Drought induced decline could portend widespread pine mortality at the xeric ecotone in managed mediterranean pine-oak woodlands Guillermo Gea-Izquierdo a,b,⇑ , Bárbara Viguera a , Miguel Cabrera c , Isabel Cañellas a a INIA-CIFOR, Crta. La Coruña km 7.5, 28040 Madrid, Spain b CEREGE UMR 7330, CNRS/Aix-Marseille Université, Europole de l’Arbois, BP 8013545, Aix-en-Provence cedex 4, France c Aranzada Gestión Forestal S.L.P., C/Alonso Heredia, 31, 28028 Madrid, Spain article info Article history: Received 24 October 2013 Received in revised form 12 February 2014 Accepted 19 February 2014 Keywords: Quercus pyrenaica Pinus sylvestris Global change Dendroecology Vulnerability to water stress abstract There is a need to better understand how different biotic and abiotic factors interact to determine climate change enhanced tree mortality. Here, we investigated whether rising water stress determined enhanced Pinus sylvestris L. mortality at the species low-elevation limit in Central Spain. We analyzed the factors determining the health status of pines and compared with co-occurring and more drought-tolerant Quer- cus pyrenaica Willd along one transect following an elevation gradient. We used ordinal logistic regres- sion to model the susceptibility of a tree to decline in relation to variability in stand competition and individual growth-patterns. The mortality pattern differed with local site conditions. Pine growth was faster but life-span shorter at drier and warmer low-elevations than at high-elevations. However, within stands, healthy trees exhibited less abrupt growth reductions and higher growth-rates but not as a con- sequence of lower competition, which under present stand conditions did not seem to increase adult mortality risk. Low moisture availability reduced tree-growth and, although P. sylvestris is less tolerant to drought, Q. pyrenaica was more sensitive to year-to-year moisture variability. Previous growth of dead trees from both species declined with rising water stress after the 1970s at low-elevations, which sug- gests that water stress intensity limited particularly tree-growth of dead trees in the long-term. For pines, widespread symptoms of crown decline (expressed by mistletoe infestation and defoliation) were only observed at low-elevation stands where, in opposition to oaks, weakened and healthy pines also exhib- ited recent negative growth-trends parallel to those of dead trees. The pervasive growth decline with enhanced water stress in pines from all health status at the species sampled xeric ecotone combined with the abundant crown decline symptoms observed, suggest pine vulnerability and could portend wide- spread mortality at its current low-elevation limit. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Climate change related increases in drought frequency and severity have a negative impact on forest ecosystem productivity and tree performance (Boisvenue and Running, 2006; Lenoir et al., 2008; Choat et al., 2012). Therefore it is crucial to better de- scribe the underlying processes governing forest acclimation to water stress (Bréda et al., 2006; Niinemets, 2010; McDowell et al., 2011) particularly at those ecosystems where sustainability is threatened by enhanced mortality caused by recent climatic changes (Adams et al., 2009; Van Mantgem et al., 2009; Allen et al., 2010). Accurate prediction of tree vulnerability to increasing water stress is challenging because many biotic and abiotic factors interact at different time scales and trees with symptoms of low vitality can recover from stress provided they do not fall below thresholds where irreversible damage occurs (Suárez et al., 2004; Dobbertin, 2005; Breshears et al., 2009). In addition, it can be par- ticularly complex to isolate the long-term effect of climate at those sites where land-use has shaped the current state of forests and determine forest dynamics (Gimmi et al., 2010; Van Bogaert et al., 2011; Wischnewski et al., 2011). The interdependent factors that determine species-specific sus- ceptibility to drought induced decline and mortality must be inves- tigated at different temporal scales in order to understand forest vulnerability. Growth can be used as a direct proxy to the tree net carbon pool. Using dendrochronological methods it is possible to analyze long-term environmental stressors (like water stress) predisposing trees to decline and short-term agents or events inciting posterior death of individuals (Suárez et al., 2004; http://dx.doi.org/10.1016/j.foreco.2014.02.025 0378-1127/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: CEREGE UMR 7330, CNRS/Aix-Marseille Université, Europole de l’Arbois, BP 8013545, Aix-en-Provence cedex 4, France. Tel.: +33 (0) 442971532. E-mail addresses: gea-izquierdo@cerege.fr, guigeiz@gmail.com (G. Gea-Izquierdo). Forest Ecology and Management 320 (2014) 70–82 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco