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During extensive periods without rain, known as dry-downs, decreasing soil moisture (SM) induces plant water stress at the point when it limits evapotranspiration, defining a critical SM threshold (θcrit). Better quantification of θcrit is needed for improving future projections of climate and water resources, food production, and ecosystem vulnerability. Here, we combine systematic satellite observations of the diurnal amplitude of land surface temperature (dLST) and SM during dry-downs, corroborated by in-situ data from flux towers, to generate the observation-based global map of θcrit. We find an average global θcrit of 0.19 m3/m3, varying from 0.12 m3/m3 in arid ecosystems to 0.26 m3/m3 in humid ecosystems. θcrit simulated by Earth System Models is overestimated in dry areas and underestimated in wet areas. The global observed pattern of θcrit reflects plant adaptation to soil available water and atmospheric demand. Using explainable machine learning, we show that aridity index, leaf area and soil texture are the most influential drivers. Moreover, we show that the annual fraction of days with water stress, when SM stays below θcrit, has increased in the past four decades. Our results have important implications for understanding the inception of water stress in models and identifying SM tipping points. Using systematic satellite observations of land surface temperature and soil moisture during soil dry-downs, the spatially-explicit global distribution of the critical soil moisture threshold of plant water stress and its drivers is uncovered. [ABSTRACT FROM AUTHOR]

The vulnerability of plant xylem to embolism can be described as the water potential at which xylem conductivity is lost by 50% (P50). According to the traditional hypothesis of hydraulic vulnerability segmentation, the difference in vulnerability to embolism between branches and roots is positive (P50 root−branch > 0). It is not clear whether this occurs broadly across species or how segmentation might vary across aridity gradients. We compiled hydraulic and anatomical datasets from branches and roots across 104 woody species (including new measurements from 10 species) in four biomes to investigate the relationships between P50 root–branch and environmental factors associated with aridity. We found a positive P50 root–branch relationship across species, and evidence that P50 root–branch increases with aridity. Branch xylem hydraulic conductivity transitioned from more efficient (e.g., wider conduit, higher hydraulic conductivity) to safer (e.g., narrower conduit, more negative P50) in response to the increase of aridity, while root xylem hydraulic conductivity remained unchanged across aridity gradients. Our results demonstrate that the hydraulic vulnerability difference between branches and roots is more positive in species from arid regions, largely driven by modifications to branch traits. [ABSTRACT FROM AUTHOR]

A central role for nature-based solution is to identify optimal management practices to address environmental challenges, including carbon sequestration and biodiversity conservation. Inorganic fertilization increases plant aboveground biomass but often causes a tradeoff with plant diversity loss. It remains unclear, however, whether organic fertilization, as a potential nature-based solution, could alter this tradeoff by increasing aboveground biomass without plant diversity loss. Here we compile data from 537 experiments on organic and inorganic fertilization across grasslands and croplands worldwide to evaluate the responses of aboveground biomass, plant diversity, and soil organic carbon (SOC). Both organic and inorganic fertilization increase aboveground biomass by 56% and 42% relative to ambient, respectively. However, only inorganic fertilization decreases plant diversity, while organic fertilization increases plant diversity in grasslands with greater soil water content. Moreover, organic fertilization increases SOC in grasslands by 19% and 15% relative to ambient and inorganic fertilization, respectively. The positive effect of organic fertilization on SOC increases with increasing mean annual temperature in grasslands, a pattern not observed in croplands. Collectively, our findings highlight organic fertilization as a potential nature-based solution that can increase two ecosystem services of grasslands, forage production, and soil carbon storage, without a tradeoff in plant diversity loss. Inorganic fertilization reduces plant biodiversity. Here, the authors conduct a global meta-analysis on the use of organic and inorganic fertilizer in croplands and grasslands, showing that while both fertilizers increase plant biomass, only organic fertilizer increases biodiversity. [ABSTRACT FROM AUTHOR]

