Inbreeding and neighbouring vegetation drive drought-induced die-off within juniper populations

Drought-induced episodes of forest die-off are being reported world-wide, and recent evidence shows that increases in temperature due to climate change are contributing to this decline. Several ecological mechanisms have been invoked at the plant level to explain the vulnerability of trees to this widespread drought-induced mortality. Apart from ecological factors, some genetic factors might also influence population responses to disturbances: it has been reported, for example, that genetic diversity enhances population resilience through genotypic complementarity, whereas genetic relatedness between conspecific individuals exerts a negative influence on variations in fecundity. However, we lack empirical data that prove any functional relationship between levels of individual inbreeding or kinship and patterns of drought-induced forest die-off resulting from field conditions. Here, we investigated the role of ecological, phenotypic and genetic factors in determining the response of trees to prolonged drought conditions. We focused on populations of the small tree Juniperus phoenicea L. that recently experienced severe drought-induced die-off in semi-arid woodlands from central Spain. We correlated mean individual inbreeding (f) and mean kinship between individuals within a plot (f(ij)) with remaining green plant canopy and crop size. We found that f was negatively correlated with plant performance, whereas f(ij) was not. Healthier canopy also tended to correspond with large plants and neighbourhoods dominated by trees and shrubs, suggesting a role for facilitative interactions and microsite heterogeneity. Accordingly, the probability of severe damage increased linearly with f, and individuals growing in open habitats with f values > 0625 only had 15% chance of remaining unaffected after a period of prolonged drought. We show that both vegetative and reproductive responses to prolonged drought episodes are determined by multiple factors, including individual inbreeding. Therefore, any fuller understanding of phenomena such as drought-induced forest die-off needs to incorporate the contribution of individual genetic variations in conjunction with plant-plant interactions.