A large number of observational and experimental studies have shown that nitrogen (N) enrichment and changes in precipitation can alter plant community structure and composition. These studies have found that N addition tends to reduce species richness, whereas increasing water availability tends to elevate species richness, especially in arid and semi-arid area where water availability is the primary limiting resource. The mechanisms underlying their effects, however, remain unclear. Traditional studies of community structure and species turnover do not consider the evolutionary relationships among species. Incorporating information on phylogenetic relatedness among species may improve our understanding of how and why communities vary in their responses to N addition and increased precipitation. Here, we investigated the influence of increased N and precipitation on plant phylogenetic diversity and phylogenetic structure, based on a seven-year field experiment with N fertilization and precipitation addition in the temperate semi-arid grassland in Inner Mongolia, China.
Results/Conclusions
Our results showed that, species richness and Faith’s phylogenetic diversity increased with increasing precipitation, but decreased in both N addition and control plots. Declines in species diversity in both N addition and control plots were likely due to the random extinctions of locally rare species, because the initial abundances of extinct species were significantly lower than the remaining species, and N addition accelerated this process. Community phylogenetic structure remained clustered in control plots, but changed towards phylogenetical overdispersion after N addition. This was driven by the emergence of species that are distantly related to the local species after N fertilization. Increasing precipitation prevented some species from extinction, and facilitated the emergence of new colonists, without altering community phylogenetic structure. Our findings suggest that locally rare species are most susceptible to extinction under elevated N deposition. Our findings also indicate that it is species gain, rather than the widely believed species loss, that plays the most important role in changing community phylogenetic structure after N fertilization.