Ecological responses to climate change will likely occur in a nonlinear way. In fact, our limited understanding of ecological thresholds to combined effects of climate and disturbance regimes constrain our ability to predict community and ecosystem trajectories under global change. We therefore examined the responses from plant species to community-level (linear vs. nonlinear) to a precipitation gradient (+50%, 0% (control), -20 %, -40 %, -60 %, -80%, -100%) crossed with aboveground biomass clipping treatment (clipped vs. unclipped) in a fully factorial design (n=3, N=21). Annual clipping (hereafter, management) was applied to mimic hay harvest. We tracked responses at the plant-species, functional group (forbs and graminoids) and community (richness, evenness, plant species composition) levels. We then specifically asked: (1) What is the linearity (linear vs. nonlinear) of ecological responses across levels of organization in managed vs. unmanaged conditions?; and (2) In nonlinear relationships, what are the estimated thresholds at precipitation levels?
Results/Conclusions
Plant richness responded nonlinearly to a gradient in precipitation in unmanaged relative to managed conditions. When analyzing thresholds along the nonlinear fit, at the community-level, plant richness declined at different estimated breakpoints in the two years (2017 and 2108, respectively): -28% precipitation and -35% precipitation. Forbs had a similar response, however, in managed conditions only. Among forbs, three species were responsible to drive the nonlinear trend in managed plots. Breakpoints at precipitation occurred at drier conditions for the two common forbs: -59% precipitation (in 2018) and -76% (in 2017) precipitation, except for a third forb species: -20% precipitation (in 2018); graminoids showed no response to the precipitation gradient. Lastly, precipitation consistently led to an increase in over-dispersion (higher variability) in species composition in extreme drought than mild drought in the first year (2017: PERMDISP, F=3.93, P=<0.01), by contrast, opposite pattern occurred in the following year (2018: PERMDISP, F=4.47, P=<0.01). Taken together, functional group and community-level responses differed in response to management under climate change. Forbs responded to management under drought conditions, but overall plant community richness declines under drought and is not influenced by its interaction with management.