The relationship between stomatal traits and environmental drivers across plant communities has important implications for ecosystem carbon and water fluxes, but it has remained unclear. Here, we measure the stomatal morphology of 4492 species-site combinations in 340 vegetation plots across China and calculate their community-weighted values for mean, variance, skewness, and kurtosis. We demonstrate a trade-off between stomatal density and size at the community level. The community-weighted mean and variance of stomatal density are mainly associated with precipitation, while that of stomatal size is mainly associated with temperature, and the skewness and kurtosis of stomatal traits are less related to climatic and soil variables. Beyond mean climate variables, stomatal trait moments also vary with climatic seasonality and extreme conditions. Our findings extend the knowledge of stomatal trait–environment relationships to the ecosystem scale, with applications in predicting future water and carbon cycles.The relationship between stomatal traits and environmental drivers across plant communities has important implications for ecosystem fluxes. Here, the authors explore community-scale stomatal trait-environment relationships, which are important for predicting future water and carbon cycles. [ABSTRACT FROM AUTHOR]

Background: It has been long thought that nitrogen (N), phosphorus (P) concentrations and their ratios (N:P) in metabolically active or functional organs (i.e., leaves) are less responsive to environmental changes. Little attention, however, has been paid to the reproductive organs—seeds, while seeds may maintain their nutrients more stable for the evolutionary fitness of next generation. Methods: Here, we conducted a field experiment of N, P addition and drought in an alpine meadow, aiming to compare the difference of leaf and seed nutrients and stoichiometric ratios in response to these resource treatments and their interactions. Four dominant species were selected among grass and forb functional groups, including Elymus nutans, Deschampsia caespitosa, Artemisia roxburghiana and Polygonum viviparum. Results: Under natural conditions, leaf N and P concentrations were consistently lower than seed among species. However, leaf nutrients were much more sensitive than seed nutrients to N and P addition. Specifically, N or P addition accordingly increased leaf N or P concentration by 22.20–44.24% and 85.54–93.61%, while only enhanced seed N or P concentration by 5.15–17.20% and 15.17–32.72%, respectively. Leaf N or P concentration was significantly reduced by P or N addition, but seed nutrients remained unchanged. In contrast, drought did not change both organ nutrients. Similarly, nutrient addition and drought had synergistic interactions on leaf nutrients, but not on seed nutrients. Conclusions: This study highlights that seed nutrient concentrations could be more stable than metabolically active leaf organ when facing multidimensional resource changes. This complements the traditional view on the ‘Stable Leaf Nutrient Hypothesis’ with the involvement of reproductive organs. The less responsiveness of seed nutrients suggests the adaptive strategy to ensure the success of next generations and long-term plant demographic stability. [ABSTRACT FROM AUTHOR]

Quantifying the stomatal responses of plants to global change factors is crucial for modeling terrestrial carbon and water cycles. Here we synthesize worldwide experimental data to show that stomatal conductance (gs) decreases with elevated carbon dioxide (CO2), warming, decreased precipitation, and tropospheric ozone pollution, but increases with increased precipitation and nitrogen (N) deposition. These responses vary with treatment magnitude, plant attributes (ambient gs, vegetation biomes, and plant functional types), and climate. All two-factor combinations (except warming + N deposition) significantly reduce gs, and their individual effects are commonly additive but tend to be antagonistic as the effect sizes increased. We further show that rising CO2 and warming would dominate the future change of plant gs across biomes. The results of our meta-analysis provide a foundation for understanding and predicting plant gs across biomes and guiding manipulative experiment designs in a real world where global change factors do not occur in isolation. Stomatal conductance is an important plant ecophysiological trait and a common parameter in earth system models. This global meta-analysis shows how CO2, warming and other global change factors affect stomatal conductance individually and interactively. [ABSTRACT FROM AUTHOR]

Quantifying and predicting variation in gross primary productivity (GPP) is important for accurate assessment of the ecosystem carbon budget under global change. Scaling traits to community scales for predicting ecosystem functions (i.e., GPP) remain challenging, while it is promising and well appreciated with the rapid development of trait-based ecology. In this study, we aim to integrate multiple plant traits with the recently developed trait-based productivity (TBP) theory, verify it via Bayesian structural equation modeling (SEM) and complementary independent effect analysis. We further distinguish the relative importance of different traits in explaining the variation in GPP. We apply the TBP theory based on plant community traits to a multi-trait dataset containing more than 13,000 measurements of approximately 2,500 species in Chinese forest and grassland systems. Remarkably, our SEM accurately predicts variation in annual and monthly GPP across China (R2 values of 0.87 and 0.73, respectively). Plant community traits play a key role. This study shows that integrating multiple plant functional traits into the TBP theory strengthens the quantification of ecosystem primary productivity variability and further advances understanding of the trait-productivity relationship. Our findings facilitate integration of the growing plant trait data into future ecological models. Integration of plant functional traits into trait-based productivity theory helps quantify primary productivity in terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

