The precedents and the responses of trees to extreme droughts reveal the processes involved in the die-off of Mediterranean conifer forests

Abstract

Camarero, J.J., Sangüesa Barreda, G., Alla, A.Q,. González de Andrés, E., Maestro Martínez, M.,Vicente-Serrano, S.V. (2012). The precedents and the responses of trees to extreme droughts reveal the processes involved in the die-off of Mediterranean conifer forests. Ecosistemas 21(3):000-000. Doi.: 10.7818/ECOS.2012.21-3.04

What drives the tree species’ vulnerability against extreme climatic events within the current context of global warming? We are aware that severe droughts contribute to forest die-off but we do not know the processes involved in the growth decline leading to a loss of tree vigour, which may ultimately cause tree death. To get a better understanding of those processes we combine retrospective secondary-growth data, obtained using dendrochronology, and functional variables which may be potential indicators of changes in vigour (crown density, sapwood production, changes in the concentrations of non-structural carbohidrates in needles and stem sapwood) in Mediterranean forests affected by drought induced die-off. We evaluate how three tree species (Abies alba, Pinus sylvestris, Pinus halepensis) responded to the extreme drought which occurred in 2012 by calculating a drought index which takes into account the effect of temperature. In spring and summer 2012 the drought was very intense because the high maximum temperatures caused very arid conditions which were unprecedented since 1950. Considering the three species, the trees with less dense crowns (more defoliated) formed less sapwood, presenting a lower concentration of soluble sugars than less defoliated trees did. The more defoliated trees were also characterized by showing a higher persistence in radial growth than less defoliated trees. The growth patterns differed among species. These patterns suggest that the P. sylvestris die-off was caused by successive and extreme droughts during the past century, whereas A. alba and P. halepensis growth patterns may correspond to a progressive die-off, which started during the past decade in response to rising temperatures. In the latter two species, declining individuals showed the highest growth rates during past climatically mild periods. The diverse described patterns indicate the need to define concepts such as tree death and post die-off tree recovery within an objective, quantitative and verifiable research framework.