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Manipulative experiments typically show a decrease in dryland biocrust cover and altered species composition under climate change. Biocrust-forming lichens, such as the globally distributed Diploschistes diacapsis, are particularly affected and show a decrease in cover with simulated climate change. However, the underlying mechanisms are not fully understood, and long-term interacting effects of different drivers are largely unknown due to the short-term nature of the experimental studies conducted so far. We addressed this gap and successfully parameterised a process-based model for D. diacapsis to quantify how changing atmospheric CO2, temperature, rainfall amount and relative humidity affect its photosynthetic activity and cover. We also mimicked a long-term manipulative climate change experiment to understand the mechanisms underlying observed patterns in the field. The model reproduced observed experimental findings: warming reduced lichen cover, whereas less rainfall had no effect on lichen performance. This warming effect was caused by the associated decrease in relative humidity and non-rainfall water inputs, which are major water sources for biocrust-forming lichens. Warming alone, however, increased cover because higher temperatures promoted photosynthesis during early morning hours with high lichen activity. When combined, climate variables showed non-additive effects on lichen cover, and effects of increased CO2 levelled off with decreasing levels of relative humidity. Synthesis. Our results show that a decrease in relative humidity, rather than an increase in temperature, may be the key factor for the survival of the lichen D. diacapsis under climate change and that effects of increased CO2 levels might be offset by a reduction in non-rainfall water inputs in the future. Because of a global trend towards warmer and drier air and the widespread global distribution of D. diacapsis, this will affect lichen-dominated dryland biocrust communities an, European Research Council, Ministerio de Economía y Competitividad, Depto. de Farmacología, Farmacognosia y Botánica, Fac. de Farmacia, TRUE, pub

Acknowledgements We thank K. Spellman, S. Schwinning and two anonymous reviewers for their constructive comments on a previous version of the manuscript and the Spanish ‘Guardia Civil’ for their technical assistance during the field work. This research was funded by the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no. 242658 (BIOCOM) awarded to FTM. JLQ acknowledges support from the BIOCOM project. FTM acknowledges support from the Spanish Ministerio de Educación (‘Salvador de Madariaga program’, PR2010-0230) during the writing of the manuscript., 1. Habitat filtering (

Summary Despite the important role that biocrust communities play in maintaining ecosystem structure and functioning in drylands world-wide, few studies have evaluated how climate change will affect them. Using data from an 8-yr-old manipulative field experiment located in central Spain, we evaluated how warming, rainfall exclusion and their combination affected the dynamics of biocrust communities in areas that initially had low (< 20%, LIBC plots) and high (> 50%, HIBC plots) biocrust cover. Warming reduced the richness (35 ± 6%), diversity (25 ± 8%) and cover (82 ± 5%) of biocrusts in HIBC plots. The presence and abundance of mosses increased with warming through time in these plots, although their growth rate was much lower than the rate of lichen death, resulting in a net loss of biocrust cover. On average, warming caused a decrease in the abundance (64 ± 7%) and presence (38 ± 24%) of species in the HIBC plots. Over time, lichens and mosses colonized the LIBC plots, but this process was hampered by warming in the case of lichens. The observed reductions in the cover and diversity of lichen-dominated biocrusts with warming will lessen the capacity of drylands such as that studied here to sequester atmospheric CO2 and to provide other key ecosystem services associated to these communities., Ministerio de Economía y Competitividad, Gobierno de España, European Research Council (ERC), Depto. de Farmacología, Farmacognosia y Botánica, Fac. de Farmacia, TRUE, pub

Photoautotrophic soil cyanobacteria play essential ecological roles and are known to experience large changes in their diversity and abundance throughout early succession. However, much less is known about how and why soil cyanobacterial communities change as soil develops from centuries to millennia, and the effects of vegetation on them. We combined an extensive field survey including 16 global soil chronosequences across contrasting ecosystems (from deserts to tropical forests) with molecular analyses to investigate how the diversity and abundance of photosynthetic and non-photosynthetic soil cyanobacteria under vegetation change during soil development from hundreds to thousands of years. We show that, in most chronosequences, the abundance, species richness and community composition of soil cyanobacteria were relatively stable as soil develops (from centuries to millennia). Regardless of soil age, forest chronosequences were consistently dominated by non-photosynthetic cyanobacteria (Vampirovibrionia), while grasslands and shrublands were dominated by photosynthetic cyanobacteria. Chronosequences undergoing drastic vegetation shifts (e.g. transitions from grasslands to forests) experienced significant changes in the composition of soil cyanobacteria communities. Our results advance our understanding of the ecology of cyanobacterial classes, specially the understudied non-photosynthetic ones and highlight the key role of vegetation as a major driver of their temporal dynamics as soil develops.