Aims: Global environmental changes are known to affect terrestrial ecosystems functions (i.e., plant growth and carbon storage). However, how plants and soil microorganisms respond to warming and nitrogen deposition in dry ecosystems with strong seasonal precipitation remains largely unexplored. Methods: Based on a 13-year manipulative field experiment, we investigated the effects of warming and nitrogen addition on soil microbial communities and plant net photosynthetic rates during dry and wet months in a desert steppe in Inner Mongolia. Results: We found that in the wet month, warming and nitrogen addition significantly increased soil microbial biomass. Yet, warming and nitrogen addition significantly increased soil inorganic nitrogen and leaf nitrogen concentration, thus promoting an increase in the net photosynthetic rate of Stipa breviflora. Moreover, warming and nitrogen addition significantly shifted soil microbial composition with an increase in soil bacterial phospholipid fatty acids (PLFAs) but a reduction in fungal PLFAs. The increased soil inorganic nitrogen indirectly enhanced leaf nitrogen and plant photosynthesis by changing soil microbial community structure. These changes were not significant in the dry month. Conclusions: Our study indicates that warming and nitrogen addition can promote plant photosynthesis by increasing soil nitrogen availability and changing soil microbial community structure. These changes only occurred when there was sufficient precipitation. These results highlight the crucial role of the soil microbial community and precipitation availability in influencing plant growth responses to global change drivers such as warming and nitrogen deposition. These findings suggest the importance of manipulating multiple factors rather than single factors in global change experiments and the role of plant and soil microbial interactions in ecosystem functions (i.e., plant growth and carbon storage) under global change. [ABSTRACT FROM AUTHOR]

Warming of northern high latitude regions (NHL, > 50 °N) has increased both photosynthesis and respiration which results in considerable uncertainty regarding the net carbon dioxide (CO2) balance of NHL ecosystems. Using estimates constrained from atmospheric observations from 1980 to 2017, we find that the increasing trends of net CO2 uptake in the early-growing season are of similar magnitude across the tree cover gradient in the NHL. However, the trend of respiratory CO2 loss during late-growing season increases significantly with increasing tree cover, offsetting a larger fraction of photosynthetic CO2 uptake, and thus resulting in a slower rate of increasing annual net CO2 uptake in areas with higher tree cover, especially in central and southern boreal forest regions. The magnitude of this seasonal compensation effect explains the difference in net CO2 uptake trends along the NHL vegetation- permafrost gradient. Such seasonal compensation dynamics are not captured by dynamic global vegetation models, which simulate weaker respiration control on carbon exchange during the late-growing season, and thus calls into question projections of increasing net CO2 uptake as high latitude ecosystems respond to warming climate conditions. The northern high latitude permafrost region has been an important contributor to the carbon sink since the 1980s. A new study finds that as tree cover increases, respiratory CO2 loss during late-growing season offsets photosynthetic CO2 uptake and leads to a slower rate of increasing annual net CO2 uptake. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This document is a correction notice for an article titled "Relationships of stomatal morphology to the environment across plant communities" published in Nature Communications. The correction addresses an error in one of the references cited in the original article. The correct reference is provided, and the correction has been made in both the PDF and HTML versions of the article. The authors of the original article are Congcong Liu, Lawren Sack, Ying Li, Jiahui Zhang, Kailiang Yu, Qiongyu Zhang, Nianpeng He, and Guirui Yu. [Extracted from the article]

