Tatiana A. Shestakova, J. Julio Camarero, Juan P. Ferrio, Anastasia A. Knorre, Emilia Gutiérrez and Jordi Voltas
Whereas tree growth is primarily constrained by water availability in Mediterranean lowlands, Mediterranean mountain forests are subjected to varying degrees of water stress depending mainly on their position along altitudinal gradients. Unravelling physiological responses along such gradients provides clues to how forests will react to emerging combinations of temperature and precipitation under climate warming. Particularly the characterisation of long-term reactions of coexisting tree species is especially relevant to contextualise the impacts of ongoing climate change in complex ecosystems.
This study investigates the extent of regionally coherent tree responses to an intensified warming-induced drought stress as imprinted in wood rings. We hypothesised that increasing drought stress has homogenized responses to climate among five pine species over the last 60 years, as drought effects have spread from low to high elevations along an altitudinal gradient (up to 2,020 m) in eastern Spain. We predicted a consistent reliance of pine performance on water conservation strategies at low elevations throughout the study period. Conversely, we expected a weaker dependence on water saving mechanisms higher up the gradient, but a gradual convergence at low- and mid-elevations (from 1,100 to 1,600 m) as sensitivity to drought became more limiting for photosynthesis.
We observed enhanced growth synchrony (that is, more coherent changes in ring-width patterns) between low- and mid-elevation pinewoods, a phenomenon attributable to increasingly negative annual water balances from the 1950s. Our results also indicated that nowadays drought effects on pine performance start earlier in the growing season (early spring) at low and mid elevations (driest sites). Particularly, we observed an increasing dependence of radial growth on needle control of water losses spreading upwards in mountain forests, hence resembling lowland forests. All together, these findings demonstrate that drought is gaining relevance over stand effects (e.g., topography, nutrient availability) as the main factor controlling multispecies tree physiology and productivity and is likely to increase pines’ sensitivity to competition. Our observations constitute non-visible early warning signals of forest vulnerability to climate change traceable back in time through the analysis of tree rings.
Image caption: : High-elevation mixed pine stand in the Gúdar mountain range composed of Pinus sylvestris and Pinus uncinata. Photo credit: Anastasia A. Knorre.