Intraspecific trait variation and species turnover mediate grazing impacts on above- and below-ground functional trait composition of plant communities

Although grazing has significant impacts on plant functional traits and
community composition in grasslands, few studies have simultaneously
explored how plant above- and below-ground traits and community functional
composition respond to grazing. Using a grazing manipulation experiment
with seven levels of grazing intensity (0, 1.5, 3.0, 4.5, 6.0, 7.5, and 9
sheep ha-1) in the Inner Mongolia grassland, we partitioned the roles of
intraspecific trait variation (ITV) and species turnover underlying the
grazing induced changes in above- and below-ground functional trait
composition of plant communities.  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-)
were measured at the plant individual level.  At the community
level, plant above-ground traits shifted towards grazing avoidance
strategy (e.g. plant height and leaf area decreased), and below-ground
traits shifted towards conservative strategy (i.e. 1st-SRL and 2nd-SRL
decreased, 1st-RTD and 2nd-ARD increased) with increasing grazing
intensity.  Functional tradeoffs were found between plant
individual biomass and plant density, and between leaf area and leaf
density under grazing.  However, community-weighted mean SRL
(SRLCWM) and ARD (ARDCWM) of different root orders exhibited functional
coordination under grazing pressure.  SLACWM and SRLCWM also
showed synergistic responses to grazing.  The ITV plays a
predominant for the changes in above- and below-ground functional trait
composition at the community level.  However, changes in mean
trait values among plant species with different resource use strategies
were mainly triggered by species turnover.  For species with
different resource use strategies, grazing exhibited a coordinated effects
on ITV but an offset effect on species turnover. Synthesis. Our results
demonstrate that both the above- and below-ground trait composition of
plant communities shifted toward conservative strategy under long-term
grazing.  This study highlights the effects of ITV and species
turnover that govern the grazing-induced changes in functional trait
composition of plant communities, and has important implications for
grazing management in semiarid grasslands.