Fungi are highly diverse organisms, which provide multiple ecosystem services. However, compared with charismatic animals and plants, the distribution patterns and conservation needs of fungi have been little explored. Here we used high-resolution sequencing to assess endemicity patterns, global change vulnerability and conservation priority areas for functional groups of soil fungi based on six global surveys using a high-resolution, long-read metabarcoding approach. We found that the endemicity of all fungi and most functional groups peaks in tropical habitats, including Amazonia, Yucatan, West-Central Africa, Sri Lanka and New Caledonia, with a negligible island effect compared with plants and animals. We also found that fungi are predominantly vulnerable to drought, heat and land cover change, particularly in dry tropical regions with high human population density. Fungal conservation areas of highest priority include herbaceous wetlands, tropical forests and woodlands. We stress that more attention should be focused on the conservation of fungi, especially root symbiotic arbuscular mycorrhizal and ectomycorrhizal fungi in tropical regions as well as unicellular early-diverging groups and macrofungi in general. Given the low overlap between the endemicity of fungi and macroorganisms, but high conservation needs in both groups, detailed analyses on distribution and conservation requirements are warranted for other microorganisms and soil organisms.

Drylands are important reservoirs of soil phosphorus (P) at the global scale, although large uncertainties remain regarding how climate change will affect P cycling in these ecosystems. Biocrust-forming lichens are important regulators of abiotic and biotic processes occurring in the soil surface, including nutrient availability and redistribution, across global drylands. However, their role as modulators of climate change impacts on soil P cycling is poorly known. We conducted a manipulative microcosm experiment to evaluate how six biocrust-forming lichens (Buellia zoharyi, Diploschistes diacapsis, Fulgensia subbracteata, Psora decipiens, Squamarina lentigera, and Toninia sedifolia) with diverse morphology and chemistry affect soil available P concentration and the activity of acid phosphatase after 50 months of simulated ~2°C warming and 35% rainfall reduction. Lichens increased soil available inorganic and total available P, and the activity of acid phosphatase, although the magnitude of these effects was highly species-specific. Climate change treatments increased available organic P regardless of lichen species. Our findings provide novel experimental evidence on the importance of biocrusts as modulators of P cycling in drylands and highlight the necessity to take into account the identity of biocrust constituents when evaluating their effects on soil fertility.

1. With ongoing climate change, the probability of crossing environmental thresholds promoting abrupt changes in ecosystem structure and functioning is higher than ever. In drylands (areas where it rains less than 65% of what could be potentially evaporated), recent research has shown how the crossing of three aridity thresholds (at aridity [1-Aridity Index] values of 0.54, 0.70 and 0.80) leads to abrupt changes on ecosystem structural and functional attributes. Despite the importance of these findings and their implications to develop effective monitoring and adaptation actions to combat climate change and desertification, we lack a proper understanding of the mechanisms unleashing these abrupt shifts. 2. Here we review multiple mechanisms that may explain the existence of aridity thresholds observed across global drylands, and discuss the potential amplification mechanisms that may underpin hypothetical abrupt temporal shifts with climate change. 3. We propose that each aridity threshold is caused by different and specific mechanisms. The first threshold is mainly caused by physiological mechanisms of plant adaptation to water shortages. The second threshold is unleashed by different mechanisms involving soil processes and plant-soil interactions such as soil erosion, plant community shifts and nutrient cycling and circulation. The collapse of vegetation observed once the third aridity threshold (0.8) is crossed is caused by mechanisms related to the survival limits of plants that may cause sudden cover and diversity losses and plant-atmospheric feedbacks that link vegetation collapse with further climate aridification. 4. By identifying, revising, and linking relevant mechanisms to each aridity threshold observed, we provide a set of specific hypotheses and identify knowledge gaps concerning the study of threshold emergence in drylands. We also were able to establish plausible factors that are context dependent and may influence the occurrence of abrupt ecosystem

Despite their extent and socio-ecological importance, a comprehensive biogeographical synthesis of drylands is lacking. Here we synthesize the biogeography of key organisms (vascular and nonvascular vegetation and soil microorganisms), attributes (functional traits, spatial patterns, plant–plant and plant–soil interactions) and processes (productivity and land cover) across global drylands. These areas have a long evolutionary history, are centers of diversification for many plant lineages and include important plant diversity hotspots. This diversity captures a strikingly high portion of the variation in leaf functional diversity observed globally. Part of this functional diversity is associated with the large variation in response and effect traits in the shrubs encroaching dryland grasslands. Aridity and its interplay with the traits of interacting plant species largely shape biogeographical patterns in plant–plant and plant–soil interactions, and in plant spatial patterns. Aridity also drives the composition of biocrust communities and vegetation productivity, which shows large geographical variation. We finish our review by discussing major research gaps, which include: studying regular vegetation spatial patterns; establishing large-scale plant and biocrust field surveys assessing individual-level trait measurements; knowing whether the impacts of plant–plant and plant–soil interactions on biodiversity are predictable; and assessing how elevated CO2 modulates future aridity conditions and plant productivity.

