Crown plasticity of five pine species in response to competition along an aridity gradient

Abstract

Crown dimensions such as diameter or crown projection area are important indicators for tree vigour, growing stage or production efficiency and can be used as proxies for the assessment of wood quality, leaf area and leaf biomass, biodiversity, wildlife habitat and disturbance risk to forest stability. Crown allometry can vary with site and competition conditions, but variation patterns are not still well understood.The main aim of this study was to analyze this allometry variation in the most common pine species along the aridity gradient of the Iberian Peninsula, studying the crown response to aridity and to competition, and determining whether the presence of inter-specific competition modifies crown allometry.A set of 40,402 sample trees from 12,283 sample plots, along with the Martonne aridity index as an indicator of climatic conditions, were used to develop allometric models, the parameters of which were generalized by including aridity and competition indicators. The plots were located in monospecific or mixed stands of Pinus sylvestris, Pinus nigra, Pinus pinaster, Pinus pinea and Pinus halepensis.According to our results, both aridity conditions and competition status are important factors modifying the crown allometry of the studied species. Moreover, we found that both size-symmetric competition and size-asymmetric competition should be taken into account when modelling the crown size, both types leading to smaller crown diameter. The crown plasticity of the studied pine species in response to competition presented a low range of variation, the greatest being that of maritime pine, while Scots pine and black pine showed the smallest. The way in which aridity and competition influence crown size differs among the studied species, indicating differences in crown plasticity patterns. Moreover, in humid sites, where competition for light is more important, the crown allometry response to changes in competition is greater, being most evident in P. pinea and P. halepensis.The species mixing effects had less influence on pine crown allometry than the total competition effect, although the effect was always significant. The effect of size-symmetric competition from admixed species was positive or neutral, except for the effect of P. sylvestris on P. nigra crowns. Size-asymmetric competition also plays an important role in the species mixing effects, these effects on intermediate or suppressed trees varying depending on the species mixture. The models developed reveal significant species-specific effects on crown allometry, pointing to the ability of pine species to adapt their crowns to different growing conditions.