Extreme drought alters competitive dominance within and between tree species in a mixed forest stand

1. The effect of extreme climate events on ecosystems is an important driver of biotic responses to climate change. For forests, extreme drought has been linked to negative effects such as large-scale mortality and reduced primary production. However, the response of plant communities to extreme drought events remains poorly understood. 2. We used mortality data from a long-term monitoring programme in the core of the focal species' ranges, in combination with annual growth data from tree-rings, to study the effect of, and recovery from, an extreme drought event. We examined both the intraspecific and interspecific drought response and explored how differential responses affect competitive dominance between the dominant species Fagus sylvatica and Quercus petraea. 3. Mortality for the most drought-susceptible species, F.sylvatica, occurred alongside a temporary reduction in competition-induced mortality of Q.petraea, resulting in the long-term alteration of the relative abundance of the two species. 4. Significant intraspecific variation occurred in post-drought recovery in surviving F.sylvatica, with two distinct cohorts identified. A prolonged recovery period was coupled with the failure to regain pre-drought growth levels in this species, whereas for Q.petraea, no severe drought impacts were observed. This species instead experienced competitive release of growth. 5. Our results demonstrate that ecosystem responses to extreme drought can involve rapid, nonlinear threshold processes during the recovery phase as well as the initial drought impact. These sudden changes can lead to the reordering of dominance between species within communities, which may persist if extreme events become more frequent.