Biodiversity promotes ecosystem functioning despite environmental change.

Three decades of research have demonstrated that biodiversity can promote the functioning of ecosystems. Yet, it is unclear whether the positive effects of biodiversity on ecosystem functioning will persist under various types of global environmental change drivers. We conducted a meta-analysis of 46 factorial experiments manipulating both species richness and the environment to test how global change drivers (i.e. warming, drought, nutrient addition or CO2 enrichment) modulated the effect of biodiversity on multiple ecosystem functions across three taxonomic groups (microbes, phytoplankton and plants). We found that biodiversity increased ecosystem functioning in both ambient and manipulated environments, but often not to the same degree. In particular, biodiversity effects on ecosystem functioning were larger in stressful environments induced by global change drivers, indicating that high-diversity communities were more resistant to environmental change. Using a subset of studies, we also found that the positive effects of biodiversity were mainly driven by interspecific complementarity and that these effects increased over time in both ambient and manipulated environments. Our findings support biodiversity conservation as a key strategy for sustainable ecosystem management in the face of global environmental change.

Risk of short-term biodiversity loss under more persistent precipitation regimes.

Recent findings indicate that atmospheric warming increases the persistence of weather patterns in the mid-latitudes, resulting in sequences of longer dry and wet periods compared to historic averages. The alternation of progressively longer dry and wet extremes could increasingly select for species with a broad environmental tolerance. As a consequence, biodiversity may decline. Here we explore the relationship between the persistence of summer precipitation regimes and plant diversity by subjecting experimental grassland mesocosms to a gradient of dry-wet alternation frequencies whilst keeping the total precipitation constant. The gradient varied the duration of consecutive wet and dry periods, from 1 up to 60 days with or without precipitation, over a total of 120 days. An alternation of longer dry and wet spells led to a severe loss of species richness (up to -75% relative to the current rainfall pattern in W-Europe) and functional diversity (enhanced dominance of grasses relative to nitrogen (N)-fixers and non-N-fixing forbs). Loss of N-fixers and non-N-fixing forbs in severe treatments was linked to lower baseline competitive success and higher physiological sensitivity to changes in soil moisture compared to grasses. The extent of diversity losses also strongly depended on the timing of the dry and wet periods. Regimes in which long droughts (≥20 days) coincided with above-average temperatures showed significantly more physiological plant stress over the experimental period, greater plant mortality, and impoverished communities by the end of the season. Across all regimes, the duration of the longest period below permanent wilting point was an accurate predictor of mortality across the communities, indicating that increasingly persistent precipitation regimes may reduce opportunities for drought stress alleviation. We conclude that without recruitment, which was precluded in this experiment, summer precipitation regimes with longer dry and wet spells will likely diminish plant diversity, at least in the short term.

Photoperiod decelerates the advance of spring phenology of six deciduous tree species under climate warming.

Vegetation phenology in spring has substantially advanced under climate warming, consequently shifting the seasonality of ecosystem process and altering biosphere-atmosphere feedbacks. However, whether and to what extent photoperiod (i.e., daylength) affects the phenological advancement is unclear, leading to large uncertainties in projecting future phenological changes. Here we examined the photoperiod effect on spring phenology at a regional scale using in situ observation of six deciduous tree species from the Pan European Phenological Network during 1980-2016. We disentangled the photoperiod effect from the temperature effect (i.e., forcing and chilling) by utilizing the unique topography of the northern Alps of Europe (i.e., varying daylength but uniform temperature distribution across latitudes) and examining phenological changes across latitudes. We found prominent photoperiod-induced shifts in spring leaf-out across latitudes (up to 1.7 days per latitudinal degree). Photoperiod regulates spring phenology by delaying early leaf-out and advancing late leaf-out caused by temperature variations. Based on these findings, we proposed two phenological models that consider the photoperiod effect through different mechanisms and compared them with a chilling model. We found that photoperiod regulation would slow down the advance in spring leaf-out under projected climate warming and thus mitigate the increasing frost risk in spring that deciduous forests will face in the future. Our findings identify photoperiod as a critical but understudied factor influencing spring phenology, suggesting that the responses of terrestrial ecosystem processes to climate warming are likely to be overestimated without adequately considering the photoperiod effect.

Ecotrons: Powerful and versatile ecosystem analysers for ecology, agronomy and environmental science.