1. In recent years there has been an upsurge of studies on ecosystem multifunctionality (EMF), or the ability of ecosystems to simultaneously provide multiple functions and/or services. The concept of EMF itself, the analytical approaches used to calculate it, and its implications depending on the spatial scale and field of study have been discussed in detail. However, to date there has been little dialogue concerning the basis of EMF studies: what should or should not be considered appropriate measures for ecosystem functions. 2. To begin this discussion, we performed an in‐depth review of EMF studies across four major terrestrial ecosystems (agroecosystems, drylands, forests and grasslands) by analysing 82 studies, which together have assessed 775 ecosystem functions from a variety of field and greenhouse experiments across the globe. 3. The number of ecosystem functions analysed varied from two to 82 per study and we found large differences in the distribution of functions across ecosystem types and ecosystem service categories. Furthermore, there was little explanation of why certain variables were included in the EMF calculation or how they relate to ecosystem functioning. 4. Synthesis. Based on the literature analysis, it is clear that there is no general agreement regarding which measurements should or should not be considered functions in the field of ecology. To address this issue, we propose a general guideline for determining and measuring appropriate functions.

1. The importance of soil phosphorus (P) is likely to increase in coming decades due to the growing atmospheric nitrogen (N) deposition originated by industrial and agricultural activities. We currently lack a proper understanding of the main drivers of soil P pools in coastal dunes, which rank among the most valued priority conservation areas worldwide. 2. Here, we evaluated the joint effects of biotic (i.e. microbial abundance and richness, vegetation and cryptogams cover) and abiotic (i.e. pH and aridity) factors on labile, medium‐lability and recalcitrant soil P pools across a wide aridity gradient in the Atlantic coast of the Iberian Peninsula. 3. Climate determined the availability of medium‐lability, recalcitrant and total P, but had a minor net effect on labile P, which was positively and significantly related to the presence of plants, mosses and lichens. Medium‐lability P was significantly influenced by soil bacterial richness and abundance (positively and negatively, respectively). 4. Our results suggest that micro‐organisms transfer P from medium‐lability pool to more labile one. At the same time, increases in bacterial richness associated to biofilms might be involved in the thickening of the medium‐lability P pool in our climosequence. 5. These bacterial‐mediated transfers would confer resistance to the labile P pool under future climate change and uncover an important role of soil micro‐organisms as modulators of the geochemical P cycle.

1. Multiple ecosystem functions need to be considered simultaneously to manage and protect the several ecosystem services that are essential to people and their environments. Despite this, cost effective, tangible, relatively simple and globally relevant methodologies to monitor in situ soil multifunctionality, that is, the provision of multiple ecosystem functions by soils, have not been tested at the global scale. 2. We combined correlation analysis and structural equation modelling to explore whether we could find easily measured, field‐based indicators of soil multifunctionality (measured using functions linked to the cycling and storage of soil carbon, nitrogen and phosphorus). To do this, we gathered soil data from 120 dryland ecosystems from five continents. 3. Two soil surface attributes measured in situ (litter incorporation and surface aggregate stability) were the most strongly associated with soil multifunctionality, even after accounting for geographic location and other drivers such as climate, woody cover, soil pH and soil electric conductivity. The positive relationships between surface stability and litter incorporation on soil multifunctionality were greater beneath the canopy of perennial vegetation than in adjacent, open areas devoid of vascular plants. The positive associations between surface aggregate stability and soil functions increased with increasing mean annual temperature. 4. Synthesis and applications. Our findings demonstrate that a reduced suite of easily measured in situ soil surface attributes can be used as potential indicators of soil multifunctionality in drylands world‐wide. These attributes, which relate to plant litter (origin, incorporation, cover), and surface stability, are relatively cheap and easy to assess with minimal training, allowing operators to sample many sites across widely varying climatic areas and soil types. The correlations of these variables are comparable to the influence of climate or soil, and would all

