Large-scale remote sensing exploration of the resilience of Iberian holm oak woodlands to extreme droughts

The organization and function of forest ecosystems is changing at an unprecedent rate under the influence of external drivers, such as climate change and human uses. In this context of change, the increasing frequency, intensity and duration of droughts that affect the Mediterranean basin urges to evaluate forest vulnerability to drought and their effects for the long-term stability of these ecosystems. In this study, we explore the responses and factors that control the resilience of holm oak (Quercus ilex) woodlands to recent extreme droughts during 2000-2019, using a remote sensing approach over a broad scale climate aridity gradient that extends from sub-humid to semi-arid climate conditions over 100,000 km² in eastern Spain. Overall, our results indicated that climate aridity and forest structure largely control the resistance, recovery and resilience of the studied holm oak woodlands to drought, which are also affected by drought intensity and both pre- and post-drought hydric conditions. We found that Q. ilex woodlands located in the dry edge of the explored climate aridity gradient showed a high sensitivity to extreme drought. Their resistance, assessed as the capacity of the woodlands to maintain primary production during drought, was low. They also showed a poor resilience, characterized by a low capacity to fully recover their pre-drought production levels. Contrarily, holm oak woodlands in sub-humid areas of the study region, where wetter climate conditions may alleviate water stress during dry periods, showed a high resistance and resilience to the effects of droughts. Drought vulnerability was particularly high for dense holm oak stands developed under semi-arid climate conditions, where strong competition for scarce water resources largely increased the negative effects of extreme drought on landscape-level ecosystem production. We also discuss the implications of these results for adaptive management of holm oak forests in the present context of climate change.