Competition Within Stands of Picea sitchensis and Pinus contorta

Abstract

Competition was analysed in plots of Picea sitchensis and Pinus contorta grown in 50 × 50 hexagonal arrays at 14 cm spacing to ages 7 and 5 years, respectively. Relative growth rates in height (RHGR) became positively related to tree heights during the year before harvest. Frequency distributions of tree heights became negatively kurtotic with tendencies towards left-skew ness. By the time of harvest, dead trees were evenly dispersed over the plots. Trees with many taller neighbours had lower RHGRs than trees with few taller neighbours, and the RHGRs of intermediate-sized trees were correlated with their ‘competitive status’. Competition was confined mostly to first-order neighbours. Large trees depressed the RHGRs of smaller neighbours and not vice versa; a simple test for this ‘one-sided’ competition is described. Neighbours did not need to greatly overtop a tree to depress its RHGR, they needed only to be at least as tall. Systematic trends in RHGR across the plots, attributed to site heterogeneity, decreased with time, and accounted for only about 10 per cent of the variation in RHGR at harvest. Competitive status accounted for 25 and 38 per cent of the variation in RHGR in the Picea sitchensis and Pinus contorta plots, respectively.