutrient Accumulation, and Nutritional Efficiency of a Clonal Eucalyptus Hybrid in Competition with Grasses

Invasive grasses reduce resource availability, mainly nutrients in the soil, and the growth of eucalyptus plants. Efficient management to increase productivity depends on understanding levels of weed interference in eucalyptus plantations. The nutritional efficiency of eucalyptus plants in competition has been evaluated by plant tissue analysis. The objective was to evaluate the growth, relative accumulation of nutrients, and nutritional efficiency of the eucalyptus clonal hybrid I144 (Eucalyptus urophylla × Eucalyptus grandis), in competition with Megathyrsus maximus cv. BRS zuri, Urochloa brizantha cv. marandu, Urochloa decumbens cv. basilisk and in the control (eucalyptus plants without weed competition). The experiment was carried out with a completely randomized design, with four treatments and ten replications. The height, stem diameter, number of leaves, leaf area, dry matter of leaves and stem, nutrient content in leaves and uptake, transport, and N, P, and K utilization efficiency of the eucalyptus clonal hybrid were evaluated at 110 days after transplantation. The growth parameters and relative contents of macro and micronutrients in the eucalyptus clonal hybrid were lower in competition with M. maximus, U. brizantha and U. decumbens. The efficiency of N, P, and K uptake and transport by the eucalyptus clonal hybrid was 29.41 and 7.32% lower in competition with U. decumbens than in the control treatments, respectively. The efficiency of N, P, and K utilization by eucalypts was 13.73, 9.18, and 22.54% lower in competition with M. maximus, U. brizantha, and U. decumbens, respectively. The reduced growth and nutritional parameters of the eucalyptus clonal hybrid were more evident in competition with U. decumbens. Plant tissue analyses efficiently determined the level of competition for nutrients between species. Crop competition with grasses can decrease the efficiency and use of nutrients, which consequently reduces plant development and productivity.

Effects of Irrigation and Nitrogen Application on Soil Nutrients in Triploid Populus tomentosa Stands

Irrigation and nitrogen application directly affect the availability and distribution of soil nutrients. Understanding the response of soil nutrients to long-term water–fertilizer coupling conditions is helpful to improve the management and use efficiency. Irrigation was divided into three gradient levels, which accounted for 45%, 60%, and 75% (W1, W2, and W3) of the field water holding capacity. Based on pure nitrogen, four levels of nitrogen application were set: 0.0, 101.6, 203.2, and 304.8 kg·hm−2 (N0, N1, N2, and N3). We measured tree height and diameter at breast height (DBH), and we analyzed the chemical properties of the soil at 0–40 cm depth, from 2007 to 2020. The ranges of DBH, tree height, individual volume, and stand volume were 5.80–25.25 cm, 6.10–16.47 m, 0.01–0.37 m3, and 11.76–481.47 m3·hm−2, respectively. The contents of organic matter, total nitrogen, available phosphorus, and available potassium in the soil ranged from 8.60 g·kg−1 to 18.72 g·kg−1, from 0.21 g·kg−1 to 0.79 g·kg−1, from 8.09 mg·kg−1 to 47.05 mg·kg−1, and from 90 mg·kg−1 to 322 mg·kg−1, respectively. Soil pH value decreased rapidly at a rate of 0.31 units per year for the first five years. Irrigation and nitrogen application, and their interaction, had significant (p < 0.01) effects on soil total nitrogen, available phosphorus, available potassium, and nitrate-nitrogen. We suggest maintaining the field water holding capacity above 60%, with a nitrogen application rate of 203.2 kg·hm−2, to save water, maintain soil fertility, and optimize soil nitrogen supply. Our study aimed to achieve scientific and accurate fertilization of Populus tomentosa stands over different periods, to alleviate the decline of soil fertility, and to improve the utilization rate of water and fertilizer through long-term nutrient monitoring.

