Research report: Promoting birch in mixed stands
Want to grow quality paper birch in mixed aspen-birch stands? An article in the September 2008 issue of the Northern Journal of Applied Forestry reports dramatic results from early birch release in mixed birch-aspen stands in Minnesota.
Study purpose and design: The research was designed to evaluate the impact of early timber stand improvement (TSI) in mixed stands at the Cloquet Forestry Center. Before treatment, stands were 16-18 feet tall with 1500-3000 stems per acre of paper birch and trembling aspen. Three different treatments were implemented. In each case, birch stems were released. This means that competing aspen or other stems were removed to give the birch more growing space.
The release treatments differed in intensity, with post-treatment stem densities from 250 to 750 stems per acre. Preference was given to birch stems, but red maple and aspen were also retained where no birch was present to meet the spacing requirements.
Results and discussion: All three treatments led to major increases in the birch component relative to aspen. In the control (untreated) plots, birch formed only about 14% of stand basal area six years post-treatment. In the treatment plots, birch formed 77-87% of stand basal area. There was little difference among treatment intensities. The increase in birch basal area as a percent of the total basal area was due mostly to reduction in other species, but also to increased birch growth.
Relevance for woodland owners: If you’re trying to grow birch, this article should be of great interest. With a brushsaw in a young stand, you can quickly release seedlings of birch or other species you want to promote. This Minnesota study documents significantly reduced birch mortality and increased birch growth as a result of the treatment.
In different stand types, TSI could lead to other desirable outcomes. For example, releasing oak or other masting species can enhance habitat quality for mast-eating wildlife like deer or wild turkey.
What about financial outcomes? Early and frequent thinning is usually the best way to optimize stand productivity and value. Although these treatments may be expensive (and in some cases cost-prohibitive) with paid labor, well planned treatments can dramatically enhance the value of subsequent thinnings and the final harvest. If you can do the work yourself or with family, your costs will be low and your return on investment greater.
Thinnings and TSI do not increase overall stand growth over time. However, by focusing growth on a smaller number of the best stems in the stand, TSI can move stems into higher-value classes (e.g. sawtimber or veneer vs. pulp) more quickly.
For this study, trees were selected for retention based on the following criteria: “(1) upper crown classes, (2) vigorous and healthy stem, (3) good bole quality, and (4) uniform spacing relative to other leave trees. Birch stump sprouts were thinned to one or two sprouts that werethe most vigorous and straight and that started low on the stump” (p. 125). These same or similar criteria would apply to most TSI treatments.
TSI benefits can be quite variable, and there may be important reasons not to implement treatments like this in your woods. Every site is unique, and it’s important to talk to a local forester about species selection, optimal spacing, and other considerations before heading to the woods with your brushsaw.
More about the reviewed article:
Zenner, Eric K.; Puettmann, Klaus J. 2008. Contrasting Release Approaches for a Mixed Paper Birch (Betula papyrifera)-Quaking Aspen (Populus tremuloides) Stand. Northern Journal of Applied Forestry, Volume 25, Number 3, pp. 124-132.
Editor’s note: This is the first in a series of research reports. To comment or suggest topics, either click “leave a reply” below or email esagor-at-umn-dot-edu