Protecting forests: The buffering role of biodiversity under droughts

The occurrence of drought events has historically been a natural disturbance agent for ecosystems in many regions across the globe, and has contributed to shaping ecological processes and evolutionary adaptations. However, under anthropogenic climate change and unsustainable management of water resources, droughts are increasing in frequency, intensity and duration, causing levels of stress and amplifying feedbacks that can drive ecosystems such as forests beyond their resilience thresholds. In this context it has been suggested that more biodiversity, in its multiple dimensions, might increase forest resilience, acting as a buffer against the impacts of droughts and their associated risks for forest health, such as bark beetle outbreaks. To uncover to what extent biodiversity’s buffering role has been investigated by the scientific community, we perform a systematic literature review covering more than 300 articles over the past ten years. Through this review, we investigate the drivers of drought vulnerability and resilience for forests worldwide. Contrary to the researchers’ expectations, the review revealed an incidental rather than comprehensive body of academic literature on the drought risk mitigating effects of biodiversity. Still, interesting findings emerge from the review. These range from the direct and indirect mitigating effects of mixed forest cover on insect outbreak, to the positive influence of functional and taxonomic diversity on post-drought tree growth recovery and resilience. Different mechanisms might lie behind such effects: The first finding possibly exemplifies a case of associational resistance, in which confounding olfactory signals in mixed forests hinder bark beetles in identifying host trees. In addition, mixed forest structure can contribute to temperature cooling and preservation of water resources, reducing tree stress and susceptibility during droughts. Similarly, the latter finding is likely due to resource partitioning in functionally diverse stands and thus decreased competition between trees. Importantly, the results of the review also emphasize that the role played by biodiversity as driver of resilience is modulated by drought severity. In addition to discussing key patterns that emerged from the review, other outputs of this research include the visualization of drivers’ connections through conceptual models and an openly accessible database storing all drivers gathered during the review.