The role of nutritional impairment in carbon-water balance of silver fir drought-induced dieback

Rear-edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in Southwestern Europe. While silver fir drought-induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon-water balances. We performed a comparative analysis of radial growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (δ¹⁸ O) and nutrient concentrations in leaves of declining (DD) and non-declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical-chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate-growth relationships and lower δ¹⁸ O than ND trees. The uncoupling between iWUE and δ¹⁸ O, together with the positive correlations between P and K leaf concentrations and δ¹⁸ O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies.

The potential of groundwater-dependent ecosystems to enhance soil biological activity and soil fertility in drylands

Water availability controls the functioning of dryland ecosystems, driving a patchy vegetation distribution, unequal nutrient availability, soil respiration in pulses, and limited productivity. Groundwater-dependent ecosystems (GDEs) are acknowledged to be decoupled from precipitation, since their vegetation relies on groundwater sources. Despite their relevance to enhance productivity in drylands, our understanding of how different components of GDEs interconnect (i.e., soil, vegetation, water) remains limited. We studied the GDE dominated by the deep-rooted phreatophyte Ziziphus lotus, a winter-deciduous shrub adapted to arid conditions along the Mediterranean basin. We aimed to disentangle whether the groundwater connection established by Z. lotus will foster soil biological activity and therefore soil fertility in drylands. We assessed (1) soil and vegetation dynamics over seasons (soil CO₂ efflux and plant activity), (2) the effect of the patchy distribution on soil quality (properties and nutrient availability), and soil biological activity (microbial biomass and mineralization rates) as essential elements of biogeochemical cycles, and (3) the implications for preserving GDEs and their biogeochemical processes under climate change effects. We found that soil and vegetation dynamics respond to water availability. Whereas soil biological activity promptly responded to precipitation events, vegetation functioning relies on less superficial water and responded on different time scales. Soil quality was higher under the vegetation patches, as was soil biological activity. Our findings highlight the importance of groundwater connections and phreatophytic vegetation to increase litter inputs and organic matter into the soils, which in turn enhances soil quality and decomposition processes in drylands. However, biogeochemical processes are jeopardized in GDEs by climate change effects and land degradation due to the dependence of soil activity on: (1) precipitation for activation, and (2) phreatophytic vegetation for substrate accumulation. Therefore, desertification might modify biogeochemical cycles by disrupting key ecosystem processes such as soil microbial activity, organic matter mineralization, and plant productivity.

Field-based tree mortality constraint reduces estimates of model-projected forest carbon sinks

Considerable uncertainty and debate exist in projecting the future capacity of forests to sequester atmospheric CO₂. Here we estimate spatially explicit patterns of biomass loss by tree mortality (LOSS) from largely unmanaged forest plots to constrain projected (2015–2099) net primary productivity (NPP), heterotrophic respiration (HR) and net carbon sink in six dynamic global vegetation models (DGVMs) across continents. This approach relies on a strong relationship among LOSS, NPP, and HR at continental or biome scales. The DGVMs overestimated historical LOSS, particularly in tropical regions and eastern North America by as much as 5 Mg ha⁻¹ y⁻¹. The modeled spread of DGVM-projected NPP and HR uncertainties was substantially reduced in tropical regions after incorporating the field-based mortality constraint. The observation-constrained models show a decrease in the tropical forest carbon sink by the end of the century, particularly across South America (from 2 to 1.4 PgC y⁻¹), and an increase in the sink in North America (from 0.8 to 1.1 PgC y⁻¹). These results highlight the feasibility of using forest demographic data to empirically constrain forest carbon sink projections and the potential overestimation of projected tropical forest carbon sinks.

Here the authors use broad-scale tree mortality data to estimate biomass loss, constraining uncertainty of projected forest net primary productivity in 6 models, finding weaker tropical forest carbon sinks with climate change.

