Effects of Herbivory by Abert Squirrels (Sciurus aberti) on Cone Crops of Ponderosa Pine

Abert squirrels (Sciurus aberti) depend on ponderosa pine (Pinus ponderosa) for cover and food resources. As squirrels forage for their food from the pines by "twig clipping," they induce losses of green foliage as well as losses of developing and mature seeds by removing immature and mature ovulate cones of certain trees. This investigation quantified the impact of herbivory by squirrels on a cone crop produced by a stand of mixed-aged ponderosa pines situated on a 5.6-ha site. We found the cone production of certain trees to be greatly reduced through intense foraging on the inner bark of excised pine shoots that bore attached conelets (immature ovulate cones). Such effects reduced the potential cone crop of the stand by 20%. In addition, by later shifting their foraging from inner bark to seeds of maturing cones, squirrels further reduced the cone crop of the stand, resulting in a total reduction of 55% over the 2 years.