Ecosystems integrity and services are threatened by anthropogenic global changes. Mitigating and adapting to these changes require knowledge of ecosystem functioning in the expected novel environments, informed in large part through experimentation and modelling. This paper describes 13 advanced controlled environment facilities for experimental ecosystem studies, herein termed ecotrons, open to the international community. Ecotrons enable simulation of a wide range of natural environmental conditions in replicated and independent experimental units while measuring various ecosystem processes. This capacity to realistically control ecosystem environments is used to emulate a variety of climatic scenarios and soil conditions, in natural sunlight or through broad-spectrum lighting. The use of large ecosystem samples, intact or reconstructed, minimizes border effects and increases biological and physical complexity. Measurements of concentrations of greenhouse trace gases as well as their net exchange between the ecosystem and the atmosphere are performed in most ecotrons, often quasi continuously. The flow of matter is often tracked with the use of stable isotope tracers of carbon and other elements. Equipment is available for measurements of soil water status as well as root and canopy growth. The experiments ran so far emphasize the diversity of the hosted research. Half of them concern global changes, often with a manipulation of more than one driver. About a quarter deal with the impact of biodiversity loss on ecosystem functioning and one quarter with ecosystem or plant physiology. We discuss how the methodology for environmental simulation and process measurements, especially in soil, can be improved and stress the need to establish stronger links with modelling in future projects. These developments will enable further improvements in mechanistic understanding and predictive capacity of ecotron research which will play, in complementarity with field experimentation and monitoring, a crucial role in exploring the ecosystem consequences of environmental changes.

Climate warming increases spring phenological differences among temperate trees.

Climate warming has substantially advanced spring leaf flushing, but winter chilling and photoperiod co-determine the leaf flushing process in ways that vary among species. As a result, the interspecific differences in spring phenology (IDSP) are expected to change with climate warming, which may, in turn, induce negative or positive ecological consequences. However, the temporal change of IDSP at large spatiotemporal scales remains unclear. In this study, we analyzed long-term in-situ observations (1951-2016) of six, coexisting temperate tree species from 305 sites across Central Europe and found that phenological ranking did not change when comparing the rapidly warming period 1984-2016 to the marginally warming period 1951-1983. However, the advance of leaf flushing was significantly larger in early-flushing species EFS (6.7 ± 0.3 days) than in late-flushing species LFS (5.9 ± 0.2 days) between the two periods, indicating extended IDSP. This IDSP extension could not be explained by differences in temperature sensitivity between EFS and LFS; however, climatic warming-induced heat accumulation effects on leaf flushing, which were linked to a greater heat requirement and higher photoperiod sensitivity in LFS, drove the shifts in IDSP. Continued climate warming is expected to further extend IDSP across temperate trees, with associated implications for ecosystem function.

Invader presence disrupts the stabilizing effect of species richness in plant community recovery after drought.

Higher biodiversity can stabilize the productivity and functioning of grassland communities when subjected to extreme climatic events. The positive biodiversity-stability relationship emerges via increased resistance and/or recovery to these events. However, invader presence might disrupt this diversity-stability relationship by altering biotic interactions. Investigating such disruptions is important given that invasion by non-native species and extreme climatic events are expected to increase in the future due to anthropogenic pressure. Here we present one of the first multisite invader × biodiversity × drought manipulation experiment to examine combined effects of biodiversity and invasion on drought resistance and recovery at three semi-natural grassland sites across Europe. The stability of biomass production to an extreme drought manipulation (100% rainfall reduction; BE: 88 days, BG: 85 days, DE: 76 days) was quantified in field mesocosms with a richness gradient of 1, 3, and 6 species and three invasion treatments (no invader, Lupinus polyphyllus, Senecio inaequidens). Our results suggest that biodiversity stabilized community productivity by increasing the ability of native species to recover from extreme drought events. However, invader presence turned the positive and stabilizing effects of diversity on native species recovery into a neutral relationship. This effect was independent of the two invader's own capacity to recover from an extreme drought event. In summary, we found that invader presence may disrupt how native community interactions lead to stability of ecosystems in response to extreme climatic events. Consequently, the interaction of three global change drivers, climate extremes, diversity decline, and invasive species, may exacerbate their effects on ecosystem functioning.

Species richness effects on grassland recovery from drought depend on community productivity in a multisite experiment.

