Aridity Gradient Research Articles

Critical thresholds for nonlinear responses of ecosystem water use efficiency to drought

Climate change is expected to lead to greater variability in precipitation and drought in different regions. However, the responses of ecosystem carbon and water cycles (i.e., water use efficiency, WUE) to different levels of drought stress are not fully understood. Here, we examined the relationship between WUE and precipitation anomalies and identified the critical drought threshold (DrCW) above which WUE showed substantial decrease. The results revealed that 85.56 % of the study area had nonlinear WUE responses to drought stress; that is, the WUE decreased sustainably and steeply when the precipitation deficit exceeded the DrCW. DrCW indicates inflection points for changing ecosystem responses from relatively resistant to vulnerable to drought stress, thus providing an instructive early warning for intensifying suppressive impacts on vegetation growth. Additionally, DrCW varies across aridity gradients and among vegetation types. Based on the DrCW at the pixel level, the future eco-drought is projected to increase in >67 % of the study area under both the SSP2&RCP4.5 and SSP5&RCP8.5 scenarios by the end of the 21st century. Our study elucidates the response of the ecosystem function to drought and supports the development of accurate ecosystem adaptation policies for future drought stress.

Intra‐specific variation and climate differentially shape the thermal germination niches of three co‐occurring woodland forbs

AbstractUnderstanding plant responses to temperature is critical for predicting their vulnerability to global warming and for planning management responses. Germination is a key life‐stage, strongly regulated by temperature, that affects the potential for plant populations to persist. Here, we compared the thermal germination niches of three unrelated, declining woodland forb species – Arthropodium fimbriatum (Asparagaceae), Bulbine bulbosa (Asphodelaceae), Microseris walteri (Asteraceae) – across common temperature and precipitation gradients, to characterize the relationships with home‐site climate, and associated implications for ecological restoration in a changing climate. Open‐pollinated seed were sampled from 14–15 populations per species across an aridity gradient in south‐eastern Australia. Germination responses for each population were tested in controlled temperature cabinets under five temperature regimes encompassing contemporary and projected future temperatures. Optimum germination temperature and thermal germination niche were characterized and assessed for associations with home‐site climate and potential germination under projected climate change. The three species showed significant intra‐specific variation in the thermal germination niche. Optimum germination temperature was correlated with home‐site climate, suggesting adaptive variation among populations in germination requirements. However, the pattern of variation in optimum germination across the environmental gradients was not always consistent among the species. Future temperatures projected under climate change tended to be outside the current thermal germination niche for all species, indicating potential benefits of incorporating pre‐adapted populations in ecological restoration. Climate‐related intra‐specific variation in the thermal germination niche provides evidence for local adaptation to climate in all three forb species and suggests potential differences among populations in vulnerability to global warming. These results emphasize the importance of understanding the extent of intra‐specific variation in key life history traits to better manage and conserve populations and restore their ecosystems as climates change.

Self-organization as a mechanism of resilience in dryland ecosystems

Self-organized spatial patterns are a common feature of complex systems, ranging from microbial communities to mussel beds and drylands. While the theoretical implications of these patterns for ecosystem-level processes, such as functioning and resilience, have been extensively studied, empirical evidence remains scarce. To address this gap, we analyzed global drylands along an aridity gradient using remote sensing, field data, and modeling. We found that the spatial structure of the vegetation strengthens as aridity increases, which is associated with the maintenance of a high level of soil multifunctionality, even as aridity levels rise up to a certain threshold. The combination of these results with those of two individual-based models indicate that self-organized vegetation patterns not only form in response to stressful environmental conditions but also provide drylands with the ability to adapt to changing conditions while maintaining their functioning, an adaptive capacity which is lost in degraded ecosystems. Self-organization thereby plays a vital role in enhancing the resilience of drylands. Overall, our findings contribute to a deeper understanding of the relationship between spatial vegetation patterns and dryland resilience. They also represent a significant step forward in the development of indicators for ecosystem resilience, which are critical tools for managing and preserving these valuable ecosystems in a warmer and more arid world.

Plant canopies promote climatic disequilibrium in Mediterranean recruit communities.

