Interannual variability in competitive effects in mixed and monospecific forests of Mediterranean stone pine

Abstract

The management of species composition and competition are two of the main adaptive options that forest managers propose to cope with the expected negative impacts of climate change on forest growth in the Mediterranean basin. Species mixture can improve the resistance and resilience of forest ecosystems to face up global change. However, it seems likely that global change will modify mixed stands dynamics. Thus, studying inter-tree relationships on an annual basis is key to understanding ecosystem dynamics in the region. The aim of this paper was to evaluate the effects of tree species composition and competition on Pinus pinea annual secondary growth in mixed vs. monospecific stands over a period of 15. years with contrasting climatic conditions. We obtained basal area growth data from tree ring measurement on cores and cross section slices from 372 trees of P. pinea L. ., Juniperus thurifera L., Quercus ilex subsp. ballota (Desf.)) Samp. and Quercus faginea Lam., in the Spanish Northern plateau, approximately half of which were in monospecific stands and half in mixed stands. We analysed the effect of intra and interspecific competition on P. pinea secondary growth comparing the performance of several distance dependent competition indexes through linear mixed models. These competition indices were calculated for all trees within each plot for each year of study. The results showed competitive reduction and tree growth amelioration in mixed vs. monospecific stands of P. pinea indicating a spatial and temporal niche separation between species and size-symmetric effects for interspecific competition. Size-asymmetric results obtained for competition within pines indicated that the largest individuals obtain the majority of the contested resources suppressing the growth of their smaller pine neighbours. Intraspecific interactions were more negative than interspecific interactions. And we finally provide evidence of a growth enhancement in mixed vs. monospecific stands in water stressed years indicating that the promotion of mixtures in P. pinea stands is a powerful management tool to buffer the effects of climate change in the region.