The environmental drivers influencing spatio-temporal dynamics of oak defoliation and mortality in dehesas of Southern Spain.
Quercus ecosystems have been affected globally by decline episodes, involving extreme defoliation and mortality events, during recent decades. Both abiotic and biotic factors seem to trigger these processes, including the spread of non-native invasive pathogens (e.g., Phytophthora spp.). However, we have limited understanding of how these factors interact in a spatio-temporal context. Regional forest health monitoring networks are a relevant but rarely used tool that enables us to disentangle the patterns of tree defoliation and mortality over time in relation to environmental and silvicultural drivers. In this study, we used a combination of statistical approaches (i.e., Kaplan-Meier, Kernel Density Estimation and generalized mixed models) to analyse 3635 records of oak trees (152 plots) from the Regional Forest Health Network of Andalusia (Spain) (2001-2016, ICP Level I). In this way, we studied the spatio-temporal variation of defoliation and mortality in dehesas of Quercus spp. in relation to oomycetes occurrence and the average and annual values of environmental factors related to oak decline. The annual defoliation and mortality were correlated with the mean annual temperature, Standardised Precipitation-Evapotranspiration Index (SPEI 18 summer and 1 spring) and soil organic matter content. The effect of oomycetes on forest decline was mediated by environmental factors, having greater importance in rich soils and wet years. These results imply that variation in defoliation and mortality is clearly linked to relevant spatio-temporal changes in environmental factors. Regional forest health networks are a crucial part of adaptive forest management strategies to significantly narrow the uncertainty regarding climate change impacts on Mediterranean oak woodland ecosystems. The timely information provided here could lead to better silvicultural practices, preventive measures against the spread of the oomycete and the implementation of early warning tools, to deliver accurate assessments of the defoliation and mortality risk on a meaningful spatio-temporal scale.