Current rates of climate change are exceeding the capacity of many plant species to track climate, thus leading communities to be in disequilibrium with climatic conditions. Plant canopies can contribute to this disequilibrium by buffering macro-climatic conditions and sheltering poorly adapted species to the oncoming climate, particularly in their recruitment stages. Here we analyse differences in climatic disequilibrium between understorey and open ground woody plant recruits in 28 localities, covering more than 100,000 m2 , across an elevation range embedding temperature and aridity gradients in the southern Iberian Peninsula. This study demonstrates higher climatic disequilibrium under canopies compared with open ground, supporting that plant canopies would affect future community climatic lags by allowing the recruitment of less arid-adapted species in warm and dry conditions, but also it endorse that canopies could favour warm-adapted species in extremely cold environments as mountain tops, thus pre-adapting communities living in these habitats to climate change.

Drought may exacerbate dryland soil inorganic carbon loss under warming climate conditions

Low moisture conditions result in substantially more soil inorganic carbon (SIC) than soil organic carbon (SOC) in drylands. However, whether and how changes in moisture affect the temperature response of SIC in drylands are poorly understood. Here, we report that the temperature sensitivity of SIC dissolution increases but that of SOC decomposition decreases with increasing natural aridity from 30 dryland sites along a 4,500 km aridity gradient in northern China. To directly test the effects of moisture changes alone, a soil moisture control experiment also revealed opposite moisture effects on the temperature sensitivities of SIC and SOC. Moreover, we found that the temperature sensitivity of SIC was primarily regulated by pH and base cations, whereas that of SOC was mainly regulated by physicochemical protection along the aridity gradient. Given the overall increases in aridity in a warming world, our findings highlight that drought may exacerbate dryland soil carbon loss from SIC under warming.

Stress Induces Trait Variability across Multiple Spatial Scales in the Arid Annual Plant Anastatica hierochuntica.

Variations in plant characteristics in response to habitat heterogeneity can provide valuable insights into the mechanisms governing plant responses to environmental conditions. In this study, we investigated the role of environmental factors associated with arid conditions in shaping the phenotypic responses of an arid annual plant, Anastatica hierochuntica, across several populations found along an aridity gradient and across multiple spatial scales. Utilizing both field surveys and a net house experiment, we assessed the effects of environmental factors on trait variability within and between populations. The results indicated a significant convergence in plant height due to site aridity, reflecting growth potential based on abiotic resources. Convergence was also observed in the plant's electrolyte leakage with aridity and in plant height concerning soil salinity at specific sites. Phenotypic plasticity was pivotal in maintaining trait variability, with plant height plasticity increasing with soil salinity, SLA plasticity decreasing with aridity, and leaf number plasticity rising with aridity. In conclusion, our findings underscore the adaptive significance of phenotypic variability, especially plasticity, in arid conditions. Notably, trait variability and plasticity did not consistently diminish in stressful settings, emphasizing the adaptive value of flexible responses in such environments.

Contrasting responses of relationship between solar-induced fluorescence and gross primary production to drought across aridity gradients

Satellite solar-induced chlorophyll fluorescence (SIF) can provide valuable insights on terrestrial gross primary production (GPP) and its response to climate from region to global scales. However, the relationship between SIF and GPP under different climate conditions (such as drought) is not yet fully understood and remains a topic of debate in the literature. Here we examined the responses of the SIF–GPP relationship (i.e., the slope k of the linear SIF–GPP relationship, GPP = k × SIF) to drought along the aridity gradient using seven satellite SIF datasets, three GPP products, and 62 flux towers. The results revealed divergent SIF–GPP relationships under different aridity and drought conditions based on different SIF and GPP datasets. Overall, we found that with increasing aridity index (AI) (more humid), k derived from SIF with afternoon overpass (dailySIFPM) decreased, while k from SIF with morning overpass (dailySIFAM) increased. When there is a drought, for arid and semi-arid regions with AI from 0 to 0.4, kPM (the slope of dailySIFPM–GPP) increased, but kAM (the slope of dailySIFAM–GPP) decreased. The dailySIFPM also showed larger decreases than dailySIFAM and GPP during the drought. The contrasting responses among two groups of SIF may result from different SIF overpass time which detected different levels of water stresses on photosynthesis. The dryland ecosystems may exhibit strong midday and afternoon depression in photosynthesis under stress, which led to underestimation of daily SIF that was converted from midday/afternoon-observed SIF by assuming ideal symmetrical diurnal cycle of solar radiation. Our study highlights that in addition to multiple previously reported factors, different observation time of SIF may also affect the SIF–GPP relationship.

