1994 Proceedings
North American Conference on Savannas and Barrens

For the past several years there has been increasing controversy regarding the ecological consequences of cattle grazing on public lands in the West. The U. S. Forest Service and Bureau of Land Management, agencies that grant grazing permits to ranchers, recently have experienced considerable pressure to monitor more stringently the ecological health of public rangeland, and to take swift action in instances of resource abuse. Unfortunately, but predictably, arguments too frequently have polarized with one side demanding an end to all cattle grazing on public land and the other denying the existence of any overgrazing and opposing any change from the status quo. Secretary of Interior Babbitt has made rangeland reform a priority, and one result is likely to be greater accountability from both the permit-granting agencies and the permitees themselves.

From the standpoint of the resource, the best grazing management decisions are those that are not merely the result of political pressures, but are based on documented information regarding the effect of cattle grazing on the plant community. Many data have accumulated from grazing studies in semi-arid landscapes, but there is a dearth of quantitative data on grazing in high altitude dry-mesic habitats. The objective of this study was to assess the effect of late season cattle grazing on the herbaceous understory in open aspen forest at an elevation of 2900 m. Specific questions included: 1) Does the understory vegetation differ between grazed and ungrazed areas?; 2) Does the plant community continue to change after grazing is halted?; 3) Over time, are any directional changes evident in the grazed understory community (e.g. an increase or decrease in forb abundance)? This study is part of a larger, long-term investigation on the effects of cattle grazing in several high altitude habitats in Colorado.

STUDY AREA

The study area is in Gunnison County, Colorado in the Copper Creek drainage of the upper East River Valley. The herbaceous understory vegetation in mature aspen stands at this location is dominated by tall perennial forbs rather than grasses (Langenheim 1962). Meadow rue (Thalictrum fendleri), lovage (Ligusticum porteri), aspen sunflower (Helianthella quinquinervis), and wild sweet pea (Lathyrus leucanthus) are common species. The vegetation is luxuriant, probably due to heavy snowfall which provides a persistent source of water throughout the growing season, and deep soils (Langenheim, 1962). The growing season is relatively short; typically plants emerge from the soil in mid-May and begin to senesce in mid- to late August. Below freezing temperatures at night can occur any time during the summer.

A cattle-proof fence bisects the study area. Ungrazed aspen (Aspen Ungrazed, AUG) had been protected from grazing for seven years when the study began, while the grazed aspen area (Aspen Grazed, AG) was annually grazed between the last week in August and the first week in October. The study area on each side of the fence was approximately one acre. It was difficult to determine actual grazing pressure on the site. The cattle graze over many acres and the number of animals and the number of days spent in any area depends on many variables such as the weather, the amount of harassment by recreationists or dogs, how frequently the rancher moves the cows back up valley, etc.

METHODS

Beginning in 1992, the vegetation was randomly sampled four times between early June and mid-August. In each area (AUG and AG), data were collected from twenty 10 x 100 cm quadrats. In each quadrat, each plant was identified and the number of individuals of each species recorded. Ten per cent of the quadrats were clipped to within 1 cm of the ground. The clipped material from each quadrat was placed in a sealed plastic bag.

In the laboratory, the contents of each bag were separated by species and weighed. The vegetation was then oven-dried to a constant weight to determine percent water content and biomass. In 1993, biomass only was measured in early October (one week post-grazing). Species nomenclature and identification follows Weber (1987) and Buck and Frase (1993).

RESULTS

For the parameters measured, only slight differences were apparent between AUG and AG, nor were there major differences between years (Table 1). Sorensen's Index of Similarity (Oosting, 1956), a measure of concordance in species composition between the sites was greater than 0.8. Productivity, as measured by dry biomass, was higher in AUG, but the contribution of adventive species to the total biomass (< 25%), did not differ significantly between AUG and AG. More species were present in AG than in AUG, but species diversity was essentially the same in both areas.

DISCUSSION AND CONCLUSION

The high values for Sorensen's Index (Table 1) indicated that AUG and AG support similar floral communities. There was no obvious pattern in the list of non-overlapping taxa; i.e., one site did not have more weedy annuals or more grass species than the other.

The lack of differences in biomass between years in both areas was unexpected given the differences between growing seasons. Snowfall was much higher than average in 1993, and spring (snowmelt) was at least three weeks later than normal. Yet plant biomass was not significantly lower than in the more typical year of 1992. One hypothesis is that because of delayed emergence, plants experienced "better" growing conditions such as warmer soil temperatures and greater isolation in the spring of 1993, and these factors allowed the plants to compensate for a late start.

Biomass was only slightly higher in AUG than in AG during the summer (Table 1). Biomass in AG was approximately 7% lower than in AUG during the growing season, and after grazing in the fall, biomass in AG was 20% lower than in AUG.

Possible, not mutually exclusive, explanations for the lack of great differences between AUG and AG include: 1) the habitat can sustain current grazing pressure, i.e. the range is not overstocked; 2) fall grazing occurs after seed set and at a time when plant senescence has begun; therefore, it has little effect on growth and reproduction of these perennial species the following season,; 3) grazing affects some aspect of the vegetation community that is not measured in this study; 4) insufficient time has elapsed since grazing has been prevented in AUG for there to be an obvious response in long lived perennials, and therefore AUG is an inadequate control; 5) the effect of grazing might be such that only under conditions of stress will the community be adversely affected.

ACKNOWLEDGMENTS

The author gratefully acknowledges the able assistance of Kevin Taylor, Christine Tonielli, Kristi Nix, and Marcia Struble in the field. Thanks are due Bill Trampe, whose cows do the grazing, for his unfailing cooperation and support of the study. The assistance of Mark Hatcher and Gay Austin of the U. S. Forest Service is greatly appreciated. Partial financial support was provided by the U. S. Forest Service, the Gunnison County Stockgrowers Association, and Bradley University. Facilities were provided by the Rocky Mountain Biological Laboratory.

LITERATURE CITED

Buck, P. and Frase, B. 1993. Vascular plants of the Gothic area. Misc. Publ. Rocky Mountain Biological Lab. No. 1. 29p.

Langenheim, J. 1962. Vegetation and environmental patterns in the Crested Butte Area, Gunnison County, Colorado. Ecological Monographs 32:249-285.

Oosting, H. 1956. The Study of Plant Communities. W. H. Freeman and Co.

Weber, W. 1987. Colorado Flora: Western Slope. Colorado Association University Press.

Table 1. Summary data from grazed (AG) and ungrazed (AUG) sites in aspen understory. Biomass values are dry weights (g) from clipped quadrats; total biomass is the sum of four summer sampling sessions. The October biomass was determined one week after grazing ended. H' is the Shannon-Weiner Diversity Index using log to the base 2, averaged for the four sampling sessions. Sorensen's Index is a measure of similarity in species composition.