| Summary: | 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. === Forestry, Faculty of === Graduate
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