Grazing exclusion jeopardizes plant biodiversity effect but enhances dryness effect on multifunctionality in arid grasslands

Grassland biodiversity is vital for the provision of multiple ecosystem functions, termed ecosystem multifunctionality. As an effective practice of grassland management, grazing exclusion is widely used to restore the ecosystem multifunctionality of degraded grasslands, but it might not be always beneficial for conserving grassland biodiversity. Moreover, when grazing is excluded, it remains unknown whether grassland biodiversity promotes multifunctionality. Here, we conducted a field experiment of grazing exclusion along a gradient of climatic aridity across four major grassland types in the arid grasslands of northern China. We determined the effects of grazing exclusion on biodiversity and multifunctionality, and further investigated how grazing exclusion could modify biodiversity-multifunctionality relationships. We found that grazing exclusion increased soil fungal diversity, but had negligible effects on plant species richness and soil bacterial diversity. The effects of grazing exclusion on multifunctionality varied among grassland types: it promoted multifunctionality only in less arid grasslands. Importantly, multifunctionality was positively associated with plant species richness under grazing, but this positive association disappeared under grazing exclusion. In addition, we showed that climatic aridity affected multifunctionality indirectly through its negative effect on plant species richness under grazing, whereas such effect disappeared under grazing exclusion. Furthermore, aridity weakly reduced multifunctionality under grazing, and its negative effect was strengthened under grazing exclusion. Our results suggest that grazing exclusion restrains the capacity of plant biodiversity to sustain multiple ecosystem functions, and moreover, aggravates the negative influences of climatic dryness on ecosystem multifunctionality. Our findings illustrate the value of grazing for maintaining biodiversity-multifunctionality relationships and for mitigating the impacts of climate change in arid grasslands.

Spatial patterns of congruence or mismatch between taxonomic, functional, and phylogenetic diversity and endemism of perennial flora along the aridity gradient of Chile.

Understanding the relationships between taxonomic, functional, and phylogenetic diversity and endemism across environmental gradients is essential for elucidating the eco-evolutionary mechanisms that shape local plant communities. A database was compiled from field surveys, national herbarium records, and virtual records of perennial plant specimens collected in the aridity gradient of northern Chile, between 18 and 32°S. A large-scale dated phylogeny of available perennial plants was used, and 11 functional traits were selected to construct a dendrogram using the Unweighted Pair-Group Method with Arithmetic Mean (UPGMA) method for the species present in our database. We calculated spatial patterns of a-diversity, including taxonomic (TD), functional (FD), and phylogenetic (PD) diversity, as well as weighted (WE), functional (FE), and phylogenetic (PE) endemism. We used multiscale geographically weighted regression (MGWR) to identify spatial congruencies and discrepancies among these dimensions and to test different eco-evolutionary processes. The diversity indices TD, FD and PD showed similar geographic patterns (R2 > 0.93), with lower diversity observed in absolute desert regions. The pattern of weighted endemism (WE) showed a weak association with functional endemism (FE) and phylogenetic endemism (PE) (local R2 < 0.48). The regions with lower FD or PD than expected given the TD (i.e. FD<TD and PD<TD) are mainly located in desert areas, as well as in high Andean areas influenced by the Atacama Desert, suggesting communities with associated in situ speciation processes, as well as a limitation of morpho-functional trait diversity in response to extreme environmental conditions (environmental filter hypothesis). Similarly, where FE and PE values are higher than expected given the WE (i.e. FE>WE and PE>WE), they are found in arid, high Andean and transitional zones, at different altitudes, which would indicate a greater presence of phylogenetic lineages and species with morpho-functional traits related to extreme environmental conditions and transitional biomes (arid-semiarid). These spatial discrepancies suggest different eco-evolutionary drivers between the dimensions of diversity and endemism (taxonomic, functional, and phylogenetic). Areas of high diversity and high endemism do not necessarily coincide, and both should be addressed by conservation efforts.

