Structure and asymmetry of tree crowns in relation to local competition in a natural mature Scots pine forest

This study examined the effect of local competition, as determined by the number, size, and spatial distribution of neighboring trees, on crown base height, crown width, and crown asymmetry of trees growing in a natural mature Scotspine (Pinus sylvestris L.) forest in eastern Finland. Local competition was described with competition indices. Crown asymmetry was characterized as the distance and direction from the stem center to the visually estimated crown mass center, Nonspatial and spatial models for crown base height and crown width were derived, as well as a spatial model for the horizontal distance from the crown mass center to stem base position. The effect of neighbors on the direction of crown mass center from stem base position was studied using the center of competition and the widest free angle free of competitors closer than 5 m. The derived spatial models for crown base height and crown width, taking local competition into account, were only slightly better than the nonspatial models. This was because diameter at breast height and tree height, the independent variables used in the nonspatial models, described quite well the conditions of local competition for a tree. The spatial model for crown base height was 2.22% units (adjusted R-2) better than the nonspatial model. Correspondingly, the spatial model for crown width was 1.77% units better than the nonspatial model. Crown asymmetry characteristics were measured fur 57% of the: trees. The spatial model for the distance to the crown mass center could only explain about 10% of the total variation of the dependent variable. The model for the direction of the crown mass center from stem base position showed that the variables describing direction of local competition, i.e., the competition center and the widest free angle, could not adequately explain the direction of asymmetry. One reason for this was that tree crowns showed plastic responses to the prevailing asymmetry of solar radiation geometry. Therefore, the directional distribution of the crown asymmetry was strongly weighted toward southern and southwestern directions, i.e., the directions of most abundant incoming solar radiation. We suggest that the detected developmental plasticity in crown asymmetry was affected both by the directionality or solar radiation geometry and by that of local competition, which is influenced by the number, size, and spatial distribution of close competitors.