Pastures and Climate Extremes: Impacts of Cool Season Warming and Drought on the Productivity of Key Pasture Species in a Field Experiment

Shifts in the timing, intensity and/or frequency of climate extremes, such as severe drought and heatwaves, can generate sustained shifts in ecosystem function with important ecological and economic impacts for rangelands and managed pastures. The Pastures and Climate Extremes experiment (PACE) in Southeast Australia was designed to investigate the impacts of a severe winter/spring drought (60% rainfall reduction) and, for a subset of species, a factorial combination of drought and elevated temperature (ambient +3°C) on pasture productivity. The experiment included nine common pasture and Australian rangeland species from three plant functional groups (C3 grasses, C4 grasses and legumes) planted in monoculture. Winter/spring drought resulted in productivity declines of 45% on average and up to 74% for the most affected species (Digitaria eriantha) during the 6-month treatment period, with eight of the nine species exhibiting significant yield reductions. Despite considerable variation in species' sensitivity to drought, C4 grasses were more strongly affected by this treatment than C3 grasses or legumes. Warming also had negative effects on cool-season productivity, associated at least partially with exceedance of optimum growth temperatures in spring and indirect effects on soil water content. The combination of winter/spring drought and year-round warming resulted in the greatest yield reductions. We identified responses that were either additive (Festuca), or less-than-additive (Medicago), where warming reduced the magnitude of drought effects. Results from this study highlight the sensitivity of diverse pasture species to increases in winter and spring drought severity similar to those predicted for this region, and that anticipated benefits of cool-season warming are unlikely to be realized. Overall, the substantial negative impacts on productivity suggest that future, warmer, drier climates will result in shortfalls in cool-season forage availability, with profound implications for the livestock industry and natural grazer communities.

Nitrogen Deposition Shifts Grassland Communities Through Directly Increasing Dominance of Graminoids: A 3-Year Case Study From the Qinghai-Tibetan Plateau

Nitrogen (N) deposition has been increasing for decades and has profoundly influenced the structure and function of grassland ecosystems in many regions of the world. However, the impact of N deposition on alpine grasslands is less well documented. We conducted a 3-year field experiment to determine the effects of N deposition on plant species richness, composition, and community productivity in an alpine meadow of the Qinghai-Tibetan Plateau of China. We found that 3 years of N deposition had a profound effect on these plant community parameters. Increasing N rates increased the dominance of graminoids and reduced the presence of non-graminoids. Species richness was inversely associated with aboveground biomass. The shift in plant species and functional group composition was largely responsible for the increase in productivity associated with N deposition. Climatic factors also interacted with N addition to influence productivity. Our findings suggest that short-term N deposition could increase the productivity of alpine meadows through shifts in composition toward a graminoid-dominated community. Longer-term studies are needed to determine if shifts in composition and increased productivity will be maintained. Future work must also evaluate whether decreasing plant diversity will impair the long-term stability and function of sensitive alpine grasslands.

Disentangling water sources in a gypsum plant community. Gypsum crystallization water is a key source of water for shallow-rooted plants

BACKGROUND AND AIMS

Gypsum drylands are widespread worldwide. In these arid ecosystems, the ability of different species to access different water sources during drought is a key determining factor of the composition of plant communities. Gypsum crystallization water could be a relevant source of water for shallow-rooted plants, but the segregation in the use of this source of water among plants remains unexplored. We analysed the principal water sources used by 20 species living in a gypsum hilltop, the effect of rooting depth and gypsum affinity, and the interaction of the plants with the soil beneath them.

METHODS

We characterized the water stable isotope composition, δ 2H and δ 18O, of plant xylem water and related it to the free and gypsum crystallization water extracted from different depths throughout the soil profile and the groundwater, in both spring and summer. Bayesian isotope mixing models were used to estimate the contribution of water sources to plant xylem sap.

KEY RESULTS

In spring, all species used free water from the top soil as the main source. In summer, there was segregation in water sources used by different species depending on their rooting depth, but not on their gypsum affinity. Gypsum crystallization water was the main source for most shallow-rooted species, whereas free water from 50 to 100 cm depth was the main source for deep-rooted species. We detected plant-soil interactions in spring, and indirect evidence of possible hydraulic lift by deep-rooted species in summer.

