Intraspecific trait variation governs grazing-induced shifts in plant community above- and below-ground functional trait composition

Abstract

In grasslands, grazing can regulate the functional trait composition of plant communities through intraspecific trait variation and species turnover. However, it is not well understood how these variables affect specific plant traits. Here, we partitioned the relative roles of intraspecific trait variation and species turnover to control grazing-induced shifts in community functional composition using a field manipulation experiment with seven levels of grazing intensity in the Inner Mongolia grassland. We concurrently measured six aboveground traits (i.e. plant height, plant aboveground biomass, plant density, leaf area, specific leaf area, SLA; and leaf density) and three root traits (average root diameter, ARD; specific root length, SRL; and root tissue density, RTD) of the first-, second- and third-order roots (1st-, 2nd-, and 3rd-). Our results showed that plant above-ground traits shifted towards grazing avoidance strategies (i.e. plant height and leaf area decreased) as grazing intensity increased, and below-ground traits shifted towards conservation strategies (i.e. 1st-SRL and 2nd-SRL decreased, 1st-RTD and 2nd-ARD increased). Additionally, functional tradeoffs were found between plant individual biomass and plant density, as well as between leaf area and leaf density under grazing. However, community-weighted mean SRL (SRLCWM) and ARD (ARDCWM) exhibited functional coordination under the grazing pressures. Altogether, shifts in community functional trait composition were predominantly governed by intraspecific variation in above- and below-ground traits, while changes in mean trait values among plant species with different resource strategies were mainly triggered by species turnover. Our results demonstrated that both above- and below-ground trait compositions in plant communities shifted towards conservation strategies under high levels of grazing intensity. Ultimately, this study highlights the important role of intraspecific trait variation in governing grazing-induced changes in the community above- and below-ground functional trait composition, which advances fundamental knowledge on the mechanisms of plant community assembly in semiarid grasslands.