Increased diurnal temperature range in global drylands in more recent decades

AbstractThe diurnal temperature range (DTR) has generally decreased at global scale according to the IPCC reports and previous publications. Here, we examined the change in DTR within the past four decades and found that DTR decreased in wet areas, but it increased in dry areas. This is because the daily minimum air temperature (Tmin) increased at a slower rate than the daily maximum air temperature (Tmax) in the dry areas, while it increased more rapidly in the wet areas. The changes in cloud cover, water vapour and moisture flux were observed to have contributed to the observed change in DTR. The change in moisture flux largely contributed to this change through the indirect effect on the change in water vapour across the aridity gradient. The significant impact of moisture flux is linked to the differential change in moisture flux between the dry and wet zones.

Multiscale phenology of seasonally dry tropical forests in an aridity gradient

The leaf phenology of seasonally dry tropical forests (SDTFs) is highly seasonal, marked by synchronized flushing of new leaves triggered by the first rains of the wet season. Such phenological transitions may not be accurately detected by remote sensing vegetation indices and derived transition dates (TDs) due to the coarse spatial and temporal resolutions of satellite data. The aim of this study was to compared TDs from PhenoCams and satellite remote sensing (RS) and used the TDs calculated from PhenoCams to select the best thresholds for RS time series and calculate TDs. For this purpose, we assembled cameras in seven sites along an aridity gradient in the Brazilian Caatinga, a region dominated by SDTFs. The leafing patterns were registered during one to three growing seasons from 2017 to 2020. We drew a region of interest (ROI) in the images to calculate the normalized green chromatic coordinate index. We compared the camera data with the NDVI time series (2000–2019) derived from near-infrared (NIR) and red bands from MODIS product data. Using calibrated PhenoCam thresholds reduced the mean absolute error by 5 days for SOS and 34 days for EOS, compared to common thresholds in land surface phenology studies. On average, growing season length (LOS) did not differ significantly among vegetation types, but the driest sites showed the highest interannual variation. This pattern was applied to leaf flushing (SOS) and leaf fall (EOS) as well. We found a positive relationship between the accumulated precipitation and the LOS and between the accumulated precipitation and maximum and minimum temperatures and the vegetation productivity (peak and accumulated NDVI). Our results demonstrated that (A) the fine temporal resolution of phenocamera phenology time series improved the definitions of TDs and thresholds for RS landscape phenology; (b) long-term RS greening responded to the variability in rainfall, adjusting their timing of green-up and green-down, and (C) the amount of rainfall, although not determinant for the length of the growing season, is related to the estimates of vegetation productivity.

Probing the interplay of biophysical constraints and photosynthesis to model tree growth

Tree growth is a key uncertainty in projections of forest productivity and the global carbon cycle. While global vegetation models commonly represent tree growth as a carbon assimilation (source)-driven process, accumulating evidence points toward widespread non-photosynthetic (sink) limitations. Notably, growth biophysical potential, defined as the upper-limit to tree growth imposed by temperature and turgor constraints on cell division, has been suggested to be a potent driver of observed decoupling between tree growth and photosynthesis. Understanding the interplay between biophysical potential and photosynthesis and how to accommodate it parsimoniously in models remains a challenge.Here, we use a soil-plant-atmosphere continuum model together with a regional network of forest structure and annual, radial tree growth observations extending over three decades to simulate tree photosynthesis and biophysical potential along an aridity gradient and across five tree species in NE Spain. We then apply a linear modelling framework to quantify the relative importance of photosynthesis, biophysical potential and their interactions to predict annual tree growth along the aridity gradient.Overall similar relative importance of photosynthesis and biophysical potential was underlain by strong variations with climate, photosynthesis being more relevant at wet sites and biophysical potential at dry sites. Observed spatial and temporal trends further suggested that tree growth is primarily limited by biophysical potential under dry conditions and that disregarding it could lead to underestimating tree growth decline with increased aridity under climate change.Our results support the idea that biophysical potential is an important component of sink limitations to tree radial growth. Its representation in vegetation models could accommodate spatially and temporally dynamic source-sink limitations on tree growth.

Effects of aridification on soil total carbon pools in China's drylands.

