Warming and nitrogen deposition are critical abiotic factors to the plant community shift in Qinghai-Tibet Plateau. Fine-root is not only the organ which responds to these factors but also relevant to species co-existing. In this study, we grew three and seven species from grass, sedge and forb as single- and double-species under warming and nitrogen deposition, respectively. We compared the functional traits between single and double-species to estimate the response to neighbor and thereafter estimate how warming and fertilization change the response.
The following hypotheses were proposed: (1) Fine-root functional traits may exhibit convergence or divergence between neighboring plants compared to single species. According to the competitive hierarchy theory, coexisting species with similar traits reduce the risk of competitive exclusion. Conversely, according to the limiting similarity theory, coexisting species minimize competition by increasing niche differentiation. (2) Warming may change the degree of convergence or divergence of fine-root functional traits among neighboring plants. On one hand, warming could decrease competition by enhancing microbial activity and increasing soil nutrient availability. On the other hand, warming may increase competition by leading to asymmetric interspecific competition through promoting plant growth or by increasing soil moisture stress. (3) Nitrogen addition may also change the degree of convergence or divergence of fine-root functional traits among neighboring plants. On one hand, nitrogen addition may decrease root competition for nutrients. On the other hand, nitrogen addition may also increase competition through mechanisms such as promoting asymmetric interspecific competition via stimulated plant growth, triggering soil acidification, or other ways.
In the control of both warming and nitrogen addition, fine-root trait differences between neighboring species in mixed plantings exhibited a convergence pattern compared to the trait differences observed in monoculture. Furthermore nitrogen deposition lead to greater dissimilarity between the neighbors. On contrast, warming significantly decreased the convergence degree, decreasing the negative correlation between trait dissimilarity and neighbor effect.