Impacts of Climate Change and Human Activities on Plant Species a-Diversity across the Tibetan Grasslands

Plant species a-diversity is closely correlated with ecosystem structures and functions. However, whether climate change and human activities will reduce plant species a-diversity remains controversial. In this study, potential (i.e., potential species richness: SRp, Shannonp, Simpsonp and Pieloup) and actual plant species a-diversity (i.e., actual species richness: SRa, Shannona, Simpsona and Pieloua) during 2000-2020 were quantified based on random forests in grasslands on the Tibetan Plateau. Overall, climate change had positive influences on potential plant species a-diversity across all the grassland systems. However, more than one-third areas showed decreasing trends for potential plant species a-diversity. Climate change increased the SRp at rates of 0.0060 and 0.0025 yr(-1) in alpine steppes and alpine meadows, respectively. Temperature change predominated the variations of Shannonp and Simpsonp, and radiation change predominated the variations of SRp and Pieloup. Geography position, local temperature, precipitation and radiation conditions regulated the impacts of climate change on potential species a-diversity. On average, human activities caused 1% plant species loss but elevated the Shannon, Simpson and Pielou by 26%, 4% and 5%, respectively. There were 46.51%, 81.08%, 61.26% and 61.10% areas showing positive effects of human activities on plant species richness, Shannon, Simpson and Pielou, respectively. There were less than 48% areas showing increasing trends of human activities' impacts on plant species a-diversity. Human activities increased plant species richness by 2% in alpine meadows but decreased plant species richness by 1% in alpine steppes. Accordingly, both the impacts of climate change and human activities on plant species a-diversity were not always negative and varied with space and grassland types. The study warned that both climate change and human activities may not cause as much species loss as expected. This study also cautioned that the impacts of radiation change on plant species a-diversity should be at least put on the same level as the impacts of climate warming and precipitation change on plant a-diversity.