Species coexistence in a mixed Mediterranean pine forest: Spatio-temporal variability in trade-offs between facilitation and competition

• We studied competition and spatial patterns in a stand of Pinus pinea and Pinus pinaster.
• Facilitation and competition trade-offs allow mixed stands to be maintained.
• Competition and facilitation are species-specific, not symmetric.
• Facilitation for sapling trees development is necessary.
• Without silvicultural interventions, P. pinea will become the dominant tree.

Studying species coexistence is key to understanding the way in which forests will respond to climate change. We studied the patterns of mixed stands including two main Mediterranean pine species: Pinus pinaster Ait. and Pinus pinea L. The spatial distribution of adult trees and saplings was studied via a point pattern approach. The effect of competition on growth of adult trees was investigated by comparing the performance of several competition indexes for each pine species through generalized linear models. Adult trees formed mixed clumps in which individuals of both species appeared together. Part of the tree growth variation was explained by tree size along with tree competition. However, the effect of conspecific vs heterospecific competition on tree growth differed and reflected species-specific neighbor-asymmetric competition. Facilitation was fundamental in the early stages for tree species development. The spatial distribution of saplings was strongly related to the spatial distribution of adult trees, also being asymmetrically clustered and neighbor-species-dependent. However, the required facilitation in early life-stage trees shifted to competition among trees in the adult stage. Species mixture may be desirable in terms of increasing and diversifying productivity, although the conditions currently present in the stand are likely to lead to future dominance of P. pinea over P. pinaster due not only to the greater competition tolerance of the former but also to a greater ability to successfully recruit in the plots, forming clusters that may be in turn be impenetrable to P. pinaster. Therefore, in order to maintain mixed stands, it would be necessary to enforce adequate silvicultural management strategies which avoid future stand dominance by P. pinea.