CONCLUSIONS

Plants coexisting in gypsum communities segregate their hydrological niches according to their rooting depth. Crystallization water of gypsum represents an unaccounted for, vital source for most of the shallow-rooted species growing on gypsum drylands. Thus, crystallization water helps shallow-rooted species to endure arid conditions, which eventually accounts for the maintenance of high biodiversity in these specialized ecosystems.

Lower relative abundance of ectomycorrhizal fungi under a warmer and drier climate is linked to enhanced soil organic matter decomposition

The aboveground impacts of climate change receive extensive research attention, but climate change could also alter belowground processes such as the delicate balance between free-living fungal decomposers and nutrient-scavenging mycorrhizal fungi that can inhibit decomposition through a mechanism called the Gadgil effect. We investigated how climate change-induced reductions in plant survival, photosynthesis and productivity alter soil fungal community composition in a mixed arbuscular/ectomycorrhizal (AM/EM) semiarid shrubland exposed to experimental warming (W) and/or rainfall reduction (RR). We hypothesised that increased EM host plant mortality under a warmer and drier climate might decrease ectomycorrhizal fungal (EMF) abundance, thereby favouring the proliferation and activity of fungal saprotrophs. The relative abundance of EMF sequences decreased by 57.5% under W+RR, which was accompanied by reductions in the activity of hydrolytic enzymes involved in the acquisition of organic-bound nutrients by EMF and their host plants. W+RR thereby created an enhanced potential for soil organic matter (SOM) breakdown and nitrogen mineralisation by decomposers, as revealed by 127-190% increases in dissolved organic carbon and nitrogen, respectively, and decreasing SOM content in soil. Climate aridification impacts on vegetation can cascade belowground through shifts in fungal guild structure that alter ecosystem biogeochemistry and accelerate SOM decomposition by reducing the Gadgil effect.

Stoichiometry of C:N:P in the Roots of Alhagi sparsifolia Is More Sensitive to Soil Nutrients Than Aboveground Organs

The stoichiometry of carbon, nitrogen, and phosphorus (C:N:P) among leaves, stems, and roots reflects trade-offs in plants for acquiring resources and their growth strategy. The widely distributed plant Alhagi sparsifolia is an ideal species to study the ecological stoichiometry in different organs in response to the availability of nutrients and water in the desert ecosystem. However, which response of organs is most sensitive to environmental conditions is still unclear. To answer this question, we collected samples of plants and soils including not only aboveground leaves and stems, but also underground roots and soils from a wide range of arid areas during the growing season. The C, N, P, C:N, C:P, and N:P ratios in leaves, thorns, stems, and roots were derived to explore their relationship as well as their response mechanisms to nutrients and water spanning 1 m deep in the soil. The results showed that the order of N concentration was leaves > thorns > stems > roots, that the concentration of P in the leaves, thorns, and stems was similar, and that their values were higher than those in the roots. First, the C:N ratios in the leaves and stems were significantly positively correlated with the ratio in roots. The C:N ratios in each organ showed a significant relationship with the soil alkali hydrolyzable nitrogen (SAN) above a depth of 60 cm. In addition to SAN, soil available phosphorus (SAP) and soil organic carbon (SOC) affect the C:N ratio in the roots. Second, the C:P and N:P ratios in aboveground organs showed no correlations with the ratios in roots. The C:P and N:P ratios in the leaves and thorns have no relationship with soil nutrients, while the C:P ratio in roots was influenced by SAN and SOC in all soil layers. Finally, the N:P ratios in roots were also affected by nutrients in different soil depths at 0-20 and 60-80 cm. These results illustrate that the roots were more sensitive to soil nutrients than the aboveground parts. Our study of ecological stoichiometry also suggests a novel systematic approach for analyzing the sensitivity of responses of an organ to environmental conditions.