Leaf trait plasticity reveals interactive effects of temporally disjunct grazing and warming on plant communities

Abstract

Changes in climate and grazing intensity influence plant-community compositions and their functional structure. Yet, little is known about their possible interactive effects when climate change mainly has consequences during the growing season and grazing occurs off growing season (dormant season grazing). We examined the contribution of trait plasticity to the immediate responses in the functional structure of plant community due to the interplay between these two temporally disjunct drivers. We conducted a field experiment in the northern Mongolian steppe, where climate was manipulated by open-top chambers (OTCs) for two growing seasons, increasing temperature and decreasing soil moisture (i.e., increased aridity), and grazing was excluded for one dormant season between these two growing seasons. We calculated the community-weighted mean (CWM) and the functional diversity (FD) of six leaf traits. Based on a variance partitioning approach, we evaluated how much of the responses in CWM and FD to OTCs and dormant season grazing occur through plasticity. The interactive effect of OTCs and the dormant season grazing were detected only after considering the role of trait plasticity. Overall, OTCs influenced the responses in CWM more than in FD, but the effects of OTCs were much less pronounced where dormant season grazing occurred. Thus, warming (together with decreased soil moisture) and the elimination of dormant season grazing could interact to impact the functional trait structure of plant communities through trait plasticity. Climate change effects should be considered in the context of altered land use, even if temporally disjunct.

Keywords: Climate change; Community-weighted mean; Functional diversity; Grazing; Intraspecific trait variation.

Fig. 1

Community-weighted means (log-transformed) for six leaf functional traits: a and b Leaf area, c and d Leaf length, e and f Leaf carbon content, g and h Leaf dry matter content, i and j Leaf nitrogen content, k and l Specific leaf area, in the four combinations of OTC and grazing. The interspecific variability (Inter) averages are based on species trait values that are averaged over all treatments, so variation between treatments in (Inter) CWM reflects variation in species representation. Both differences in species abundances and treatment induced changes in intraspecific variability (i.e., plasticity) contribute to (Inter + Intra) CWM for a given combination of OTC and grazing. Error bars denote standard errors

Fig. 2

Functional diversity for six leaf functional traits: a and b Leaf area, c and d Leaf length, e and f Leaf carbon content, g and h Leaf dry matter content, i and j Leaf nitrogen content, k and l Specific leaf area, in the four combinations of OTC and grazing. As in Fig. 1, interspecific variability (Inter) averages are based on variation in species abundances but constant trait values per species across treatments, while the interspecific and intraspecific variability averages (Inter + Intra) FD reflect both variation in species abundances and also trait plasticity within a species for a given combination of OTC and grazing. Error bars denote standard errors