Drylands are important carbon pools and are highly vulnerable to climate change, particularly in the context of increasing aridity. However, there has been limited research on the effects of aridification on soil total carbon including soil organic carbon and soil inorganic carbon, which hinders comprehensive understanding and projection of soil carbon dynamics in drylands. To determine the response of soil total carbon to aridification, and to understand how aridification drives soil total carbon variation along the aridity gradient through different ecosystem attributes, we measured soil organic carbon, inorganic carbon and total carbon across a ~4000 km aridity gradient in the drylands of northern China. Distribution patterns of organic carbon, inorganic carbon, and total carbon at different sites along the aridity gradient were analyzed. Results showed that soil organic carbon and inorganic carbon had a complementary relationship, that is, an increase in soil inorganic carbon positively compensated for the decrease in organic carbon in semiarid to hyperarid regions. Soil total carbon exhibited a nonlinear change with increasing aridity, and the effect of aridity on total carbon shifted from negative to positive at an aridity level of 0.71. In less arid regions, aridification leads to a decrease in total carbon, mainly through a decrease in organic carbon, whereas in more arid regions, aridification promotes an increase in inorganic carbon and thus an increase in total carbon. Our study highlights the importance of soil inorganic carbon to total carbon and the different effects of aridity on soil carbon pools in drylands. Soil total carbon needs to be considered when developing measures to conserve the terrestrial carbon sink.

Herbivores disrupt clinal variation in plant responses to water limitation

Abstract Plasticity in plant traits, including secondary metabolites, is critical to plant survival and competitiveness under stressful conditions. The ability of a plant to respond effectively to combined stressors can be impacted by crosstalk in biochemical pathways, resource availability and evolutionary history, but such responses remain underexplored. In particular, we know little about intraspecific variation in response to combined stressors or whether such variation is associated with the stress history of a given population. Here, we investigated the consequences of combined water and herbivory stress for plant traits, including relative growth rate, leaf morphology and various measures of phytochemistry, using a common garden of Asclepias fascicularis milkweeds. To examine how plant trait means and plasticities depend on the history of environmental stress, seeds for the experiment were collected from across a gradient of aridity in the Great Basin, United States. We then conducted a factorial experiment crossing water limitation with herbivory. Plants responded to water limitation alone by increasing the evenness of UV‐absorbent secondary metabolites and to herbivory alone by increasing the richness of metabolites. However, plants that experienced combined water and herbivory stress exhibited similar phytochemical diversity to well‐watered control plants. This lack of plasticity in phytochemical diversity in plants experiencing combined stressors was associated with a reduction in relative growth rates. Leaf chemistry means and plasticities exhibited clinal variation corresponding to seed source water deficits. The total concentration of UV‐absorbent metabolites decreased with increasing water availability among seed sources, driven by higher concentrations of flavonol glycosides, which are hypothesized to act as antioxidants, among plants from drier sites. Plants sourced from drier sites exhibited higher plasticity in flavonol glycoside concentrations in response to water limitation, which increased phytochemical evenness, but simultaneous herbivory dampened plant responses to water limitation irrespective of seed source. Synthesis. These results suggest that climatic history can affect intraspecific phytochemical plasticity, which may confer tolerance to water limitation, but that co‐occurring herbivory disrupts such patterns. Global change is increasing the frequency and intensity of stress combinations, such that understanding intraspecific responses to combined stressors is critical for predicting the persistence of plant populations.

Fog controls biological cycling of soil phosphorus in the Coastal Cordillera of the Atacama Desert

AbstractSoils in hyper‐arid climates, such as the Chilean Atacama Desert, show indications of past and present forms of life despite extreme water limitations. We hypothesize that fog plays a key role in sustaining life. In particular, we assume that fog water is incorporated into soil nutrient cycles, with the inland limit of fog penetration corresponding to the threshold for biological cycling of soil phosphorus (P). We collected topsoil samples (0–10 cm) from each of 54 subsites, including sites in direct adjacency (&lt;10 cm) and in 1 m distance to plants, along an aridity gradient across the Coastal Cordillera. Satellite‐based fog detection revealed that Pacific fog penetrates up to 10 km inland, while inland sites at 10–23 km from the coast rely solely on sporadic rainfall for water supply. To assess biological P cycling we performed sequential P fractionation and determined oxygen isotope of HCl‐extractable inorganic . Total P (Pt) concentration exponentially increased from 336 mg kg−1 to a maximum of 1021 mg kg−1 in inland areas ≥10 km. With increasing distance from the coast, soil values declined exponentially from 16.6‰ to a constant 9.9‰ for locations ≥10 km inland. Biological cycling of HCl‐Pi near the coast reached a maximum of 76%–100%, which could only be explained by the fact that fog water predominately drives biological P cycling. In inland regions, with minimal rainfall (&lt;5 mm) as single water source, only 24 ± 14% of HCl‐Pi was biologically cycled. We conclude that biological P cycling in the hyper‐arid Atacama Desert is not exclusively but mainly mediated by fog, which thus controls apatite dissolution rates and related occurrence and spread of microbial life in this extreme environment.

