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Variation in the relative competitive abilities of plant species between different environments (such as climate) has been proposed as a control of the species composition of plant communities. This thesis examines whether the competitive abilities of paper birch and Douglas-fir vary with climate from a coastal site to a southern interior site in British Columbia. The overall hypothesis is that paper birch would be more competitive at the interior site and Douglas-fir would be more competitive at the coastal site. The study examined several components of this variation in competition. Seedlings of paper birch (Betula papyrifera Marsh.) and Douglas-fir {Pseudotsuga menziesii [Mirb.] Franco) were planted in pots at different densities either as pure culture (1, 2, 4 and 6 seedlings per pot) or as mixed culture (2, 4, 6 seedlings per pot in a 1:1 ratio). The study was established at each of two sites (Adams Lake in the southern interior and Malcolm Knapp Research Forest on the coast) representing one biogeoclimatic subzone in each of two different biogeoclimatic zones (Interior Cedar-Hemlock zone (ICH) and Coastal Western Hemlock zone (CWH), respectively). There was no replication of the study sites. The experiment was run for two growing seasons (1993 and 1994). Seasonal net photosynthetic rates of the two species were measured. Douglas-fir seedlings continued photosynthesis year-round at the coastal site but only photosynthesized seven months at the interior site (because of snow cover, photosynthesis was only measured for seven month but was assumed to negligible beneath the snow). Paper birch had active photosynthesis for only six months at the coastal site and five months at the interior site. Assuming that photosynthesis of mature conifers at the interior site was limited in the winter by low temperature, this suggests that winter photosynthesis by Douglas-fir at the coastal site contributes significantly to its carbon gain and competitive ability. Douglas-fir had higher photosynthetic NUE and WUE at the coastal site than at the interior site. However it had higher biomass-based NUE at the interior site. Paper birch had higher nitrogen use efficiency (NUE) and water use efficiency (WUE) at the coastal site than at the interior site. Douglas-fir seedlings had significantly larger basal diameter and height at the coastal site than at the interior site throughout the growing season. Paper birch seedlings were taller in the early part of the growing season at the coastal than at the interior site, but there was no significant difference in basal diameter between the two sites. Douglasfir had significantly larger foliage, root, shoot and total biomass per plant at the coastal site than at the interior site. However, it had a lower root/shoot ratio at the coastal site, while the opposite was true for birch. The birch foliage, shoot, root and total biomass decreased significantly with increasing density at both sites, suggesting strong intra-specific competition. Douglas-fir experienced less intra-specific competition at both sites, and had greater negative effects on paper birch in terms of growth and biomass production at the coastal site than at the interior site. Paper birch had greater negative effects on Douglas-fir at the interior site than at the coastal site. Douglas-fir had greater RGR at the coastal site than at the interior site. In contrast, paper birch had greater a RGR at the interior site than at the coastal site. Douglas-fir had significantly greater relative competitive ability at the coastal site when grown in mixed culture with paper birch. In contrast, paper birch had.greater competitive ability at the interior site. Stem analysis of height growth patterns of the two species showed that height of Douglas-fir exceeded height of paper birch at about 45-50 years after stand establishment at the interior site. In comparison, Douglas-fir exceeded paper birch in height at about 25-30 years at the coastal site. Difference in height growth patterns of the two species at the two sites contributed to the explanation of differences in the persistence of paper birch in mixtures between these two biogeoclimatic subzones. Other factors between the two different study areas, such as differences in frequencies of natural fire, insect outbreak and forest diseases, may also be important in determining why paper birch is more abundant in the ICH zone.