Biodiversity can buffer ecosystem functioning against extreme climatic events, but few experiments have explicitly tested this. Here, we present the first multisite biodiversity × drought manipulation experiment to examine drought resistance and recovery at five temperate and Mediterranean grassland sites. Aboveground biomass production declined by 30% due to experimental drought (standardised local extremity by rainfall exclusion for 72-98 consecutive days). Species richness did not affect resistance but promoted recovery. Recovery was only positively affected by species richness in low-productive communities, with most diverse communities even showing overcompensation. This positive diversity effect could be linked to asynchrony of species responses. Our results suggest that a more context-dependent view considering the nature of the climatic disturbance as well as the productivity of the studied system will help identify under which circumstances biodiversity promotes drought resistance or recovery. Stability of biomass production can generally be expected to decrease with biodiversity loss and climate change.

Variation in leaf flushing date influences autumnal senescence and next year's flushing date in two temperate tree species.

Recent temperature increases have elicited strong phenological shifts in temperate tree species, with subsequent effects on photosynthesis. Here, we assess the impact of advanced leaf flushing in a winter warming experiment on the current year's senescence and next year's leaf flushing dates in two common tree species: Quercus robur L. and Fagus sylvatica L. Results suggest that earlier leaf flushing translated into earlier senescence, thereby partially offsetting the lengthening of the growing season. Moreover, saplings that were warmed in winter-spring 2009-2010 still exhibited earlier leaf flushing in 2011, even though the saplings had been exposed to similar ambient conditions for almost 1 y. Interestingly, for both species similar trends were found in mature trees using a long-term series of phenological records gathered from various locations in Europe. We hypothesize that this long-term legacy effect is related to an advancement of the endormancy phase (chilling phase) in response to the earlier autumnal senescence. Given the importance of phenology in plant and ecosystem functioning, and the prediction of more frequent extremely warm winters, our observations and postulated underlying mechanisms should be tested in other species.

Simulated heat waves affected alpine grassland only in combination with drought.

The Alpine region is warming fast, and concurrently, the frequency and intensity of climate extremes are increasing. It is currently unclear whether alpine ecosystems are sensitive or resistant to such extremes. We subjected Swiss alpine grassland communities to heat waves with varying intensity by transplanting monoliths to four different elevations (2440-660 m above sea level) for 17 d. Half of these were regularly irrigated while the other half were deprived of irrigation to additionally induce a drought at each site. Heat waves had no significant impacts on fluorescence (Fv /Fm , a stress indicator), senescence and aboveground productivity if irrigation was provided. However, when heat waves coincided with drought, the plants showed clear signs of stress, resulting in vegetation browning and reduced phytomass production. This likely resulted from direct drought effects, but also, as measurements of stomatal conductance and canopy temperatures suggest, from increased high-temperature stress as water scarcity decreased heat mitigation through transpiration. The immediate responses to heat waves (with or without droughts) recorded in these alpine grasslands were similar to those observed in the more extensively studied grasslands from temperate climates. Responses following climate extremes may differ in alpine environments, however, because the short growing season likely constrains recovery.

Three times greater weight of daytime than of night-time temperature on leaf unfolding phenology in temperate trees.

The phenology of spring leaf unfolding plays a key role in the structure and functioning of ecosystems. The classical concept of heat requirement (growing degree days) for leaf unfolding was developed hundreds of years ago, but this model does not include the recently reported greater importance of daytime than night-time temperature. A manipulative experiment on daytime vs night-time warming with saplings of three species of temperate deciduous trees was conducted and a Bayesian method was applied to explore the different effects of daytime and night-time temperatures on spring phenology. We found that both daytime and night-time warming significantly advanced leaf unfolding, but the sensitivities to increased daytime and night-time temperatures differed significantly. Trees were most sensitive to daytime warming (7.4 ± 0.9, 4.8 ± 0.3 and 4.8 ± 0.2 d advancement per degree Celsius warming (d °C-1 ) for birch, oak and beech, respectively) and least sensitive to night-time warming (5.5 ± 0.9, 3.3 ± 0.3 and 2.1 ± 0.9 d °C-1 ). Interestingly, a Bayesian analysis found that the impact of daytime temperature on leaf unfolding was approximately three times higher than that of night-time temperatures. Night-time global temperature is increasing faster than daytime temperature, so model projections of future spring phenology should incorporate the effects of these different temperatures.

Increased heat requirement for leaf flushing in temperate woody species over 1980-2012: effects of chilling, precipitation and insolation.