Quantifying the drought sensitivity of grassland under different climate zones in Northwest China

Grassland is essential for maintaining the stability and functionality of terrestrial ecosystems. Although previous research has explored how grassland responds to drought, the drought sensitivity of grassland (DSG) across climate zones and aridity gradients remains uncertain. In this study, we conducted a comprehensive investigation spanning 1982 to 2015 in Northwest China. To assess the time-cumulative effect (TCE) and the time-lag effect (TLE) of drought on grassland, we employed Spearman rank correlation analysis, utilizing long-term datasets of the normalized difference vegetation index (NDVI) and the standardized precipitation evapotranspiration index (SPEI). This analysis allowed us to quantify the DSG in the region and further examine its variations across climate zones and aridity gradient. Our results revealed that 81.2 % and 99.7 % of the grassland in Northwest China was influenced by the TCE and TLE of drought, respectively, with 38.2 % and 60.9 % of these effects being statistically significant (p < 0.05). The mean accumulated and lagged timescales of drought on grassland were 7.89 and 9.41 months, respectively. Remarkably, the highest DSG was observed in the semi-arid zone (0.58), followed by the arid (0.54), sub-humid (0.51), and humid (0.44) zones. Furthermore, we identified significant nonlinear variation patterns of DSG along the aridity gradient, characterized by several discernible trend breaks. These findings contribute to our understanding of the impacts of drought on vegetation, particularly in ecologically fragile regions.

A water stress factor based on normalized difference water index substantially improved the accuracy of light use efficiency model for arid and semi-arid grasslands

High-accuracy simulation of gross primary productivity (GPP) is crucial for the monitoring and evaluation of the ecosystem services and the adaptive management of grassland. The light use efficiency (LUE) model is one of the most widely-used methods to simulate GPP, given its simple structure and low input requirements. Current LUE models are less applicable to grasslands than other vegetation types and have lower overall estimation accuracy in arid and semi-arid regions. A grassland-specific light use efficiency model (GRASS-LUE), which optimizes three important parameters (the fraction of absorbed photosynthetically active radiation FPAR, optimum temperature Topt and water stress factor f(W)), has been developed to improve the accuracy of GPP simulation for grasslands along aridity gradients. GPP simulated by the GRASS-LUE agreed well with the eddy covariance (EC) GPP estimates for grasslands along the aridity gradient at 8-day (coefficient of determination (R2) = 0.85, Bias = −0.67 gC m−2 day−1), monthly (R2 = 0.88, Bias = −22.33 gC m−2 month−1) and annual time scales (R2 = 0.95, Bias = −118.91 gC m−2 year−1). Compared with five state-of-the-art GPP products (PML, MOD17, rEC-LUE, VPM and BESS), GRASS-LUE had the best and most stable performance in reproducing EC GPP, especially for semi-arid grassland, with the highest global performance indicator (GPI) value. Sensitivity tests further revealed that: 1) modifying f(W) to be based on the Normalized Difference Water Index (NDWI) substantially improved the model accuracy for arid and semi-arid grasslands and 2) using the minimum of temperature and water stress factors (i.e., min⁡(f(W),f(T))) to represent environmental stress in GRASS-LUE was better than that from the multiplication of temperature and water stress factors (i.e., f(W)×f(T)).

