Deadwood and Tree-related Microhabitat's abundance and diversity are determined by the interplay of drought-induced die-off and local climate

Global climate change is increasing the frequency and severity of drought events that cause tree mortality worldwide, particularly in temperate and Mediterranean regions. Researchers have extensively studied the impact of such drought events on forest growth and tree mortality. However, knowledge on the aftermath of such mortality on habitat provision for biodiversity (i.e., abundance, richness, and diversity of deadwood in different decaying stages and tree-related microhabitats) is limited. In this study, we aimed to quantify the impact of drought die-off events on these biodiversity habitat proxies by sampling twenty sites comprised of paired plots (i.e., drought die-off vs control) in Scots pine (Pinus sylvestris) dominated forests in Catalonia (NE Spain), also considering the influence of forest structure and local climate. We used deadwood and Tree-related Microhabitats (TreM) as proxies for potential biodiversity, basal area as a descriptor of forest structure, and precipitation and temperature to characterize local climate. We found that die-off plots exhibited higher abundance (83%), richness (23%), and diversity (20%) of deadwood than control ones. We also observed that standing (snags) and downed (logs) deadwood compartment showed high abundance (45% and 56%, respectively), richness (26% and 25%, respectively) and diversity (18% for both snags and logs) of TreM. Yet, we found that the presence of deadwood in different decaying stages was also shaped by local climate factors: i.e., wetter sites contained higher proportion of recent deadwood and lower proportion of decaying deadwood, while colder sites contain higher deadwood abundance. Differences in the timing of tree death together with local climate resulted in higher richness of deadwood types in wetter sites, which can likely support higher biodiversity through the presence of more abundant TreM. However, these benefits may be temporary, due to faster deadwood decomposition in wetter environments. Conversely, the observed more long-lasting presence of deadwood in colder and drier sites, may promote structural complexity to persist, although these more extreme climatic conditions may also hinder this benefit if they affect the species relaying on this resource. Ultimately, our study pinpoints that at least during a certain period following a drought-induced die-off event, forest structure becomes more heterogeneous and complex, potentially supporting higher biodiversity, with local climate further shaping the duration of these beneficial effects. These results may help forest managers in guiding their decision regarding the management of deadwood following die-off episodes, with the aim of promoting heterogeneous forest structures and enhancing biodiversity conservation.