Relative humidity predominantly determines long-term biocrust-forming lichen survival under climate change

1. Manipulative experiments show a decrease in dryland biological soil crust cover and altered species composition under 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 studies conducted so far. 2. We addressed this gap and successfully parameterized a process-based model for the biocrust-forming lichen Diploschistes diacapsis as a common and globally distributed representative of biocrust communities 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. 3. The model reproduced observed experimental findings: warming reduced lichen cover whereas less rainfall had no effect. This warming effect was caused by the associated decrease in relative humidity and non-rainfall water inputs as major water sources for lichens. Warming alone, however, increased cover because higher temperatures promoted photosynthesis during the cool morning hours with high lichen activity. When combined, climate variables showed non-additive effects on lichen cover, and fertilization effects of CO2 leveled off with decreasing levels of relative humidity. 4. 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 dryland lichens under climate change and that CO2 fertilization effects might be offset by a reduction in non-rainfall water inputs in the future. Because of a global trend towards warmer and thus drier air, this will affect lichen-dominated dryland biocrust communities and their role in regulating ecosystem functions, worldwide. ### Competing Interest Statement The authors have declared no competing interest.