Adaptation to climate change: the importance of tree diversity for the resilience of forest ecosystems

Resilience is increasingly becoming a guiding principle for forest management, commonly used to describe the ability of a forest to maintain its fundamental structure and functioning despite altered by disturbances. Biotic and abiotic disturbances are an intrinsic element of forest ecosystems. However, climate change is creating substantial new challenges. Forest resilience is closely tied to tree species diversity, anchored in the notion that a wide range of species, which respond in different ways to changes, confers greater ecological stability. Droughts, in particular, are widely recognized as a major threat to forests. Recent drought events have been related to increased defoliation in European forests, as well as to significant reductions in gross primary production. Consequently, with droughts expected to become increasingly more frequent and severe in many regions, including in Belgium, there is a dire need to enhance our understanding of how forests respond to drought and how species dynamics change with major shifts in climate, so as to develop adaptive management strategies to increase the resilience of forest ecosystems.

In this research, we addressed this knowledge gap by providing a comprehensive assessment of drought impacts on forest health and forest growth, as well as of the effects of tree diversity on the resilience of forests to disturbances. To do so, we started by analyzing the temporal variation in the defoliation of Fagus sylvatica, Quercus petraea and Quercus robur trees, using crown defoliation as a proxy for forest health. Next, we investigated the growth dynamics of these species during recent drought events, and across a gradient of site conditions. Special attention was paid to differences between trees growing in mixed and pure stands.  This study builds on data from existing forest monitoring networks in Belgium, under ICP Forests programme, and from forest inventory plots in Flanders and Wallonia (Belgium).

To overcome the well-documented mismatch between  research and management, in a first step, we conducted a survey with the objective of identifying the extent to which specific adaptation measures to climate change are being implemented and, from the perspective of forest managers, which impediments limit their ability to prepare and respond to these changes. Our analysis highlighted a lack of adaptation responses so far, despite relatively high awareness of climate change as an issue in forest management. We suggest that providing clear, but detailed, and accessible information on how to manage forests as climate changes is likely to be the most effective strategy to mainstream adaptation.

In a second step, we demonstrate that the crown condition of the three species has declined appreciably over the past decades, in parallel with increasing water stress. This change has triggered a regime shift from healthy monocultures to resilient tree species mixtures. The observed tipping point in the relationship between species richness and the health of forests, suggesting that species interactions shifted from competition to facilitation, though implicit in the literature, has never been previously reported from real ecosystems.

Finally, the well documented, but not yet fully understood, relationship between species diversity and forest productivity was investigated. We describe individual tree and stand level growth rates, and examine how species richness influences tree growth response to drought. On the one hand, a strong and clear link emerged between a decline in tree productivity and the occurrence of drought events, associated with longer recovery times. On the other hand, we found that trees growing in mixtures have greater capacity to recover after drought events than trees growing in monocultures. Similarly, both F. sylvatica and Quercus spp. trees grew better in mixed stands than in pure stands. However, when we calculated the effect of species mixing on the productivity of each species at stand level, we did not confirm the previous finding. Here, using data from national forest inventories, we found that the positive effect of species mixing on the growth of F. sylvatica was counterbalanced by an underyielding of Quercus spp., and that the effect was not strong enough to significantly increase overall stand productivity.

Taken together, our results suggest that mixed species forests can better withstand drought stress, which confirms their greater resilience to projected changes in climate extremes and disturbance regimes. Managing forests to retain or increase diversity has the additional potential benefit to provide managers with more options for future stand development.