Functional diversity and spatial association analyses at different spatial scales reveal no changes in community assembly processes along an aridity gradient in the Atacama Desert

The structuring of plant assemblages along environmental gradients is typically explained by shifts from competition (limiting similarity) to environmental filtering as the environment becomes more stressful. However, facilitation, weaker-competitor exclusion, environmental heterogeneity, and the colonization-competition tradeoff can also structure plant assemblages along gradients. These assembly processes act on different plant traits and organs, and their prevalence varies with respect to the spatial scale. Using patterns of functional diversity, coupled with patterns of species association at two spatial scales, here we discern the assembly processes that structure shrub communities in four localities along an aridity gradient of the Atacama Desert. At each site, we calculated functional dispersion indexes for above- and below-ground traits, and patterns of species association at a patch and neighborhood scale. Our results revealed that at the patch scale in intermediate levels of aridity, the dominant assembly process was within-site environmental heterogeneity. At the neighborhood scale, communities are assembled mainly through random processes. Nonetheless, in some communities, the dominant assembly process was competition via limiting similarity or exclusion of the weaker competitor, and these did not change along the gradient. Together, these results reveal that environmental heterogeneity and competition are the main drivers of plant community assembly in a hyper-arid environment.

Root carbon and soil temperature may be key drivers of below-ground biomass in grassland following prescribed fires in autumn and spring

Under global warming, fire and the season in which the fire occurs both have important impacts on grassland plant biomass. Still, the effect of fire on below-ground biomass (BB) along a natural aridity gradient and the main impact factors remain unclear. Here, we conducted a fire manipulation experiment (including un-fired, autumn fire and spring fire treatments) to investigate the effects of prescribed fire on BB and its critical determinants along a transect of grassland in northern China. BB had different response strategies in different aridity regions and fire seasons, despite above-ground biomass (AB) and root-shoot ratio were not significantly affected by fire. General linear regression models revealed that the fire changed the trend of increasing BB to decreasing along increasing aridity (p ＜ 0.05). Random forest model (RFM) and partial correlations revealed that the BB was primarily influenced by aridity, followed by the nitrogen (N) and phosphorus (P) concentration ratio of AB under un-fired disturbance. For autumn fire, the BB was primarily influenced by below-ground biomass carbon concentration (BB c), followed by the C and N concentration ratio of BB. For spring fire, the BB was primarily influenced by soil temperature (ST), followed by aridity and soil total phosphorus concentration (Soil p). Furthermore, partial least squares path model (PLS-PM) revealed that autumn fires weakened the effects of environmental factors on BB, while spring fires enhanced the effects of soil nutrients on BB. These suggested that fire disrupted the original stable nutrient dynamics of BB. Our results suggested that fire promoted the growth of BB in relatively humid areas (aridity = 0.51–0.53) while inhibited the growth of BB in relatively arid areas (aridity = 0.68–0.74). BB c and ST may be key drivers of BB after prescribed fire in autumn and spring.

Short-term recovery of post-fire vegetation is primarily limited by drought in Mediterranean forest ecosystems

BackgroundClimate change is altering the fire regime and compromising the post-fire recovery of vegetation worldwide. To understand the factors influencing post-fire vegetation cover restoration, we calculated the recovery of vegetation in 200,000 hectares of western Mediterranean forest burned by 268 wildfires over a 27-year period (1988–2015). We used time series of the Tasseled Cap Transformation Brightness (TCTB) spectral transformation over Landsat imagery to calculate vegetation recovery. Then, we quantified the importance of the main drivers of post-fire vegetation recovery (climate, fire severity, and topography) along an aridity gradient (semi-arid, sub-humid, and humid) using Random Forest models.ResultsIn most models (99.7%), drought duration was the most important factor, negatively affecting post-fire recovery especially in the extremes of the aridity gradient. Fire severity was the second most important factor for vegetation cover recovery, with its effect varying along the aridity gradient: there was a positive relationship between fire severity and recovery in sub-humid and humid areas, while semi-arid areas showed the opposite pattern. Topographic variables were the least important driver and had a marginal effect on post-fire recovery. Additionally, semi-arid areas exhibited a low mean recovery rate, indicating limitations in the short-term recovery after a fire.ConclusionsOur study highlights the key role that drought duration plays in the recovery of vegetation after wildfires in the Mediterranean basin and, particularly, in forests located in climatically extreme areas. The results suggest that the predicted increase in drought duration coupled with a higher frequency and intensity of large fires may modify the structure and composition of Mediterranean forest ecosystems. Our analysis provides relevant information to evaluate and design adaptive management strategies in post-fire recovery hotspots of Mediterranean forest ecosystems.