Seeing the trees for the forest: Drivers of individual growth responses to climate in Pinus uncinata mountain forests

Individual trees, not forests, respond to climate. Such an individual-scale approach has seldom been used to retrospectively track the radial growth responses of trees to climate in dendrochronology. The aim of this study was to adopt this individual view to retrospectively assess tree sensitivity to climate warming, and to evaluate and compare the potential drivers of tree growth responses to climate acting at species, site and individual scales. Following a dendroecological framework, we sampled a network of 29 Pinus uncinata forests in NE Spain and obtained tree-ring widths series from 642 trees. Individual features as northness, elevation, slope, basal area, sapwood area, tree height and tree age were used to evaluate the potential drivers of tree growth responses to climate. The analysed data set includes diverse ecological and biogeographical conditions. The tree growth responses to climate were assessed by relating growth indices to climatic variables using linear-mixed effects models. Maximum November temperatures during the year prior to tree-ring formation enhanced P. uncinata growth mainly in mid-elevation sites, whereas at higher elevations growth was more dependent on the positive effect of warmer minimum May temperatures during the year of tree-ring formation. Current June precipitation was the positive main climatic driver of growth in sites prone to water deficit such as the southernmost limit of the species distribution area or very steep sites. Elevation was the main factor controlling how much growth variability is explained by climate at the site and tree scales. Climate warming was more intense during the early 20th century, when the importance of elevation as an indirect modulator of growth declined as compared with the late 20th century. Synthesis. The individual-scale approach taken in this study allowed detecting that trees growing at southern and low-elevation sites were the most negatively affected by warm and dry summer conditions. Our results emphasize that both (i) an individual-scale approach to quantify tree growth responses to climate and (ii) a detailed evaluation of the potential biotic and abiotic drivers of those individual responses are necessary to understand climate sensitivity of trees.. © 2014 British Ecological Society.This study was supported by projects 012/2008 and 387/2011 (Organismo Autónomo Parques Nacionales, Spain) and by a JAE-CSIC grant to the first author. J.J.C. thanks the support of ARAID. We also acknowledge funding by projects which contributed to build this data set (FoRmat EU ENV4-CT97-0641, AMB95-0160 and CGL2011-26654). We thank the comments made by two reviewers and the associate editor. We are indebted to all people who helped us sampling in the field: A.Q. Alla, G. Sangüesa-Barreda, E. Muntan, M. Ribas, P. Sheppard, M.A. Rodríguez-Arias, J. Tardif, and many other colleagues and friends.Peer Reviewe