Shrub removal suppresses the effects of warming on nematode communities in an alpine grassy ecosystem

Climate warming is a key driver of changes in ecosystem structure and function, with well-documented effects on the vegetation aboveground. Warming can also influence soil organisms both directly and indirectly through impacts on vegetation composition and edaphic properties. The predicted increase in shrub encroachment in grassy alpine ecosystems on the Tibetan plateau due to warming is therefore likely to cause significant impacts belowground. We explored how a dominant shrub moderates the effect of warming on soil nematode richness and abundance in a grassy ecosystem on the Qinghai-Tibet Plateau. We used structural equation modelling (SEM) to examine effects on nematode assemblages through shifts in understory composition, edaphic properties, and soil microbial communities. We found that warming increased nematode richness and abundance, but only when shrubs were present. Similarly, warming changed nematode community composition, mainly due to increased abundance of fungivores and omnivores, only when shrubs were present. In addition, warming impacted nematode-based indices indicative of changes in ecosystem structure and function. The SEM revealed that warming and shrub removal interactively reduced nematode richness and the enrichment index directly. Shrub removal thus suppresses the positive effects of warming on nematode richness, abundance, and nematode-based indices in alpine grassy ecosystems. By inference, our results indicate that the effect of warming on soil fauna community diversity and structure in grassy alpine ecosystems will be exacerbated by shrub encroachment.