Warming decreases the desert ecosystem functioning of global drylands by altering biocrust cryptogams

Climate projections predict that warming, drought frequency and severity, and water stress will increase in drylands at rates faster than the global means. Notably, drylands, which cover more than 41% of Earth’s terrestrial surface, support various desert ecosystems. An increasing amount of solid evidence indicates that considering biocrusts is of paramount importance when assessing the direct and indirect impacts of climate change on global desert functioning and highlights the significance of biocrusts as modulators of these impacts. Changes in the precipitation regime have stronger negative impacts than warming on the biocrust component and structure, as well as on the linked ecosystem functioning; however, the impacts of warming coupled with precipitation alterations are more prominent. Climate change induces inconsistent responses to warming and precipitation alteration in the biocrust cryptogams, such as mosses and lichens. Warming coupled with precipitation alterations contributes to reducing the wet period and biocrust water availability for lichens and mosses to fix atmospheric CO₂ and N₂, leading to drought stress and a potential biocrust backslide from the late successional stage to the early stage, which in turn results in global decreases in dryland carbon and nitrogen due to deficits in biocrust carbon sequestration and nitrogen fixation. Alterations of the water balance can also occur when warming influences infiltration, runoff, non-rainfall water entrapment, evaporation and soil water reallocation. These changes in the biocrust ecosystem functioning are unlikely to be conducive to both passive and artificially facilitated eco-restoration of drylands. Conversely, to a large extent, the presence of well-developed biocrusts regulates and alleviates the negative impacts of climate change on dryland ecosystems.