Plant species α-diversity is closely correlated with ecosystem structures and functions. However, whether climate change and human activities will reduce plant species α-diversity remains controversial. In this study, potential (i.e., potential species richness: SRp, Shannonp, Simpsonp and Pieloup) and actual plant species α-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 α-diversity across all the grassland systems. However, more than one-third areas showed decreasing trends for potential plant species α-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 α-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 α-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 α-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 α-diversity should be at least put on the same level as the impacts of climate warming and precipitation change on plant α-diversity.

The Qinghai-Tibet Plateau has experienced an increase in N deposition/input due to global change. However, it remains unclear how the responses and whether the responses of soil bacterial diversity to short-term N input are consistent at different levels. Here, we investigated soil bacterial species and phylogenetic α-diversity and community composition based on a short-term nitrogen input experiment (five levels: 0, 2.5, 5, 10, and 20 g N m−2 y−1) in an alpine steppe at the source of Brahmaputra, using high-throughput sequencing technology. Short-term nitrogen input did not affect the species α-diversity and β-diversity of soil bacteria. However, soil bacterial phylogenetic α-diversity and dissimilarity increased with increasing nitrogen input. Different relative contributions and correlations of primary factors to species and phylogenetic diversity under short-term nitrogen input may result in different responses, in which ecological processes also play a role. Therefore, studying the response of soil bacteria to short-term nitrogen input should take into account not only the species level but also the phylogenetic level. We should pay close attention to the potential influence of short-term nitrogen deposition/fertilization on the soil bacterial community in the alpine steppe on the Tibetan Plateau.