Recent observational evidence suggests that nighttime temperatures are increasing faster than daytime temperatures, while in some regions precipitation events are becoming less frequent and more intense. The combined ecological impacts of these climatic changes on crassulacean acid metabolism (CAM) plants and their interactions with other functional groups (i.e., grass communities) remain poorly understood. Here we developed a growth chamber experiment to investigate how two CAM–grass communities in desert ecosystems of the southwestern United States and northern Mexico respond to asymmetric warming and increasing rainfall variability. Grasses generally showed competitive advantages over CAM plants with increasing rainfall variability under ambient temperature conditions. In contrast, asymmetric warming caused mortality of both grass species (Bouteloua eriopoda and Bouteloua curtipendula) in both rainfall treatments due to enhanced drought stress. Grass mortality indirectly favored CAM plants even though the biomass of both CAM species Cylindropuntia imbricata and Opuntia phaeacantha significantly decreased. The stem's volume-to-surface ratio of C. imbricata was significantly higher in mixture than in monoculture under ambient temperature (both P < 0.0014); however, the difference became insignificant under asymmetric warming (both P > 0.1625), suggesting that warming weakens the negative effects of interspecific competition on CAM plant growth. Our findings suggest that while the increase in intra-annual rainfall variability enhances grass productivity, asymmetric warming may lead to grass mortality, thereby indirectly favoring the expansion of co-existing CAM plants. This study provides novel experimental evidence showing how the ongoing changes in global warming and rainfall variability affect CAM–grass growth and interactions in dryland ecosystems. [ABSTRACT FROM AUTHOR]

Urban heat islands (UHIs) exacerbate the risk of heat-related mortality associated with global climate change. The intensity of UHIs varies with population size and mean annual precipitation, but a unifying explanation for this variation is lacking, and there are no geographically targeted guidelines for heat mitigation. Here we analyse summertime differences between urban and rural surface temperatures (ΔTs) worldwide and find a nonlinear increase in ΔTs with precipitation that is controlled by water or energy limitations on evapotranspiration and that modulates the scaling of ΔTs with city size. We introduce a coarse-grained model that links population, background climate, and UHI intensity, and show that urban–rural differences in evapotranspiration and convection efficiency are the main determinants of warming. The direct implication of these nonlinearities is that mitigation strategies aimed at increasing green cover and albedo are more efficient in dry regions, whereas the challenge of cooling tropical cities will require innovative solutions. The effect of cities on urban climate (often warmer but sometimes cooler than their surroundings) is largely explained by local hydroclimate and patterns of city development. [ABSTRACT FROM AUTHOR]

Background and aims: The intensity of extreme rainfall events is increasing in many regions of the world, with important impacts on community dynamics and ecosystem functioning especially in water-limited ecosystems. The impact of the intensification of extreme precipitation on mixed communities of CAM plants and dryland C4 grasses remains poorly understood. Methods: Through a set of greenhouse experiments we investigated the effect of increasing intraseasonal rainfall variability on two separate pairs of CAM and C4 grass species (C. imbricata and B. eriopoda; O. phaeacantha and B. curtipendula) that coexist in arid grasslands across the southwest USA. Results: The increased rainfall variability did not significantly change the biomass of C. imbricata while increasing the biomass of B. eriopoda when these two species coexisted. More extreme rainfall regimes caused a 24.8% decrease in B. curtipendula (grass) biomass, compared with a 71.3% decline in the O. phaeacantha CAM plant. Significantly lower nocturnal carbon assimilation rates and higher stem volume-to-surface area ratios were found in CAM plants in mixture than in monoculture. Conclusions: Our study suggests that C4 grasses may outcompete CAM plants through greater access to deep soil water under increased rainfall variability and highlights the important role of hydrologic conditions as determinants of the competitive relations between CAM plants and grasses, their community composition, and ecological resilience in dryland ecosystems. [ABSTRACT FROM AUTHOR]

