Effect of crown asymmetry on size-structure dynamics of plant populations

Abstract The effect of crown asymmetry on the size–structure dynamics of populations was evaluated using a spatial competition model incorporating crown asymmetry. Computer simulations were carried out with various combinations of density levels, spatial patterns, and degrees of asymmetry in competition to assess how they modify the effect of crown asymmetry on size–structure dynamics. In the model, crown asymmetry is expressed by the crown-vector, or the vector linking the stem base and the centre of the projected area of the crown on the horizontal plane. Crown-vectors are assumed to develop in the manner by which crowns repel each other. As crown-vectors develop, the positions of the crown-centres move. Competition between individuals is expressed by a neighbourhood model, in which individual growth is determined by the distance from, and size of, the neighbours' crown-centres. Generally, populations of individuals which developed asymmetric crowns had larger survivorship, larger mean size, smaller coefficients of variation and skewness, and a more regular spatial pattern than populations of individuals which developed symmetric crowns. The effect of crown symmetry is generally stronger in populations with high density and a clumped spatial pattern. The effect of mortality caused by one-sided competition on size-structure dynamics was similar to that of crown asymmetry; mortality increased mean size, reduced size hierarchy, and made the spatial pattern more regular. Because mortality was heavier in populations without crown asymmetry, its effect on size-structure dynamics cancelled out, or overwhelmed, the effect of crown asymmetry in later growth stages. If crown asymmetry is associated with a reduction in growth, the effect of crown asymmetry is reduced. Nevertheless, the resultant population structure is different from that of populations without crown asymmetry.