Functional leaf traits are likely to play a major role in how plant communities respond or not to changes in climate and land use. At the community level, functional leaf traits may respond through either species turnover or intraspecific trait variation, but little research has evaluated the relative contribution of the two. We set up a field experiment to examine how climate change will affect the semiarid, northern Mongolian steppe, which supports pastoral nomads as it has for millennia. We looked at the consequence of short term warming, which is predicted to occur more frequently in the future, and also the consequence of livestock grazing cessation. We examined the effect of grazing and warming by open top chambers (OTCs) on six common leaf functional traits (leaf area, specific leaf area, leaf length, leaf dry matter content, leaf nitrogen content and leaf carbon content). Using a single trait approach, we calculated both the community weighted mean (CWM) and community level functional diversity (FD) or trait dispersion, using the Rao index. Based on a variance partitioning approach, we evaluated to what extent the responses of CWM and FD to OTC and grazing are attributable to intraspecific trait variation and species turnover.
Results/Conclusions
We found that the relative importance of intraspecific and interspecific variation depends on the trait and differs in response to the OTC and grazing. For interspecific trait variation, the OTC negatively impacted CWM for four of the six traits and had no effect on FD for any trait, while grazing did not affect either index. When intraspecific and interspecific variation were considered together in CWM and FD calculations, the OTC still negatively impacted CWM for all the traits, but the effect was less severe where grazing also occurred. Including intraspecific trait variation also helped us capture a positive response of FD in some traits to grazing, which was not detected when focusing on interspecific trait variation alone. Our results imply that neglecting intraspecific trait variation could underestimate community responses to environmental changes and show that grazing can mitigate the effects of climate change on plant traits. Both the effect of intraspecific trait variation and changes in species turnover must be taken into account when assessing community level trait responses to environmental factors. Moreover, we emphasize that climate change effects must be considered in the context of altered land use to gain a better understanding of its likely impacts.