The transient response of ecosystems to climate change is amplified by trophic interactions

Studies of ecosystem responses to climate change often focus on potential equilibria in species or community distributions, overlooking the transitions to new equilibrium states. Transient phases can however last for decades or centuries, during which both demography and interspecific interactions are expected to play a crucial role. Here, we investigate the response of vegetation to climate warming at high latitudes involving a shift from open vegetation to either boreal (mainly coniferous) or temperate (mainly deciduous) forests. We specifically address how interactions among browsers and vegetation could affect the shift in dominance of vegetation type after climate warming. We characterize the transient dynamics using five measures: 1) asymptotic resilience, i.e. the rate at which equilibrium is restored, 2) initial resilience, the short-term rate of change of the ecosystem after climate change, 3) ecosystem exposure, i.e. the shift of the equilibrium due to climate change, 4) sensitivity, or the time to recover equilibrium, and 5) vulnerability, measured as the cumulative amount of changes in vegetation states during the transient phase. We find that plant-herbivore interactions usually extend the length of the transient period and induce more cumulative changes in vegetation types. This result implies that the consideration of multiple interacting species is necessary to provide robust scenarios of how ecosystems will respond to global changes. We furthermore show that plant-herbivore interactions disrupt the correlation between the five measures characterizing the transient dynamics, highlighting the need for a full multidimensional characterization of transients.