Predicting hydroclimatic changes on the Tibetan Plateau (TP) is crucial for managing water and ecosystems for the well-being of millions of people. Our understanding of the synoptic conditions on the TP is, however, still limited due to the paucity of meteorological measurements and proxy-based, high-resolution climate reconstructions. Here, we use state-of-the-art dendroclimatological techniques to investigate the paleoclimatic potential of drought-sensitive Picea likiangensis var. balfouriana forests between 4000 and 4500 m asl on the southeastern TP (SETP). The newly developed tree-ring width chronology correlates significantly with yearly changes in regional relative air humidity (RH) (r = 0.85, P < 0.001, 1978–2011). A new 407-year-long reconstruction of RH over the hydrological year from previous year August to July of the year of ring formation shows that, despite the generally humid conditions, four of the ten driest years are observed in the twentieth century with 1983 having been the driest. On the other hand, seven out of the ten most humid years were found in the eighteenth century. Our reconstruction reveals that the Pacific Decadal Oscillation (PDO) is the dominant climate driver at multi-decadal scales, but the relationships are not stable over time, with unknown underlying mechanisms. Although our study demonstrates the importance of the PDO for hydroclimate projections on the TP, caution is advised when considering only its most recent fluctuations. [ABSTRACT FROM AUTHOR]

Aims: Global climate change may greatly alter the structure and stability of drylands, creating an urgent need to recover their functions and services. Biological soil crust (biocrust), an interface between the soil and atmosphere, plays a crucial role in ecohydrological processes, and thus in influencing the restoration dynamics of dryland ecosystems. Previous studies have generally investigated the influences of biocrust on ecohydrological processes as an exogenous factor. However, it remains unclear how biocrusts, as an integral part of many ecosystems (i.e., as a system state variable), will change under global climate change.Methods: This study developed a new ecohydrological model with biocrust cover as a system state variable, and explored the response of dryland ecosystems to altered rainfall regimes.Results: Biocrust cover responded with an inverted U-shaped curve relationship to increasing annual rainfall and linearly to increasing rainfall frequency. Vascular plant (grass and shrub) cover showed an increasing trend with increasing annual rainfall and a decreasing trend to increasing rainfall frequency. Therefore, biocrust usually dominated over vascular plants (i.e., high biocrust cover and low vascular plant cover) under low annual rainfall. Furthermore, an increasing rainfall frequency would amplify the range of environmental (rainfall) conditions dominated by biocrust from an annual rainfall 0-100 mm under a rainfall frequency of 0.025 day−1 to 0-500 mm under a rainfall frequency of 1 day−1.Conclusions: This study developed a model framework to predict dryland dynamics for surfaces covered by biocrust under global climate change. We suggest that restoration efforts could target at biocrust-dominated state in deserts, especially in a (future) drier climate. [ABSTRACT FROM AUTHOR]

Plants influence the atmosphere through fluxes of carbon, water and energy1, and can intensify drought through land-atmosphere feedback effects2-4. The diversity of plant functional traits in forests, especially physiological traits related to water (hydraulic) transport, may have a critical role in land-atmosphere feedback, particularly during drought. Here we combine 352 site-years of eddy covariance measurements from 40 forest sites, remote-sensing observations of plant water content and plant functional-trait data to test whether the diversity in plant traits affects the response of the ecosystem to drought. We find evidence that higher hydraulic diversity buffers variation in ecosystem flux during dry periods across temperate and boreal forests. Hydraulic traits were the predominant significant predictors of cross-site patterns in drought response. By contrast, standard leaf and wood traits, such as specific leaf area and wood density, had little explanatory power. Our results demonstrate that diversity in the hydraulic traits of trees mediates ecosystem resilience to drought and is likely to have an important role in future ecosystem-atmosphere feedback effects in a changing climate. The diversity in the hydraulic traits of trees mediates ecosystem resilience to drought and will probably have an important role in future ecosystem-atmosphere feedback effects. [ABSTRACT FROM AUTHOR]

Background and aims: We investigated the response of a model facultative CAM plant (Mesembryanthemum crystallinum) and its competition with a C3 grass (Bromus mollis) to changes in rainfall regime.Methods: Seedlings of M. crystallinum and B. mollis in both monoculture and mixtures growing in shallow and deep pots were subjected to three levels of intra-seasonal rainfall variability and rainfall seasonality in both high water and low water conditions. Response of plants were evaluated by nocturnal carboxylation and biomass.Results: A high rate of water drainage beneath root zones in coarse soil led to a negative response of M. crystallinum and B. mollis in monoculture under increased intra-seasonal rainfall variability. Seasonal rainfall shifts to later dates during the growing season generally favored the growth of M. crystallinum and B. mollis in monoculture, with the exception of high water stress conditions whereby drought-intolerant species B. mollis was disfavored. Rainfall seasonality but not intra-seasonal rainfall variability affected nocturnal carboxylation by M. crystallinum in monoculture.Conclusions: We suggest that soil texture, root depth, and rainfall gradient are important mediators of plant growth under increased intra-seasonal rainfall variability. Drought severity and the ability of a plant to tolerate drought and can greatly affect its response to the seasonal timing of rainfall. Nocturnal carboxylation by M. crystallinum in response to rainfall variability depends on the timescale. [ABSTRACT FROM AUTHOR]

