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Biocrusts are critical biological components of drylands and play an important role in soil carbon (C) cycling. However, the effect of biocrusts on soil CO 2 exchange across global gradients of temperature and moisture is poorly understood. Moreover, their response to climate change remains highly uncertain. Bayesian hierarchical meta-analyses were performed on 47 published studies to quantify the impact of biocrusts on net soil exchange (NSE) of carbon- the difference between respiration and photosynthesis. Meta-analyses were also used on 23 studies to examine the effects of experimental warming on NSE in biocrusts. Meta-regressions further explored the thermal and wetness sensitivities of biocrust NSE and potential adaptation of biocrust responses to climate change. The development of biocrusts in dryland soils significantly increased NSE by 66.5 [22.2, 112.2] g C m−2yr−1, despite seasonal fluctuations, indicating a net loss of carbon to the atmosphere. Experimental warming, on average, increased biocrust NSE by 22.9 [-0.1, 40.8] g C m−2yr−1 per °C. However, across the spatial climate gradient, aridity limited the effects of warming, while high temperature decreased the thermal sensitivity of biocrust NSE, thus supporting the thermal adaptation of biocrusts. These results emphasize the critical role of biocrusts in modulating soil carbon exchange in response to climate warming across drylands, with particularly high thermal sensitivity in cool and moist regions. This highlights the need to incorporate biocrusts into global carbon budgets and models for a comprehensive understanding of their impact on the carbon cycle. • Biocrusts increase soil CO2 efflux by 66.5 g C m-2yr-1 in drylands worldwide. • Climate warming increases CO2 efflux in biocrust soil by 22.9 g C m-2yr-1 per °C. • Thermal adaptation reduces thermal sensitivity of biocrust carbon efflux. [ABSTRACT FROM AUTHOR]

• Effect of sowing date on biomass partition varied with forage crop species. • Aboveground biomass at flowering was dependent on rainfall and solar radiation. • Aboveground biomass at maturity more reliant on temperature and rainfall. • Allometric approach should be used in the analysis of allocation pattern. Biomass production and partitioning are important for crop production and is influenced by environmental conditions. Sowing date can profoundly influence biomass production and portioning in crop plants depending upon the local climatic conditions. However, the accumulation of biomass in different plant components in annual forages in response to changes in sowing date has not been quantified, especially for semiarid farming systems. A field experiment was conducted during 2011–2013 to investigate how sowing date (early, mid, late) influences biomass partitioning and the interrelationships with environments in four warm-season annual crops in the Loess Plateau of China. The aboveground biomass of all four crops was more reliant on rainfall and solar radiation at flowering and temperature and rainfall at maturity. Solar radiation, rainfall, and temperature had a combined effect on belowground biomass (0–0.3 m), depending on the forage crop. Reductions in biomass in the late-sown treatment were mostly attributed to plant size; the 'allometry' among plant components did not change. The changes of shoot: root and leaf: stem ratios were related to the relationship between biomass accumulation and rainfall, temperature and intercepted radiation. It is evident that dry matter allocation patterns in crops must be analyzed using a dynamic approach (allometry) and relate with environmental conditions, rather than a static approach using ratios. The results from this study provide references for crop management in forage production and breeding in semiarid areas. [ABSTRACT FROM AUTHOR]

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

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

Complex systems can undergo abrupt state transitions near critical points. Theory and controlled experimental studies suggest that the approach to critical points can be anticipated by critical slowing down (CSD), that is, a characteristic slowdown in the dynamics. The validity of this indicator in field ecosystems, where stochasticity is important in driving transitions, remains unclear. We analyze long-term data from a dryland ecosystem in the Shapotou region of China and show that the ecosystem underwent an abrupt transition from a nearly bare to amoderate grass cover state. Prior to the transition, the systemshowed no (or weak) signatures of CSD but exhibited expected increasing trends in the variability of the grass cover, quantified by variance and skewness. These surprising results are consistent with the theoretical expectation of stochastically driven abrupt transitions that occur away from critical points; indeed, a driver of vegetation--annual rainfall--showed rising variance prior to the transition. Our study suggests that rising variability can potentially serve as a leading indicator of stochastically driven transitions in real-world ecosystems. [ABSTRACT FROM AUTHOR]

When restoring dryland ecosystems, growing biological soil crust (biocrust) may greatly change the redistribution of rainfall in layered soils. However, ecohydrological modelling studies generally ignore biocrust and thus, the ecohydrological effects of biocrust on restorations remain largely unexplored. Using a long-term restoration case (located in the southeast edge of the Tengger Desert, northern China), we developed an ecohydrological model with explicit consideration of the infiltration in three layered soils (biocrust, shallow and deep sand layers) to investigate influences of biocrust on restoration dynamics in drylands. The proportion of infiltration that reaches ‘annual grass’ (including biocrust and shallow sand layers, 0–30 cm) and ‘shrub’ layers (30–150 cm) with biocrust significantly increased and decreased relative to the values without biocrust, respectively. Meanwhile, biocrust significantly decreased soil water content in deep sand layer, but not in shallow sand layer. As more water was used by transpiration than evaporation, the ecosystem with biocrust reached a final grass-dominated state (high grass cover of 40%, low shrub cover of 4%) rather than a shrub-dominated state (grass cover of 3%, shrub cover of 20%). This study suggests that we need to account for the roles of biocrust on rainfall infiltration to better understand vegetation and restoration dynamics in dryland ecosystems. [ABSTRACT FROM AUTHOR]