Sensitivity to water stress drives differential decline and mortality dynamics of three co-occurring conifers with different drought tolerance

Abstract

Abstract Drought-induced tree decline and accelerated mortality under global change threaten forests worldwide. Identifying environmental and structural factors driving tree mortality of species with different ecology is essential to understand forest dynamics. We compared three coniferous species and trees with different health status (i.e. healthy, declining and dead) on a xeric Mediterranean ecotone where Pinus pinaster shows widespread decline (defoliation, mistletoe, high mortality) while more drought-tolerant Pinus pinea and Juniperus oxycedrus do not. We analysed individual tree and stand characteristics and regeneration of all species to understand how different abiotic factors at different time scales affected stand species dynamics in relation to tree decline and mortality. Only dead and declining pine trees showed recent multiannual negative radial growth trends. The growth decline started after two extreme droughts, being 10\xa0years longer in the declining species, P. pinaster, than in non-declining P. pinea. We observed no signs of decline or mortality in the most drought-tolerant species, J. oxycedrus. Variables related to moisture availability, like stand dominant height and the tree response to precipitation, increased survival probability in P. pinaster but not in P. pinea. Despite P. pinaster dominating the canopy, its regeneration was very scarce and less abundant than that of co-occurring more drought-tolerant species. Our results suggest that drought-induced mortality was related to species-specific sensitivity to water stress and may be inducing compositional changes towards dominance of more drought-tolerant species. Consequently, at its lower elevation limit P. pinaster might be able to persist only in microsites with higher moisture availability.