Savannas are characterized by the coexistence of trees and flammable grasses. Yet, tree-grass coexistence has been labeled as paradoxical-how do these two functional groups coexist over such an extensive area, despite being generally predisposed to excluding each other? For instance, many trees develop dense canopies that limit grass growth, and many grasses facilitate frequent/intense fires, increasing tree mortality. This study revisits tree-grass coexistence with a model of hierarchical competition between pyrogenic grasses, 'forest trees' adapted to closed-canopy competition, and 'savanna trees' that are inferior competitors in closed-canopy communities, but more resistant to fire. The assumptions of this model are supported by empirical observations, including a systematic review of savanna and forest tree community composition reported here. In general, the model simulations show that when savanna trees exert weaker competitive effects on grasses, a self-reinforcing grass community is maintained, which limits forest tree expansion while still allowing savanna trees to persist (albeit as a subdominant to grasses). When savanna trees exert strong competitive effects on grasses, savanna trees cover increases initially, but as grasses decline their inhibitory effect on forest trees weakens, allowing forest trees to expand and exclude grasses and savanna trees. Rather than paradoxical, these results suggest that having weaker competitive effects on grasses may be advantageous for savanna trees, leading to greater long-term abundance and stability. We label this the 'enemy of my enemy hypothesis,' which might apply to species coexistence in communities defined by hierarchical competition or with species capable of generating strong ecological feedbacks. [ABSTRACT FROM AUTHOR]

Precipitation variability has been predicted to increase in a global warmer climate, and is expected to greatly affect plant growth, interspecies interactions, plant community composition, and other ecosystem processes. Although previous studies have investigated the effect of intra-annual rainfall variability on plant growth and ecosystem dynamics, the impacts of interannual rainfall variability remain understudied. This paper uses satellite data and develops a new mechanistic model to investigate the response of tree-grass composition to increasing interannual rainfall variability in arid to sub-humid ecosystems along the Kalahari Transect in Southern Africa. Both satellite data and model results show that increasing interannual rainfall fluctuations favor deep-rooted trees over shallow-rooted grasses in drier environments (that is, mean annual rainfall, MAP < 900-1000 mm) but favor grasses over trees in wetter environments (that is, MAP > 900-1000 mm). Trees have a competitive advantage over grasses in dry environments because their generally deeper root systems allow them to have exclusive access to the increased deep soil water resources expected to occur in wet years as a result of the stronger interannual rainfall fluctuations. In relatively wet environments, grasses are favored because of their high growth rate that allows them to take advantage of the window of opportunity existing in years with above average precipitation and thus increase fire-induced tree mortality. Thus, under increasing interannual rainfall fluctuations both direct effects on soil water availability and indirect effects mediated by tree-grass interactions and fire dynamics are expected to play an important role in determining changes in plant community composition. [ABSTRACT FROM AUTHOR]

Abiotic drivers of environmental stress have been found to induce CAM expression (nocturnal carboxylation) in facultative CAM species such as Mesembryanthemum crystallinum. The role played by biotic factors such as competition with non-CAM species in affecting CAM expression, however, remains largely understudied. This research investigated the effects of salt and water conditions on the competition between M. crystallinum and the C grass Bromus mollis with which it is found to coexist in California's coastal grasslands. We also investigated the extent to which CAM expression in M. crystallinum was affected by the intensity of the competition with B. mollis. We found that M. crystallinum had a competitive advantage over B. mollis in drought and saline conditions, while B. mollis exerted strong competitive effects on M. crystallinum in access to light and soil nutrients in high water conditions. This strong competitive effect even outweighed the favorable effects of salt or water additions in increasing the biomass and productivity of M. crystallinum in mixture. Regardless of salt conditions, M. crystallinum did not switch to CAM photosynthesis in response to this strong competitive effect from B. mollis. Disturbance (i.e., grass cutting) reduced the competitive pressure by B. mollis and allowed for CAM expression in M. crystallinum when it was grown mixed with B. mollis. We suggest that moderate competition with other functional groups can enhance CAM expression in M. crystallinum, thereby affecting its plasticity and ability to cope with biological stress. [ABSTRACT FROM AUTHOR]

