Adam Olson and Dr. Sam St. Clair, Plant and Wildlife Sciences

Introduction

Browse damage to the quaking aspen (Populus Tremuloides) has dramatically increased in recent years due to proliferate wildlife densities, especially elk, in the central Rocky Mountains (Kaye et al. 2005). To protect itself from this, the quaking aspen produces chemical compounds called phenolic glycosides which deter wildlife from browsing the leaves. While the quaking aspen is known to increase the levels of these compounds in response to herbivory (Young et al. 2010), it is unknown whether these increases effect the entire canopy or only locations that are vulnerable to browsing. The purpose of this project was to test whether highly browsed areas of the aspen canopy are more strongly defended than other, more out-of-reach locations. By varying defense compound concentrations throughout the canopy, the quaking aspen can theoretically conserve resources that would otherwise be used for full canopy defense. The hypothesis states that mature and juvenile aspen trees will show higher concentrations of defense chemistry in the lower versus the upper canopy as this area is more easily browsed by local wildlife. Aspen suckers were hypothesized to have increased phenolic glycoside concentrations in the upper canopy where they are more heavily browsed.

Methodology

Three different aspen classifications were used in this study so as to test for concentration variance within the aspen canopy in addition to variations as a function of age. The age classes were chosen on the basis of tree height and are defined as: sucker (1-3 feet), juvenile (4-10 feet), or mature (12+ feet high with a browse line no less than 5 feet in height). The samples from each classification of quaking aspen were collected from both the tops and bottoms of each tree. A 15’ telescoping tree pruner was used to reach the top of mature aspen trees at an average height of 20’. These samples were separated into individual envelopes and stored on dry ice to preserve the chemical compounds and prevent tissue decomposition. The samples were collected from 10 different locations spanning over 250 miles from southern to northern Utah. Sites were selected on the basis that all three aspen types were identifiable in close proximity to one another so as to maintain a consistent environment between each sample classification.

All of the samples were freeze dried upon return from collection, so as to completely preserve the concentration of chemical compounds within each sample. These samples were then ground into small tissue particles and weighed out at 40 mg each. Using extraction and analysis methods described by Richard L. Lindroth et al. (see references below), the phenolic glycoside concentrations for each aspen sample were collected. Subsequent analysis of this data in terms of both location and aspen classification became the basis for the results of this study.

Results

Initial results from this study show significant differences between each age class of quaking aspen. Mature aspen trees show lower concentrations of phenolic glycosides as compared to younger trees while aspen suckers show the greatest percentage of compound concentration. In addition, aspen suckers show an overall higher concentration of defense chemistry in the upper canopy where as mature and juvenile aspen trees show higher concentrations in the lower canopy. These differences are in line with the hypothesis of this study for each classification of aspen tree. While the differences between upper and lower canopy concentrations exist for each classification, p-values show that these differences are not statistically significant from one another.

Discussion

The results of this study show that further research is required before any conclusions can be made with regard to the proposed hypothesis. This is primarily because of the conflict that exists between observed differences and statistical significance. Each aspen classification shows variance between the upper and lower canopy, however, the data from this study alone does not show statistical significance overall. Nonetheless, an interesting correlation deserves to be explored in the way these differences lay in accordance with the proposed hypothesis. For instance, even though concentration variances were smaller than expected, the effects of even these small differences could mean important changes in terms of herbivory and browse damage to the quaking aspen. Future research regarding the effects of small concentration differences, as well as additional studies verifying the statistical significance of canopy variance, will prove important in the future progress of this study.

Conclusion

The results of this study show support both for and against the proposed hypothesis. There is statistical evidence that varying concentrations of defense chemistry exist within the quaking aspen canopy. However, whether these differences have any significant effect on wildlife browsing is currently unknown. Further research will provide more information regarding the consistency of canopy variation in the quaking aspen and its role in resource conservation and wildlife browsing.

Scholarly Sources

• Clair, S. B. S., S.D. Monson, E. A. Smith, D. G. Cahill, & W. J. Calder. 2009. Altered leaf morphology, leaf resource dilution and defense chemistry induction in frost-defoliated aspen (populus tremuloides). Tree Physiology. 29.10:1259–1268.
• Donaldson, J. R., M. T. Stevens, H. R. Barnhill, R. L Lindroth. 2006. Age-related shifts in leaf chemistry of clonal apsen (populus tremuloides). J. Chem. Ecol. 32.7:1415-1429.
• Hemming, J.D. & R.L. Lindroth. 1999. Effects of light and nutrient availability on aspen: growth, phytochemistry, and insect performance. J. Chem. Ecol. 25.7:1687–1714.
• Kaye, M. W., D. Binkley, & T. J. Stohlgren. 2005. Effects of conifers and elk browsing on quaking aspen forests in the central Rock Mountains, USA. Ecol. Appl. 15.4:1284-1295.
• Lindroth R., R. Kinney, & C. Platz. 1993. Responses of deciduous trees to elevated atmospheric CO2: productivity, phytochemistry and insect performance. Ecology 74.3:763-777.
• Osier, T. L., & R. L. Lindroth. 2006. Genotype and environment determine allocation to and costs of resistance in quaking aspen. Oecologia. 148.2:293-303.
• Smith, E. A., S. B. Collette, T. A Boynton, T. Lillrose, M. R. Stevens, M.F. Bekker, D. Eggett, S. B. S. Clair. 2011. Developmental contributions to phenotypic variation in functional leaf traits within quaking aspen clones. Tree Physiology. 31.1:68-77.
• Young, B., D. Wagner, P. Doak, & T. Clausen. 2010. Induction of phenolic glycosides by quaking aspen (populus tremuloides) leaves in relation to extrafloral nectaries and epidermal leaf mining. J. Chem. Ecol. 36.4: 369-377.