Adaptive Management Projects

Project Summary: Effect of Stand Thinning on Feeding Damage by Red Squirrels

Management Issue:

Will thinning juvenile stands of lodgepole pine reduce feeding damage by red squirrels, and enhance stand productivity?

Red squirrels damage trees in juvenile stands of lodgepole pine by feeding on the vascular tissue, resulting in both suppressed tree growth and tree mortality. Thousands of hectares in the Interior of B.C. are susceptible to feeding damage.

Stand thinning may reduce feeding damage by reducing cover, thus creating marginal habitat for squirrels.

Thinning also stimulates growth of young lodgepole pine, increasing stand productivity. In addition, past studies suggest that feeding damage may decline when trees reach a dbh of greater than 20 cm; by increasing the rate of diameter growth, thinning may decrease the length of time that juvenile trees are vulnerable to feeding damage.

However, these benefits may be offset because squirrels preferentially feed on vigorous trees.

Thinning could contribute to the maintenance of biological diversity. There are few low density stands of lodgepole pine in B.C.. Thinning could maintain understory vegetation for longer periods, creating stands with greater structural diversity, and providing a greater diversity of wildlife habitats.

Most pre-commercial thinning prescriptions call for a target density of 1600-2000 stems per ha, but stands of this density still suffer significant feeding damage.

Can we define a thinning regime that will reduce feeding damage, optimize stand productivity, and provide a greater diversity of habitats for wildlife?

Management Strategy:
A previous short term, small scale study indicated that thinning reduced populations of red squirrels, but yielded no clear information about the impact on feeding damage.

Researchers designed and implemented a large (i.e., operational) scale experiment to test the impact of 3 levels of thinning on damage, productivity, and habitat diversity. The least severe thinning (i.e., highest density) was the level normally prescribed for precommercial thinning (2000 stems/ha).

Treatments were applied to juvenile stands of lodgepole pine, which ranged in size form 15-30 ha, in 3 study areas. The study areas were located in 3 different biogeoclimatic subzones -IDFdk, MSdm, and SBSdw. All sites had experienced substantial feeding damage. In each study area, indicators were also monitored in an old-growth stand and an unthinned juvenile stand. Sites were monitored for 5 years.

Benefits:
In all 3 biogeoclimatic subzones, thinning reduced feeding damage and enhanced diameter growth, and did not negatively affect diversity of small mammals or understory vegetation. Thinned stands are increasing in structural complexity, and hence are providing a greater diversity of wildlife habitats.

Researchers argue that results from this large scale experiment are more useful for management than results from previous small scale experiments, because the movements and home ranges of the squirrels are accommodated within treatment units.

Limitations:
Monitoring has not continued long enough to determine whether the faster diameter growth decreases the length of time that juvenile trees are vulnerable to feeding damage.

The prescriptions for management are not clear. In order to determine which level of thinning maximizes timber productivity, managers must know whether the increase in volume resulting from reduced feeding damage and faster diameter growth offsets any decrease in volume resulting from fewer stems/ha.

Managers must also assess whether any increase in volume is sufficient to offset the cost of thinning. Finally, in making decisions about whether, or at what level to thin, managers must consider the non- monetary benefits - of increased stand diversity, and the contribution of the stand to landscape diversity. None of these questions is answered by this management experiment.

References:

Sullivan, T. 1993. Influence of lodgepole pine stand density on red squirrel populations, feeding damage, and wildlife habitat diversity. Draft manuscript, 64 pp. FRDA II.