Background and aims: Determining which strategies confer a competitive advantage in variable environments with strong short-term resource fluctuations (i.e., seasonal drought and flooding) is critical for understanding ecosystem structure and function, and predicting how ecosystems will respond to global change. Here, we ask how functional traits of plant species vary in response to drought and flooding and whether trait plasticity is a better predictor of plants' cover than average functional trait values. Methods: We transplanted vegetative recruits (similar size and age) of eight common species from a moderately grazed alpine wetland in the Eastern Tibetan Plateau to a common garden where we evaluated the trait responses of these species to drought and flooding. We also measured the coverage, biomass, and height of these plant species in the field communities. Results: The leaf morphological and physiological traits of these eight species generally showed different responses to drought and flood conditions. Variation (plasticity) of leaf morphological or physiological traits rather than average values of these traits were better predictors of cover and biomass in the field community. Species with lower plasticity in leaf morphological traits had higher plasticity in leaf physiology and the higher coverage and biomass in the field community. Conclusions: Our results suggest that the ability to quickly adjust leaf physiology to changing environmental conditions provides an advantage in this ecosystem, where intra-annual rainfall variability is high and thus resource fluctuations are pulsed in time. In contrast, greater functional trait plasticity in leaf morphology was a disadvantage, possibly because the benefits of these adjustments did not outweigh their initial costs. [ABSTRACT FROM AUTHOR]

Past studies have largely focused on hydraulic redistribution (HR) in trees, shrubs, and grasses, and recognized its role in interspecies interactions. HR in plants that conduct crassulacean acid metabolism (CAM), however, remains poorly investigated, as does the effect of HR on transpiration in different vegetation associations (i.e., tree-grass, CAM-grass, and tree-CAM associations). We have developed a mechanistic model to investigate the net direction and magnitude of HR at the patch scale for tree-grass, CAM-grass, and tree-CAM associations at the growing season to yearly timescale. The modeling results show that deep-rooted CAM plants in CAM-grass associations could perform hydraulic lift at a higher rate than trees in tree-grass associations in a relatively wet environment, as explained by a significant increase in grass transpiration rate in the shallow soil layer, balancing a lower transpiration rate by CAM plants. By comparison, trees in tree-CAM associations may perform hydraulic descent at a higher rate than those in tree-grass associations in a dry environment. Model simulations also show that hydraulic lift increases the transpiration of shallow-rooted plants, while hydraulic descent increases that of deep-rooted plants. CAM plants transpire during the night and thus perform HR during the day. Based on these model simulations, we suggest that the ability of CAM plants to perform HR at a higher rate may have different effects on the surrounding plant community than those of plants with C or C photosynthetic pathways (i.e., diurnal transpiration). [ABSTRACT FROM AUTHOR]

Quantifying soil P fractions is essential for understanding soil P cycling because these fractions are potential sources of bioavailable P. Here, we investigated whether soil P fractions (i.e., labile inorganic P [P], intermediately available P, organic P, occluded P, and apatite P) differed among three tropical forests at different stages of succession (early, middle, and late) in the Dinghushan Biosphere Reserve (DBR), Southern China. We also determined which soil P fractions was closely related to soil microbial and chemical properties in these forests. Soil microbial biomass and chemical properties (except pH, exchangeable Ca, and Mn) were higher in the late successional forest than those in the other two forests. We found that soil organic and occluded P were the dominant fraction in all three forests and together accounted for 78.9, 84.2, and 78.6 % of total P in the early, middle, and late successional forests, respectively. Soil P fractions and acid phosphomonoesterase activity differed significantly among forest successional stages; intermediately available P and organic P (potential sources of labile P) were highest in the late successional forest, occluded P was highest in the middle successional forest, and acid phosphomonoesterase activity significantly increased in middle and late successional forest. Soil labile P fraction and its potential sources (i.e., intermediately available P and soil organic P) were both positively correlated with soil microbial and chemical properties, suggesting that these properties may play critical roles in maintaining high levels of available or potentially available P fractions during this forest succession. Overall, our study indicates that succession in tropical forest ecosystems may not always lead to lower soil P availability. [ABSTRACT FROM AUTHOR]

