Annual variations in competition symmetry in even-aged sitka spruce

Understanding of the temporal development of competition symmetry, size hie rarchy and the role of episodically supplied resources in forest stands is limited. Because of intrinsic differences in the symmetry of competition fo r light and below-ground resources, it is hypothesized that the overall exp ression of competition varies as a consequence of variations in water avail ability. Historical growth and precipitation data from an unthinned stand o f Sitka spruce (Picea sitchensis (Bong.) Carr.) in Denmark are analysed wit h respect to the effects of precipitation on competition symmetry. The shap e and linearity of the size-increment relationship are analysed, and the te mporal development of diameter distribution and size hierarchy is modelled. The test value of lack of linear fit through the origin for the diameter/i ncrement relationship plotted against precipitation in the growth period fo rms a triangular envelope. indicating a more curvilinear relationship for y ears with high water availability. This indicates that dominant trees appar ently benefit more from increased precipitation than suppressed trees which is also shown in the analysis of the temporal development of the diameter distribution-and as a consequence the size hierarchy increases. Finally. it is proposed that temporal and spatial variations in the availability of wa ter and other below-ground resources may have similar effects on competitio n symmetry. (C) 2001 Annals of Botany Company.