Recent studies have revealed large unexplained variation in heat requirement-based phenology models, resulting in large uncertainty when predicting ecosystem carbon and water balance responses to climate variability. Improving our understanding of the heat requirement for spring phenology is thus urgently needed. In this study, we estimated the species-specific heat requirement for leaf flushing of 13 temperate woody species using long-term phenological observations from Europe and North America. The species were defined as early and late flushing species according to the mean date of leaf flushing across all sites. Partial correlation analyses were applied to determine the temporal correlations between heat requirement and chilling accumulation, precipitation and insolation sum during dormancy. We found that the heat requirement for leaf flushing increased by almost 50% over the study period 1980-2012, with an average of 30 heat units per decade. This temporal increase in heat requirement was observed in all species, but was much larger for late than for early flushing species. Consistent with previous studies, we found that the heat requirement negatively correlates with chilling accumulation. Interestingly, after removing the variation induced by chilling accumulation, a predominantly positive partial correlation exists between heat requirement and precipitation sum, and a predominantly negative correlation between heat requirement and insolation sum. This suggests that besides the well-known effect of chilling, the heat requirement for leaf flushing is also influenced by precipitation and insolation sum during dormancy. However, we hypothesize that the observed precipitation and insolation effects might be artefacts attributable to the inappropriate use of air temperature in the heat requirement quantification. Rather than air temperature, meristem temperature is probably the prominent driver of the leaf flushing process, but these data are not available. Further experimental research is thus needed to verify whether insolation and precipitation sums directly affect the heat requirement for leaf flushing.

Can permanent grassland soils with elevated organic carbon buffer negative effects of more persistent precipitation regimes on forage grass performance?

Agricultural practices enhancing soil organic carbon (SOC) show potential to buffer negative effects of climate change on forage grass performance. We tested this by subjecting five forage grass varieties differing in fodder quality and drought/flooding resistance to increased persistence in summer precipitation regimes (PR) across sandy and sandy-loam soils from either permanent (high SOC) or temporary grasslands (low SOC) in adjacent parcels. Over the course of two consecutive summers, monoculture mesocosms were subjected to rainy/dry weather alternation either every 3 days or every 30 days, whilst keeping total precipitation equal. Increased PR persistence induced species-specific drought damage and productivity declines. Soils from permanent grasslands with elevated SOC buffered plant quality, but buffering effects of SOC on drought damage, nutrient availability and yield differed between texture classes. In the more persistent PR, Festuca arundinacea FERMINA was the most productive species but had the lowest quality under both ample water supply and mild soil drought, whilst under the most intense soil droughts, Festulolium FESTILO maintained the highest yields. The hybrid Lolium × boucheanum kunth MELCOMBI had intermediate productivity and both Lolium perenne varieties showed the lowest yields under soil drought, but the highest forage quality (especially the tetraploid variety MELFORCE). Performance varied with plant maturity stage and across seasons/years and was driven by altered water and nutrient availability and related nitrogen nutrition among species during drought and upon rewetting. Moreover, whilst permanent grassland soils showed the most consistent positive effects on plant performance, their available water capacity also declined under increased PR persistence. We conclude that permanent grassland soils with historically elevated SOC likely buffer negative effects of increasing summer weather persistence on forage grass performance, but may also be more sensitive to degradation under climate change.

Soil environmental anomalies dominate the responses of net ecosystem productivity to heatwaves in three Mongolian grasslands.

Climate change is causing more frequent and intense heatwaves. Therefore, it is important to understand how heatwaves affect the terrestrial carbon cycle, especially in grasslands, which are especially susceptible to climate extremes. This study assessed the impact of naturally occurring, simultaneous short-term heatwaves on CO2 fluxes in three ecosystems on the Mongolia Plateau: meadow steppe (MDW), typical steppe (TPL), and shrub-grassland (SHB). During three heatwaves, net ecosystem productivity (NEP) was reduced by 86 %, 178 %, and 172 % at MDW, TPL, and SHB, respectively. The changes in ecosystem respiration, gross primary production, evapotranspiration, and water use efficiency were divergent, indicating the mechanisms underlying the observed NEP decreases among the sites. The impact of the heatwave in MDW was mitigated by the high soil water content, which enhanced evapotranspiration and subsequent cooling effects. However, at TPL, insufficient soil water led to combined thermal and drought stress and low resilience. At SHB, the ecosystem's low tolerance to an August heatwave was heavily influenced by species phenology, as it coincided with the key phenological growing phase of plants. The potential key mechanism of divergent NEP response to heatwaves lies in the divergent stability and varying importance of environmental factors, combined with the specific sensitivity of NEP to each factor in ecosystems. Furthermore, our findings suggest that anomalies in soil environment, rather than atmospheric anomalies, are the primary determinants of NEP anomalies during heatwaves. This challenges the conventional understanding of heatwaves as a discrete and ephemeral periods of high air temperatures. Instead, heatwaves should be viewed as chronologically variable, compound, and time-sensitive environmental stressors. The ultimate impact of heatwaves on ecosystems is co-determined by a complex interplay of environmental, biological, and heatwave features.

