FRDA Report: Black Cottonwood and Balsam Poplar Managers ...

stands with scattered large cottonwood of the same age or large bigleaf maple left from
previous forests. The large maple and cottonwood provide an important habitat
component for wildlife species that utilize larger living or dead wood that is otherwise
rare or unavailable in alder stands.
Mixed stands of cottonwood and red alder are expected to benefit from alder’s
contribution to site nutrient capital and long-term productivity. In experimental plantings
of black cottonwood and red alder on a uniform site, a harvest at age 4 years revealed
average tree heights of 10.2 m for cottonwood grown in pure stands, 8.4 m for alder
grown in pure stands, and cottonwood average height of 11.0 m at age 4 when grown in
mixture with alder. At the end of the first year, the alder and cottonwood were about
equal in height but at the end of 4 years, height of alder averaged 2.6 m less than the
associated cottonwood. Despite cottonwood mixed with alder being taller than
cottonwood in pure stands at age 4, average biomass yield after 4 years was greater in
pure stands than in mixed stands (Table 24). The patterns of mean annual increment
(MAI) and periodic annual increment (PAI) show higher yield from the mixed planting
in the first 2 years. However, in the third year, PAI of pure cottonwood surpassed PAI of
the mixed stand. Consequently MAI and PAI were greater for the pure stand in the
fourth year. This change in relative productivity probably results from the declining
growth rate of alder. Because of the low nitrogen fixation by overtopped red alder,
there seems to be little justification for closely spaced mixtures of red alder and
cottonwoods. More widely spaced planting arrangements for the cottonwood should
reduce the shading of the alder and increase nitrogen fixation. Such plantations will be
advantageous on soils low in nitrogen and for longer rotations. This indicates that
managers may be able to grow a small component of cottonwood (50–100 trees per
hectare) in otherwise alder-dominated stands.
In upland areas of northeastern British Columbia and western Alberta, balsam
poplar is geographically commonly associated with aspen, but there is typically some
site differentiation in their relative abundances. Wet depressions are more likely to be
occupied by balsam poplar than by aspen. Therefore, cutblocks that have substantial
areas left unharvested because of excessive soil moisture are commonly dominated by
balsam poplar. The effect is that on an area basis, balsam poplar makes up a higher
proportion of residual standing stems than was the case before harvesting. In the early
phases of harvesting for oriented strandboard production, this tendency was further
intensified by preferential harvest of aspen, with balsam poplar left standing even on
sites that had no harvesting limitations because of excessive soil moisture (Figure 29).
One suggested approach is to identify wet depressions during silviculture prescription
and to leave these as uncut areas of green-tree retention. These wet areas commonly
do not regenerate readily to aspen, and the retained islands of mature trees — usually
balsam poplar — assist with transpirational removal of water from the wet sites, as well
as providing wildlife habitat and biodiversity benefits.
The recent tendency to bypass balsam poplar and to harvest mainly aspen for
oriented strandboard production has raised concerns about possible negative effects
upon aspen reproduction as a result of depressed soil temperatures under the shade of
residual balsam poplars. Furthermore, the retention of balsam poplar when aspen is
harvested raises questions about the overall effects on post-harvest aspen stand
development, growth, and yield. Some information is available from west-central Alberta
where aspen regeneration within and adjacent to clumps of balsam poplar residuals
87