Drought impacts on tree growth of two pine species along an altitudinal gradient and their use as early-warning signals of potential shifts in tree species distributions.

Abstract

Mediterranean pine forests are at risk of experiencing a decline in tree growth in response to climatewarming if rising temperatures amplify drought stress. In mountain areas, tree growth could be enhancedin temperature-limited high elevations, whilst it might decline at water-constrained low elevations.Species differential responses could, however, modulate the impact of drought on forests along altitudinalgradients. To test for evidence of species differential drought impacts along an altitudinal gradient, westudied the growth responses of two Iberian pine species (Pinus sylvestris and Pinus nigra) subjected toMediterranean conditions in Eastern Spain. We analysed the stability of growth (basal area increment)responses to climate and drought during the 1950&-2014 period by using resistance and resilience indices.Pinus sylvestris growth was enhanced by warm spring temperatures, while Pinus nigra growth wasimproved by a positive spring water balance. Pinus sylvestris growth decreased temporally at the lowerend of its altitudinal range, whereas Pinus nigra growth decreased at the upper end. Pinus sylvestris exhibitedlow growth stability at its low-elevation limit. Pinus nigra resistance also decreased along its altitudinalrange, but this effect was compensated by a high resilience. In mixed stands the results werecontrasting, with Pinus sylvestris (at the lower altitudinal range) being more vulnerable to droughtinducedgrowth decline than Pinus nigra (at the upper altitudinal range).