Tree diversity mitigates defoliation after a drought‐induced tipping point

Abstract

Understanding the processes that underlie drought-related tree vitality loss is essential for anticipating future forest dynamics, and for developing management plans aiming at increasing the resilience of forests to climate change. Forest vitality has been continuously monitored in Europe since the acid rain alert in the 1980s, and the intensive monitoring plots of ICP Forests offer the opportunity to investigate the effects of air pollution and climate change on forest condition. By making use of over 100 long-term monitoring plots, where crown defoliation has been assessed extensively since 1990, we discovered a progressive shift from a negative to a positive effect of species richness on forest health. The observed tipping point in the balance of net interactions, from competition to facilitation, has never been reported from real ecosystems outside experimental conditions; and the strong temporal consistency of our observations with increasing drought stress emphasizes its climate change relevance. Furthermore, we show that higher species diversity has reduced the severity of defoliation in the long term. Our results confirm the greater resilience of diverse forests to future climate change-induced stress. More generally, they add to an accumulating body of evidence on the large potential of tree species mixtures to face manifold disturbances in a changing world.

Discerning Effects of Climate Variability, Local Land Degradation and Tree Species Diversity on Remote Sensing Derived Resilience and Resistance of Dry Afromontane Forest 

“…Generally, tree diversity was associated with resilience, yet the Shannon and evenness indicators had a different impact. In the literature, there are contradicting findings on the effect of diversity on stability, where positive effect of species diversity has been reported in grasslands (Tilman et al 2006;Van Ruijven and Berendse 2010), and in forests across Europe (Guyot et al 2016, Sousa-Silva et al 2018, Vannoppen et al 2019, while others argue that there is no true positive diversity effect found so far on resilience (Bauhus et al 2017). We found a positive association of Shannon diversity with resilience, but saturating eventually.…”

“…Forest stability was successfully characterized using resilience, resistance and variability from remotely sensed imagery in different forests (Sousa-Silva et al 2018;Frazier et al 2018). In Desa'a, a dry tropical Afromontane forest, the four stability metrics were modelled.…”

Increasing the broad-leaved tree fraction in European forests mitigates hot temperature extremes

“…Potential benefits of an increased BTF are a decreased risk of fires, wind throw and bark beetle outbreaks 15 , 17 , 59 . An increase in the BTF may be particularly beneficial if it leads to higher biodiversity 16 , 60 . Thus, rather than a complete conversion from needle-leaved to broad-leaved-based forests, a more moderate increase in BTF preserving species diversity may perform better across various sustainability indicators 14 .…”

Forest resilience under global environmental change: Do we have the information we need? A systematic review

“…Interestingly, there is almost no empirical or experimental evidence about how diversity may affect plant performance after disturbance (but see [23,25,79]). This highlights an area for future research, as diverse ecosystems are thought to be more resistant to disturbances caused by insect pests, diseases, and drought [21,78,80,81]. If recovery were also to be affected by diversity, then management practices geared toward increasing diversity should be promoted [82].…”

“…Furthermore, forests can be highly resilient to a disturbance they evolved with, e.g., fire-prone environments, but highly susceptible to new stressors, e.g., introduction of nuisance species [17], or to changes in the disturbance regime [18,19]. Diverse communities have higher chances of containing species that can contribute to recovery processes [20]; for example, higher tree species diversity mitigates defoliation in forests after drought [21]. The functional diversity associated with species richness also seems to be tightly related to resilience [22], e.g., in temperate and boreal forests hydraulic diversity increases forest resilience to drought [23].…”