Drought rather warming increase the ratio of rare species by producing a larger quantity of low-quality foliar litter

The decomposition of foliar litter and the release of nutrients, which are sensitive to climate changes, are critical regulators of forest ecosystem function. Yet, how litter nutrient allocation responds to frequently elevated temperature and drought, and whether they mediate the formation of plant species richness at the community level under predicted climate patterns are unknown. In this study, we monitored the long-term dynamic changes in concentrations and amounts of nitrogen (N) and phosphorus (P) of fallen foliar by 132 litter traps, and explored the associations with changes in plant species richness with 870 seedling quadrats surrounding from 2015 to 2022 in a 60-ha tropical rainforest plot in China. During the eight years, as the annually averaged temperature increased, foliar litter N concentration significantly decreased. When annual precipitation decreased, foliar litter P concentration also significantly decreased. Correspondingly, with the decrease in precipitation, the ratios of N:P concentrations only showed a marginally significant increase. This N and P nutrient depletion significantly decreased the litter quality, but did not change the total nutrient amount allocated in litters. More interestingly, the rare species ratios of surrounding seedlings increased significantly and positively correlated with the foliar litter N:P ratios by the indirect regulation of decreasing precipitation, which might cause the increase of fresh leaves N:P to facilitate the rare species. Therefore, under the climate change scenario, there is a subtle balance to maintain the nutrient cycles and species richness, which was unnoticed before.