Annual variations in competition symmetry in even-aged Sitka spruce

Abstract

Understanding of the temporal development of competition symmetry, size hierarchy and the role of episodically supplied resources in forest stands is limited. Because of intrinsic differences in the symmetry of competition for light and below-ground resources, it is hypothesized that the overall expression of competition varies as a consequence of variations in water availability. Historical growth and precipitation data from an unthinned stand of Sitka spruce (Picea sitchensis (Bong.) Carr.) in Denmark are analysed with respect to the effects of precipitation on competition symmetry. The shape and linearity of the size-increment relationship are analysed, and the temporal development of diameter distribution and size hierarchy is modelled. The test value of lack of linear fit through the origin for the diameter/increment relationship plotted against precipitation in the growth period forms a triangular envelope, indicating a more curvilinear relationship for years with high water availability. This indicates that dominant trees apparently 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 water and other below-ground resources may have similar effects on competition symmetry. © 2001 Annals of Botany Company.