ELK-WILLOW INTERACTIONS IN YELLOWSTONE’S NORTHERN RANGE

by

Print Friendly, PDF & Email

Stuart C. Wooley and Dr. Rex G. Cates, Botany & Range Science

In the last century an alarming decrease in willow (Salix spp.) populations and stature has occurred in the northern range in Yellowstone National Park (YNP) (Singer et al. 1994). The reasons for these declines have been heavily debated, and no consensus has been reached as to the actual causes. The most favored ideas are: 1) heavy elk browsing is occurring due to too large of an elk population (Wagner et al. 1995, see also Kay 1990), and 2) the willow decline is due to a combination of biotic and abiotic factors (Singer et al. 1994, Singer and Cates 1995). Singer and Cates (1995) propose that causal factors in the current willow decline are changes in climate (increased temperature and reduced precipitation) and in hydrology (lower water table, fewer floods) which stress willows by reducing the availability of habitat favorable for willow growth, reproduction, and seedling establishment and growth.

On the northern range over the last century annual precipitation has decreased 61 mm, temperature has increased 0.5-1.0 (C, and large-scale flooding which creates favorable willow habitat has decreased. Beaver dams are no longer established due to the extirpation of beaver (Wagner et al. 1995, Singer et al. 1994, see also Balling et al. 1992, Houston 1982). The net result is that the habitat has become drier and less favorable to willows. This in turn has decreased the ability of willows to produce defensive compounds against elk (e.g. carbon-based tannins and phenolics) (Singer and Cates 1995). Consequently, willows on the northern range are growing in a stressful environment which increases their susceptibility to elk browsing.

Elk heavily browse willows year round, but especially during the winter when food is in short supply (Singer et al. 1994). The objective of my research was to determine the relationship between willow carbohydrate levels in stem and leaf tissue and simulated levels of elk utilization in Yellowstone’s northern range. The hypothesis is that heavily clipped willows (50- 100% clipping current year’s growth) will have higher levels of soluble carbohydrates than the less heavily clipped willows (0% clipped).

Plant samples and experimental design
Available for carbohydrate analysis are 360 samples collected during 1989, 1990, and 1996 from 20 willow communities. Ten communities were classified by level of browsing as heavily browsed (HB) and 10 as less heavily browsed (LHB). For each sample year 120 samples are available, 60 of which are from HB communities and 60 from LHB communities. In addition, 52 plants in three exclosure (replicates) were used in a mechanical clipping study which involved 0%, 50% and 100% clipping of current year’s growth. The samples were collected in 4 oz. bottles, frozen in liquid nitrogen and then freeze-dried in the Chemical Ecology Lab at BYU. The 52 samples from the clipping study, were analyzed for soluble carbohydrate concentrations (e.g. glucose, galactose, fructose, mannose, sucrose).

Capillary Gas Chromatography (GC)
The analytical methods followed Zou and Cates (1995) with slight modifications. Each 250 mg sample was extracted in 20 ml of methanol and water (80% MeOH/20% water). Five mls of the resulting extraction were taken to dryness. The carbohydrates were then derivatized with 0.2 ml HMDS and 0.1 ml TMCS so the carbohydrates could evaporate in the GC. GC temperature programming was identical to Zou and Cates (1995). Sample peaks from the chromatograph were identified by co-chromatography using known standards. Concentrations each were expressed as milligrams of total carbohydrate per gram dry weight of tissue.

The data show that between all levels of clipping (0%, 50% and 100% of the current year’s growth) there were no significant differences in total carbohydrates between the clipping levels within a species nor among species. Consequently the hypothesis was rejected. However, this result is consistent with the idea that plants are growing in a poor habitat and therefore, they do not have the carbohydrate reserves to respond to clipping. It may also indicate that the plants have been stressed for an extended period of time reducing their stored carbohydrate content. The overall result is a decreased ability to mobilize carbohydrates as carbon sources for other metabolic processes such as carbon-based defenses and energy-expensive reproduction.

More study is needed and will be completed as a MS project at BYU under the direction of Dr. Rex Cates. Pooled with these data will be additional data on carbohydrate, phenolic and tannin content. Further conclusions will be made and possible solutions or management recommendations will be suggested.

References

  • Singer, F., L. Mack, R. Cates. 1994. Ungulate herbivory on Yellowstone’s northern winter range. J. Range Manage. 47:435-43.
  • Singer, F. R. Cates. 1995. Response to comment: Ungulate herbivory on Yellowstone’s northern winter range. J. Range. Manage. 48:563-5.
  • Wagner, F. H., R. Kiegley, C. Wambolt. 1995. Comment: Ungulate herbivory on Yellowstone’s northern winter range: Response to Singer, et al. (1994). J. Range Manage. 48:475-7.
  • Zou, J., R. Cates. 1995. Foliage constituents of Douglas Fir (Pseudotsuga menziesii (Mirb.) Franco (Pinaceae)): Their seasonal variation and potential role in Douglas Fir resistance and silviculture. J. Chem. Ecol. 21:387-402.