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This Extension Note is based on a presentation by Teresa Newsome at the Northern Interior Vegetation Management Association (NIVMA) 1995 annual general meeting in Williams Lake.
Trembling aspen (Populus tremuloides) and other hardwoods have traditionally been regarded as weeds and treated as noncommercial species. At best, aspen suckers in young stands have only been retained where they were not inhibiting conifer growth. Aspen, however, has become an economically valuable species in some areas of the British Columbia interior, and requirements for the maintenance and enhancement of biodiversity under the Forest Practices Code are also encouraging forest resource managers to consider natural aspen regeneration as a managed component of a new stand.
While serious competition problems are treated or best avoided altogether, some advantages of retaining aspen and other hardwoods as a component of conifer stands are:
COMPETITION INDICES
A competition index is used to predict levels of competition between plant species for making treatment decisions. The decision to treat a stand rests on whether or not a certain threshold of competition has been exceeded. Relative treatment costs are also a factor in the treatment decision.
Many competition indices have been developed by researchers with the intention to produce a simple but reliable indicator of competition levels. In this retrospective study, 11 competition indices were tested using measurements from some natural pine/aspen stands.
STUDY METHODS
Fifty lodgepole pine were selected from three naturally regenerated stands between 9 and 15 years of age in the SBSdw1 subzone. The number of aspen growing around a pine within a 1.8 m radius varied from zero to over 20 stems. Within each 1.8 m radius plot, the following data were collected:
RESULTS AND DISCUSSION
Initial correlations were high between pine basal area and basal area growth and three variables: aspen density, the sum of the pine stem to aspen stem distances, and the sum of the pine stem to aspen crown distances. Aspen density was chosen for further analysis as it showed the clearest relationship and is easily measured operationally.
CONCLUSIONS
Pine diameter and diameter growth become significantly reduced at more than 2000 aspen stems/ha compared to pine growing free of competition. Therefore, up to two aspen may be left in a 1.8m radius plot without seriously affecting pine diameter growth.
At more than 5000 aspen stems/ha, pine total height and leader growth were significantly reduced when compared to the largest pine height and leader measurements found at 1000 aspen stems/ha.
Leaving 1000 stems/ha of aspen will not significantly reduce pine diameter growth and may actually increase pine height growth while encouraging self-pruning of the pine through the minor aspen competition.
This retrospective study of 9 to 15-year-old naturally regenerated lodgepole pine stands in the SBSdw1 has yielded some preliminary recommendations. These recommendations should be used with caution, however, as they are based on measurements taken on only a few sites and may be affected by stand age.
RECOMMENDATIONS
If retention of aspen as tall or taller than the pine is prescribed, leave one to two of these aspen within a 1.8m radius from the pine.
To insure optimal pine growth, remove aspen that have diameters equal to or greater than 0.75 times the pine diameter.
Minimize whipping damage by removing any aspen that could potentially abrade the conifers if their crowns were to interact in wind.
CONTACT
For more Information, contact Teresa
Newsome, Research Silviculturist, at (250) 398-4408
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