Understanding whether soil microbial respiration adapts to the ambient thermal climate with an enhanced or compensatory response, hence potentially stimulating or slowing down soil carbon losses with warming, is key to accurately forecast and model climate change impacts on the global carbon cycle. Despite the interest in this topic and the plethora of recent studies in natural ecosystems, it has been seldom explored in croplands. Using two recently published independent datasets of soil microbial metabolic quotient (MMQ; microbial respiration rate per unit biomass) and carbon use efficiency (CUE; partitioning of C to microbial growth vs. respiration), we find a compensatory thermal adaptive response for MMQ in global croplands. That is, mean annual temperature (MAT) has a negative effect on MMQ. However, this compensatory thermal adaptation is only half or less of that found in previous studies for noncultivated ecosystems. In contrast to the negative MMQ‐MAT pattern, microbial CUE increases with MAT across global croplands. By incorporating this positive CUE‐MAT relationship (greater C partitioning into microbial growth rather than respiration with increasing temperature) into a microbial‐explicit soil organic carbon model, we successfully predict the thermal compensation of MMQ observed in croplands. Our model‐data integration and database cross‐validation suggest that a warmer climate may select for microbial communities with higher CUE, providing a plausible mechanism for their compensatory metabolic response. By helping to identify appropriate representations of microbial physiological processes in soil biogeochemical models, our work will help build confidence in model projections of cropland C dynamics under a changing climate.

1. Climate change will increase heat and drought stress in many dryland areas, which could reduce soil nutrient availability for plants and aggravate nutrient limitation of primary productivity. Any negative impacts of climate change on foliar nutrient contents would be expected to negatively affect the photosynthetic capacity, water use efficiency and overall fitness of dryland vegetation. 2. We conducted a 4‐year manipulative experiment using open top chambers and rainout shelters to assess the impacts of warming (~2°C, W), rainfall reduction (~30%, RR) and their combination (W + RR) on the nutrient status and ecophysiological performance of six native shrub species of contrasting phylogeny in a semi‐arid ecosystem. Leaf nutrient status and gas exchange were assessed yearly, whereas biomass production and survival were measured at the end of the study. 3. Warming (W and W + RR) advanced shoot growth phenology and reduced foliar macro‐ (N, P, K) and micronutrient (Cu, Fe, Zn) concentrations (by 8%–18% and 14%–56% respectively), net photosynthetic rate (32%), above‐ground biomass production (28%–39%) and survival (23%–46%). Decreased photosynthesis and growth in W and W + RR plants were primarily linked to enhanced nutritional constraints on carbon fixation. Poor leaf nutrient status in W and W + RR plants partly decoupled carbon assimilation from water flux and led to drastic reductions in water use efficiency (WUEi; ~41%) across species. The RR treatment moderately decreased foliar macro‐ and micronutrients (6%–17%, except for Zn) and biomass production (22%). The interactive impacts of warming and rainfall reduction (W + RR treatment) on plant performance were generally smaller than expected from additive single‐factor effects. 4. Synthesis. Large decreases in plant nutrient pool size and productivity combined with increased mortality during hotter droughts will reduce vegetation cover and nutrient retention capacity, thereby disrupting biogeochemical processes a

The spatial configuration of vascular vegetation has been linked to variations in land degradation and ecosystem functioning in drylands. However, most studies on spatial patterns conducted to date have focused on a single or a few study sites within a particular region, specific vegetation types, or in landscapes characterized by a certain type of spatial patterns. Therefore, little is known on the general typology and distribution of plant spatial patterns in drylands worldwide, and on the relative importance of biotic and abiotic factors as predictors of their variations across geographical regions and habitat types. We analyzed 115 dryland plant communities from all continents except Antarctica to: 1) investigate the general typology of spatial patterns, and 2) assess the relative importance of biotic (plant cover, frequency of facilitation, soil amelioration, height of the dominant species) and abiotic (aridity, rainfall seasonality and sand content) factors as predictors of spatial patterns (median patch size, shape of patch‐size distribution and regularity) across contrasting habitat types (shrublands and grasslands). Precipitation during the warmest period and sand content were particularly strong predictors of plant spatial patterns in grasslands and shrublands, respectively. Facilitation associated with power‐law like and irregular spatial patterns in both shrublands and grasslands, although it was mediated by different mechanisms (respectively soil ammelioration and percentage of facilitated species). The importance of biotic attributes as predictors of the shape of patch‐size distributions declined with aridity in both habitats, leading to the emergence of more regular patterns under the most arid conditions. Our results expand our knowledge about patch formation in drylands and the habitat‐dependency of their drivers. They also highlight different ways in which facilitation affects ecosystem structure through the formation of plant spatial patterns.