Investment in agricultural land in the developing world has rapidly increased in the past two decades. In Cambodia, there has been a surge in economic land concessions, in which long-term leases are provided to foreign and domestic investors for economic development. More than two million hectares have been leased so far, sparking debate over the consequences for local communities and the environment. Here we combined official records of concession locations with a high-resolution data set of changes in forest cover to quantify the contribution of land concessions to deforestation between 2000 and 2012. We used covariate matching to control for variables other than classification as a concession that may influence forest loss. Nearly half of the area where concessions were granted between 2000 and 2012 was forested in 2000; this area then represented 12.4% of forest land cover in Cambodia. Within concessions, the annual rate of forest loss was between 29% and 105% higher than in comparable land areas outside concessions. Most of the deforestation within concessions occurred after the contract date, and whether an investor was domestic or foreign had no effect on deforestation rates. We conclude that land acquisitions can act as powerful drivers of deforestation. [ABSTRACT FROM AUTHOR]

The relationship between species richness and productivity has been a central issue in community ecology, and this issue has resulted in much debate in the ecological literature. To evaluate whether species richness is consistently associated with productivity and the underlying mechanisms, a potted experiment with various combinations of three perennial plant species ( Elymus nutans, Roegneria nutans and Festuca sinensis) was conducted under three fertilization levels over three years with interannual variation in rainfall in a subalpine meadow on the Tibetan plateau, China. The additive partitioning method was used for measuring net, selection and complementarity effects. The results suggest that species richness, composition and density had significant effects on aboveground biomass, but their effects were much less than abiotic factors (fertility and year). Relative yield total (RYT), net, selection and complementarity effects were not consistently positive or negative across years in the mixed communities. Both the interaction of year and species richness and the interaction of year and composition had significant effects on aboveground biomass, selection and complementarity effects. Fertilization and density had no significant effects on net, selection and complementarity effects. Our results indicate that plant richness is not consistently associated with productivity in these experimental plant communities, considering that selection and complementarity effects can vary with complex environmental conditions, and that these factors influence plant productivity. We suggest that different forms of relationships between species richness and productivity may be exhibited based on biotic or abiotic factors in plant communities. [ABSTRACT FROM AUTHOR]

Plants with crassulacean acid metabolism (CAM) are increasing their cover in many dryland regions around the world. Their increased dominance has been related to climate warming and atmospheric CO fertilization, while the effects of interspecies interactions and the role of CAM plant facilitation by trees and grasses remain poorly understood. Woody plants are known for their ability to directly facilitate CAM plants through amelioration of the abiotic environment. Mechanisms of indirect facilitation of trees on CAM plants in tree-grass-CAM associations, however, have received less attention. It is also unclear whether grasses might facilitate CAM plants in mixed tree-grass-CAM communities. For instance, the inclusion of grasses in tree-CAM associations could enhance hydraulic lift and facilitate CAM plants in their access to shallow soil moisture at the expenses of deep-rooted trees. If this effect outweighs the competitive effects of grasses on CAM plants, grasses could overall facilitate CAM plants through hydraulic lift. Here we develop a process-based ecohydrological model to investigate the direct and indirect facilitation in tree-CAM-grass associations; the model quantifies transpiration of CAM plants when isolated as well as in associations with trees and/or grasses. It is found that woody plants having a high root overlap with CAM plants indirectly facilitate CAM plants by significantly reducing grass transpiration in shaded conditions. For situations of a low-to-moderate root overlap, facilitation may occur both directly and indirectly. Conversely, grasses are unable to indirectly facilitate CAM plants through the mechanism of hydraulic lift because the competitive effects of grasses on CAM plants outweigh the facilitation induced by hydraulic lift. [ABSTRACT FROM AUTHOR]