Competitive interactions between juvenile trembling aspen and lodgepole pine: A comparison of two interior British Columbia ecosystems

To facilitate ecosystem-specific management of juvenile mixtures of lodgepole pine ( Pinus contorta Dougl. Ex Loud. Var. latifolia Engelm.) and trembling aspen ( Populus tremuloides Michx.) in south-central British Columbia, we compared the characteristics of pine–aspen competition between a moist sub-boreal spruce and a dry interior Douglas-fir ecosystem. A total of 252 lodgepole pine and their neighbourhoods were examined across four untreated stands, each of which was sampled three times between ages 12 and 24 years. Pine diameter and height decreased with increasing density of trembling aspen at least as tall as the target pine (tall aspen) in both ecosystems. Regression analysis was used to examine the ability of tall aspen density and four competition indices (CIs) to predict pine size. Tall aspen density, which is easily assessed in the field, accounted for 63% and 69% of the variation in pine diameter and height in 20–24 year-old stands, respectively. The most successful competition index, based on the basal diameter ratio (BDR) of trembling aspen to pine accounted for, respectively, 78% and 73% of the variation. In the same stands, R 2 values were 1–5% lower when tall aspen density and BDR at age 15–19 years were used to predict size of 20–24-year-old pine. A threshold of 1000 tall aspen/ha was identified to achieve 90% open-grown pine diameter in both ecosystems. Although most pine continued to survive at age 20–24, over 50% of those in the moist sub-boreal spruce ecosystem were in poor condition where tall aspen density exceeded 1000 stems/ha. In contrast, a similar decline in vigour did not occur in the dry interior Douglas-fir ecosystem until tall aspen density exceeded 5000 stems/ha. We concluded that the competitive effects of aspen on pine were stronger and longer-lasting in the moist sub-boreal spruce ecosystem, where they were not fully expressed at stand ages of 20–24 years, than in the dry interior Douglas-fir ecosystem where they appeared to be peaking at the same age.