Leaf temperatures in glasshouses and open-top chambers.

Climate manipulation experiments are of key importance in identifying possible responses of plant communities and ecosystems to climate change. Experiments for warming the air under sunlit conditions are carried out in (partial) enclosures. These inevitably alter the energy balance inside, potentially altering tissue temperatures which affect metabolism and growth. Using an empirically validated energy balance model, we investigate effects of two widely used warming methods, climate-controlled glasshouses and passively warmed open-top chambers (OTCs), on leaf temperatures. The model applies standard energy balance formulas, supplemented with data on optical properties of glasshouse materials and wind conditions inside OTCs. Results show that the different radiation environment inside glasshouses did not produce large leaf temperature deviations compared with outside. Poor glasshouse design with significant radiation blockage by the structure or with insufficient ventilation did affect tissue temperatures more significantly. The drastic wind speed reduction inside OTCs approximately doubled the actual (canopy) warming compared with earlier reported increases in air temperature provided by this technique - an effect that was inflated if the plants' stomates closed. These results demonstrate that leaf temperatures were higher than previously considered in OTCs but not in climate-controlled glasshouses.

Quantification of excess water loss in plant canopies warmed with infrared heating.

Here we investigate the extent to which infrared heating used to warm plant canopies in climate manipulation experiments increases transpiration. Concerns regarding the impact of the infrared heater technique on the water balance have been raised before, but a quantification is lacking. We calculate transpiration rates under infrared heaters and compare these with air warming at constant relative humidity. As infrared heating primarily warms the leaves and not the air, this method increases both the gradient and the conductance for water vapour. Stomatal conductance is determined both independently of vapour pressure differences and as a function thereof, while boundary layer conductance is calculated using several approaches. We argue that none of these approaches is fully accurate, and opt to present results as an interval in which the actual water loss is likely to be found. For typical conditions in a temperate climate, our results suggest a 12-15% increase in transpiration under infrared heaters for a 1 °C warming. This effect decreases when stomatal conductance is allowed to vary with the vapour pressure difference. Importantly, the artefact is less of a concern when simulating heat waves. The higher atmospheric water demand underneath the heaters reflects naturally occurring increases of potential evapotranspiration during heat waves resulting from atmospheric feedback. While air warming encompasses no increases in transpiration, this fully depends on the ability to keep humidity constant, which in the case of greenhouses requires the presence of an air humidification system. As various artefacts have been associated with chamber experiments, we argue that manipulating climate in the field should be prioritized, while striving to limit confounding factors. The excess water loss underneath infrared heaters reported upon here could be compensated by increasing irrigation or applying newly developed techniques for increasing air humidity in the field.

Urban warming increases the temperature sensitivity of spring vegetation phenology at 292 cities across China.

Urban spring phenology changes governed by multiple biological and environmental factors significantly impact urban ecosystem functions and services. However, the temporal changes in spring phenology (i.e., the start of the vegetation growing season, SOS) and the magnitude of SOS sensitivity to temperature in urban settings are not well understood compared with natural ecosystems. Therefore, we explored warming impacts on SOS across 292 rural and urban areas from 2001 to 2016. We found that warming occurred in 79.9% of urban areas and 61.3% of rural areas. This warming advanced SOS in 78.3% of the urban settings and 72.8% of the rural areas. The accelerated rate of SOS in urban settings was significantly higher (-0.52 ± 0.86 days/year) than in rural areas (-0.09 ± 0.69 days/year). Moreover, SOS was significantly more sensitive to warming in urban areas (-2.86 ± 3.57 days/°C) than in rural areas (-1.57 ± 3.09 days/°C), driven by urban-rural differences in climatic (precipitation, temperature, and warming speed) and vegetation factors. Precipitation contributed the most had the highest relative importance for controlling SOS, at 45% and 63% for urban and rural areas, respectively. These findings provide a new understanding of the impacts of urbanization and climate change on vegetation phenology. Moreover, our results have implications for urban environment impacts on ecosystems and human health.

