Indicators of competition in a Northwestern Ontario jack pine plantation

Abstract

Competition in forestry is discussed as a prelude to the hypothesis that tree seedling growth is strongly related to that of other vegetation close to it. The hypothesis states that: a) the relationship is species-dependent; b) seedling growth is inversely proportional to the amount of other nearby vegetation; c) seedling growth is directly proportional to the distance separating the tree from other vegetation; and d) that the amount of competition is quantifiable by some measurement of the tree seedling. The study was conducted in a four year- old Pinus banksiana plantation in Northwestern Ontario. Several non-crop plant species occupied the site: Epilobium angustifolium, Calamagrostis canadensis, Carex spp., Rubus idaeus, Corylus cornuta, Prunus pensylvanica, Betula papyrifera, Acer spicatum and Alnus crispa.. The author established in the study area 110 sample plots of 1.13 m radius (4 irfi ) divided into 2 m^ inner and 2 m^ outer rings centred on jack pine (crop) tree seedlings. For each plot, a tally sheet was completed recording: (a) crop tree parameters (height, stem diameter, and crown width), and (b) non-crop species, average height, and ground cover as a percentage of the plot. Samples of jack pine from peripheral areas and non-crop species above-ground biomass samples were collected, dried, and weighed. The crop tree parameters were correlated to the non-crop species measurements and Towill and Archibald's (1991) Competition Index (T&A’s Cl) method was applied to assess the degree of competition. The correlation coefficients (r) for crop tree parameters with non-crop species vary from 0.852 for alder, 0.602 for natural jack pine, -0.572 for total plant species as a group, to 0.239 for fireweed. T&A’s CIs correlate highly with the estimated dry weight for all plant species (r = 0.845), but poorly with the jack pine parameters (r = -0.259 to -0.377). The strengths and weaknesses of this Cl are considered, and modifications are proposed, formulated and applied individually and in combination. The best correlations of the modified indices with jack pine dry weight and with any other parameters were not significantly different from the unmodified index.