Biocrust Functional Traits Exhibit Divergent Responses to Future Climate–Land Use Scenarios in an Arid Region of Northern China

Biocrusts are critical yet threatened components of dryland ecosystems, and predicting their functional trait dynamics under future scenarios is essential for conservation planning. Using 129 occurrence localities and 84 trait sampling sites across three precipitation zones in China’s Mu Us Sandland, we combined MaxEnt habitat modeling with Random Forest regression to predict biocrust functional traits—including coverage, thickness, and total volume for both moss and cyanobacterial crusts—under current conditions and 12 future climate–land use scenarios (four SSPs × three time periods: 2050s–2090s). Soil nitrogen, annual precipitation, and soil potassium emerged as key environmental drivers of biocrust habitat distribution. Currently, moss crusts cover 7.63% of the study area (thickness: 10.56 mm) and cyanobacterial crusts cover 5.88% (thickness: 4.88 mm), with a total biocrust volume of 4629 × 104 m3. Across the emission and policy gradient, functional traits exhibited contrasting responses: coverage showed scenario-dependent declines, while thickness remained relatively stable. Under SSP126, moss coverage declined by 3.32% and cyanobacterial coverage by 2.80% by the 2070s, with total volume decreasing by 2064.76 × 104 m3; by the 2090s, moss coverage partially recovered (+0.26%). In contrast, SSP370 and SSP585 projected sustained losses without recovery. A striking divergence emerged: cyanobacterial thickness increased consistently (+0.02 to +0.23 mm) even as coverage declined, while moss thickness fluctuated within ±0.13 mm. Notably, high-precipitation transitional zones (362–434 mm) exhibited the greatest vulnerability, with moss coverage declining 3× more under SSP126 than SSP585 by the 2070s and volume losses persisting through the 2090s. These findings provide spatially explicit predictions of biocrust traits and quantitative baselines for prioritizing conservation in transitional zones facing accelerating environmental pressures.