Drought-induced decline of Scots pine in central Pyrenees is mediated by stand-level predisposing factors

Background/Question/Methods Episodes of drought-induced forest dieback have been recently observed in many regions of the world. Under climate change, those effects could signal potential vegetation shifts occurring over large geographical areas, with major impacts on ecosystem form and function. The western Mediterranean basin is one of the regions where the increase in the frequency and severity of extreme droughts is forecasted to be larger. Species with their distribution limit in this area, such as Scots pine (Pinus sylvestris L.), are likely to be particularly vulnerable to increased aridity. This is consistent with the high mortality rates recently recorded in several Scots pine populations in central and southern Europe. In this paper we study the effect of a single drought episode, occurred in summer 2005, on a Scots pine population in central Pyrenees (NE Spain; 42º22’43’’ N, 01º06’29’’ E). Our main objective is to study the environmental correlates of forest decline and vegetation change at the plot level within a single Scots pine population. Thirty plots (314 m² each) were established along an altitudinal gradient from 645 to 1383 m a.s.l. within the same valley. Each of those plots was characterized in terms of stand structure, soil properties, infection by parasites (mostly the mistletoe Viscum album L.), tree defoliation and mortality, and recruitment of woody species. Generalized linear models were used to study the relationship between canopy defoliation, mortality, recruitment and plot characteristics.

Results/Conclusions The decline observed on the studied Scots pine forests was associated to the local level of drought stress experienced at each plot. In addition, stand structure, soil properties and mistletoe infection, as well as their interaction, contributed to the observed defoliation and mortality patterns, presumably acting as predisposing factors. Recruitment of Scots pine was low in all plots. In contrast, we observed an increase in the recruitment of other tree species, mostly Quercus ilex (evergreen) and Q. humilis (deciduous), in plots where Scots pine showed high defoliation and mortality. These results suggest an altitudinal upwards migration of Quercus species in the studied slopes, and that this migration is mediated by the dieback of the dominant canopy species. In the mid term, this trend is likely to result in a vegetation shift in the study area, from pine-dominated to broadleaves-dominated forests. We hypothesize that many of the scattered populations of Scots pine sheltered in the mountain environments of the Iberian Peninsula could be at risk under future climate scenarios.