Sitka alder (Alnus sinuata (Regel) Rydb.) dominates many lodgepole pine (Pinus contorta ssp. latifolia Dougl.) sites following clearcutting in the Montane Spruce zone of the southern interior of British Columbia. The objectives of this study were to examine the effects of the sitka alder-dominated shrub community on the performance of lodgepole pine and levels of environmental resources and conditions. Competitive interactions were examined in two studies: (1) among two year-old planted seedlings and plant species in various experimentally created shrub densities (0 to 2514 clumps/ha) and herb covers (0 to 100%), and (2) among eight year-old naturally regenerated saplings and plant species in an undisturbed community. In the first study, survival of seedlings among the experimentally created competition levels was 86% two years after planting. The main causes of mortality were drought and browsing by hares (Lepus sp.). Survival rate was not significantly affected by shrub and herb densities; however, survival was lowest where all vegetation had been removed. Seedling mortality in the total removal treatment may have been the result of high radiation loads and low moisture availability immediately following planting. Mean seedling size in the plantation was negatively affected by shrub and herb density. Stem diameter was the most responsive performance measure, smaller on average by 25% when seedlings were growing among maximum shrub and herb densities as compared with those growing free of competition. Height, in contrast, increased as shrub and herb densities increased. The decrease in diameter and increase in height in response to increasing vegetative competition reflected patterns in resource (particularly carbon) allocation. Several environmental factors were important to the enhancement of seedling water uptake and growth when competing vegetation was removed. Significant increases in seedling water uptake did not coincide with increases in soil water potential, but rather with increases in soil temperatures. Increases in seedling diameter corresponded with increased soil and air temperatures, light availability and mineralizable NO3-N. Individual seedling size in the plantation decreased with increasing amounts of neighboring plants. Visual estimates of percent cover of neighboring plants (extensive interspecific competition indices) explained more variation in pine size than did the more detailed measurements of alder size and proximity (intensive indices). Percent cover of all shrubs and herbs accounted for 16% of the variation in height:diameter ratio while angular dispersion and distance to neighboring sitka alder accounted for only 9%. A competition threshold, i.e. the amount of neighboring vegetation at which competition began and growth was limited, was not identified. Seedlings with the largest stem diameters, however, occurred in neighborhoods with less than 10% cover of herbs and shrubs each. The best multiple regression models developed explained 22% of the variation in pine diameter and 43% of the variation in height. The independent variables were initial height, seedling vigour, browsing damage and percent cover of all shrubs. Light and, to a lesser degree, soil water available to seedlings were reduced by neighboring vegetation. Within the experimentally created competition levels, sitka alder clumps sprouted to a mean height of 70 cm and mean diameter of 73 cm two growing seasons after manual cutting. The tallest stems (125 cm) reached 42% of the pre-treatment height (3 m). The density of sprouting alder clumps had a significant effect on the development of most neighboring shrub and herb species. Percent cover of alder, thimbleberry (Rubus parviflorus), fireweed (Epilobium angustifoliuin) and pinegrass (Calamagrostis rubescens) was greatest in the intermediate density range of 1258 to 1886 clumps/ha. Within this density range, threshold levels of environmental resources and conditions may have been reached which resulted in the greatest vegetative cover. Two growing seasons after planting, all neighboring species except grouseberry (Vacciniwn scoparium) were overtopping pine seedlings. In the second study of a 10 year-old undisturbed sitka alder dominated community, two vegetation types were identified. Type I was dominated by lodgepole pine and pinegrass while type II was dominated by sitka alder, thimbleberry and black huckleberry (Vaccinium membranaceum). The size of individual pine saplings was more negatively affected by neighboring plants in type II than I. Sitka alder, of all the dominant species in the undisturbed community, had the greatest competitive effect on pine size. The extensive competition index, percent cover of sitka alder, explained 45% of the variation in stem diameter. In contrast, the intensive indices, height of and distance to neighboring sitka alders, together explained 40% of the variation in stem diameter. A clearly defined competition threshold was not identified. Rather, pine size increased linearly as sitka alder densities decreased. Sitka alder had a negative effect on light availability to pine, particularly in type II.