Does previous exposure to extreme precipitation regimes result in acclimated grassland communities?

Climate change will likely increase weather persistence in the mid-latitudes, resulting in precipitation regimes (PR) with longer dry and wet periods compared to historic averages. This could affect terrestrial ecosystems substantially through the increased occurrence of repeated, prolonged drought and water logging conditions. Climate history is an important determinant of ecosystem responses to consecutive environmental extremes, through direct damage, community restructuring as well as morphological and physiological acclimation in species or individuals. However, it is unclear how community restructuring and individual metabolic acclimation effects interact to determine ecosystem responses to subsequent climate extremes. Here, we investigated, if and how, differences in exposure to extreme or historically normal PR induced long-lasting (i.e. legacy) effects at the level of community (e.g., species composition), plant (e.g., biomass), and molecular composition (e.g., sugars, lipids, stress markers). Experimental grassland communities were exposed to long (extreme) or short (historically normal) dry/wet cycles in year 1 (Y1), followed by exposure to an identical PR or the opposite PR in year 2 (Y2). Results indicate that exposure to extreme PR in Y1, reduced diversity but induced apparent acclimation effects in all climate scenarios, stimulating biomass (higher productivity and structural sugar content) in Y2. In contrast, plants pre-exposed to normal PR, showed more activated stress responses (higher proline and antioxidants) under extreme PR in Y2. Overall, Y1 acclimation effects were strongest in the dominant grasses, indicating comparatively high phenotypical plasticity. However, Y2 drought intensity also correlated with grass productivity and structural sugar findings, suggesting that responses to short-term soil water deficits contributed to the observed patterns. Interactions between different legacy effects are discussed. We conclude that more extreme PR will likely alter diversity in the short-to midterm and select for acclimated grassland communities with increased productivity and attenuated molecular stress responses under future climate regimes.

Climate warming shifts the time interval between flowering and leaf unfolding depending on the warming period.

The timing of flowering (FL) and leaf unfolding (LU) determine plants' reproduction and vegetative growth. Global warming has substantially advanced FL and LU of temperate and boreal plants, but their responses to warming differ, which may influence the time interval between FL and LU (∆LU-FL), thereby impacting plant fitness and intraspecific physiological processes. Based on twigs collected from two flowering-first tree species, Populus tomentosa and Amygdalus triloba, we conducted a manipulative experiment to investigate the effects of winter chilling, spring warming and photoperiod on the ∆LU-FL. We found that photoperiod did not affect the ∆LU-FL of Amygdalus triloba, but shortened ∆LU-FL by 5.1 d of Populus tomentosa. Interestingly, spring warming and winter chilling oppositely affected the ∆LU-FL of both species. Specifically, low chilling accumulation extended the ∆LU-FL by 3.8 and 9.4 d for Populus tomentosa and Amygdalus triloba, but spring warming shortened the ∆LU-FL by 4.1 and 0.2 d °C-1. Our results indicate that climate warming will decrease or increase the ∆LU-FL depending on the warming periods, i.e., spring or winter. The shifted time interval between flowering and leaf unfolding may have ecological effects including affecting pollen transfer efficiency and alter the structure and functioning of terrestrial ecosystem.

Dryland mechanisms could widely control ecosystem functioning in a drier and warmer world.

Responses of terrestrial ecosystems to climate change have been explored in many regions worldwide. While continued drying and warming may alter process rates and deteriorate the state and performance of ecosystems, it could also lead to more fundamental changes in the mechanisms governing ecosystem functioning. Here we argue that climate change will induce unprecedented shifts in these mechanisms in historically wetter climatic zones, towards mechanisms currently prevalent in dry regions, which we refer to as 'dryland mechanisms'. We discuss 12 dryland mechanisms affecting multiple processes of ecosystem functioning, including vegetation development, water flow, energy budget, carbon and nutrient cycling, plant production and organic matter decomposition. We then examine mostly rare examples of the operation of these mechanisms in non-dryland regions where they have been considered irrelevant at present. Current and future climate trends could force microclimatic conditions across thresholds and lead to the emergence of dryland mechanisms and their increasing control over ecosystem functioning in many biomes on Earth.