Influence of genotype and soil fertility on the susceptibility of Populus tremuloides to deer browsing

Background/Question/Methods

Trembling aspen (Populus tremuloides) is an important browse species for wild and domestic ungulates throughout most of its expansive range in North America. In the Interior West, ungulate browsing is contributing to the demise of aspen forests. Field observations suggest, however, that not all aspen are equally susceptible to browsing damage. This study investigated the resistance of aspen to deer browsing by examining the independent and interactive effects of tree genotype and soil fertility on deer feeding preference. To assess preference, we subjected replicates of seven unique aspen genotypes grown under low or high soil nutrient conditions to browsing by whitetail deer (Odocoilius virginianus) in southern Wisconsin. We deployed individual, 2-year-old containerized trees to a rural woodlot setting during the period of shoot elongation, and surveyed the proportion of current year shoots browsed during a 2-week exposure period. A complementary set of trees grown under the same conditions but not subjected to browsing was destructively harvested to determine chemical composition.

Results/Conclusions

The proportion of new shoots eaten by deer varied 5-fold among the aspen genotypes. Soil nutrient level did not influence browsing preference or the nitrogen concentration of young shoots. However, shoot nitrogen varied by 24% among genotypes and correlated negatively with browsing damage, suggesting a role of shoot nitrogen in modulating browsing preference. Variation in browsing was not related to carbon-based defense compounds or to carbohydrates, which varied over only a small range in our experimental trees.  Overall, our observations suggest that moderate deer browsing can potentially favor success of aspen that are less-palatable, exerting a selective pressure within wild aspen populations. The mechanism underlying genetically-determined aspen variation in susceptibility to deer browsing warrants further investigation.