Ontario’s boreal mixedwood forests, which stretch across much of northern Ontario, have a mixture of tree species, such as spruce, pine, aspen, poplar, and birch. These forests often grow on the most fertile sites and can be very productive. They also have some distinct advantages over forests dominated by a single species. For example:
- The more tree species a forest has, the greater the range of potential products for local mills.
- If a pest or disease sweeps through a mixedwood forest, it will probably affect only one or few species, leaving some or most of the trees unharmed.
- The more diverse a forest is, the more wildlife species it can support.
As a result, foresters are looking for practical ways to regenerate (get new trees to grow after harvest) mixed stands of white spruce and trembling aspen. They are especially interested in knowing the best ways to ensure that when the trees are young, competing plants do not overshadow the valuable conifers.
In 2002, a group of researchers led by Doug Pitt of the Canadian Wood Fibre Centre (and including OFRI's Bill Parker) set up a study with sites in west central Alberta and northeastern Ontario to shed light on radial and broadcast treatments to control plants that compete with planted spruce and naturally regenerating aspen. (Radial=controlling competition within a two metre radius of each spruce; broadcast=controlling all competition across the plot).
After five growing seasons, researchers report that:
- Planted spruce seedlings growing on plots that were not treated at all had only half the stem volume of those that had the lowest level of vegetation control. These trees will likely grow poorly for decades.
- Two to four years of controlling all competitor plants within a two metre radius of each spruce seedling (planted five metres apart) greatly improved the growth and development of spruce as well as aspen.
- Height growth of planted spruce can be improved when partially shaded by aspen, and seedlings will be less likely to suffer damage due to late spring frost.
The researchers point out that aspen and spruce have specific needs for water, light, and soil nutrients and that forest management objectives for boreal mixedwoods can vary as well, ranging from increasing the conifer component to creating more diverse wildlife habitat. This complexity means that foresters need a range of strategies and tools to restore and maintain boreal mixedwood forests.
For more details on study results, e-mail information.ofri@ontario.ca for a copy of Early Vegetation Control for the Regeneration of a Single-Cohort, Intimate Mixture of White Spruce and Trembling Aspen on Upland Boreal Sites, published in the Canadian Journal of Forest Research.
