Brookshire, Jack; Boone Kauffman, Danna Lytjen and Nick Otting. 2002. Cumulative effects of wild ungulate and livestock herbivory on riparian willows. Oecologia 132:559-566.
RELEVANT TO: |
GRASSLANDS/SHRUBLANDS AQUATIC/RIPARIAN LIVESTOCK WILDLIFE |
DESCRIPTION
This is a five-year, small-exclosure, riparian willow study in which three exclosures were established in an area grazed only by deer and elk; and three other exclosures were established in an area which elk and deer browsed and domestic sheep grazed lightly during summer months (i.e., herders had been instructed to keep sheep out of riparian areas except for access to water for drinking). "This resulted in very low utilization rates within riparian zones. Within the study area, it was difficult to detect current-year utilization of herbaceous plants" (p. 560). The sites were in the spring-fall range of elk and "both mule deer and elk commonly utilized the riparian zones" (p. 560).
The study was conducted along a 1 km section of Upper Meadow Creek, a tributary to the Grande Ronde River in the Blue Mountains of northeast Oregon, at elevations of 1,280-1,340 m. The willow community consisted of Salix boothii and S. geyeriana.
The exclosures, 4.7 by 4.7 m and 2.5 m tall, were built in 1996 after 159 study plants had been tagged (97 and 62 respectively of S. boothii and S. geyeriana). Height of tallest stem was taken before (1996) and after (1997, 1998, 2000) the exclosures were built. After the exclosures were built, number of tagged plants browsed inside and outside exclosures was also determined. In 1998, willows flowered for the first time; number of flowers and basal stems on each plant were counted; diameter of largest basal stem; mean stem diameter (on 5 random stems/plant); and dimensions of the leaf crown.
MAJOR FINDINGS
- The percent of plants browsed outside exclosures was 89-100%, with no significant difference between wild ungulate and sheep + wild ungulate areas.
- Exclosure effects increased over time (1998-2000).
- At the onset, mean heights of willow were significantly greater (69 cm) in the wild ungulate area than in the sheep + wild ungulate area (51 cm), but no significant differences between tagged willows inside and outside the exclosures.
- By 2000, willows inside exclosures were 58-174% taller, significantly broader in crown area, and produced significantly larger mean stem diameters compared to willows outside exclosures.
- Height of browsed S. boothii increased significantly in the wild ungulate area, but neither species increased in the wild ungulate + sheep area.
- From 1998-2000 about 70% of browsed willows in the sheep + wild ungulate area incurred reductions in main stem diameters.
- After 2 years of protection from herbivory, 22% of willows flowered with about 10 catkins/plant; 17% flowered in the wild ungulate area with about 3 catkins/plant; no flowering occurred in the wild ungulate + sheep area.
- No willows <70 cm in height flowered.
Height (the primary mechanism for escaping herbivory) was the most strongly affected by exclusion of herbivory.
"Thus, in contrast to studies of willow herbivory reporting stimulated branch growth [citation], basal stem sprouting citation], and flowering [citation] following singular herbivory events, our results suggest that repeated, long-term growth suppression can eliminate compensatory responses (i.e. increased production of basal stems or flowers) to herbivory." (p 564; emphases added).
"Ungulates strongly depressed flowering of willows, particularly within the sheep + wild ungulate area" (p. 564). Similar results were found with elk browsing in Yellowstone National Park (Kay research).
"These species reproduce sexually and do not spread vegetatively [citation], thus long-term suppression of flowering would likely affect their persistence in riparian zones" (p. 564).
"Our study suggests that even relatively light levels of livestock grazing can limit growth and reproduction of woody vegetation in riparian zones that are also browsed by wild ungulates" (p. 565).
"Although willows likely constitute a small proportion of ungulate diets [citation], they may be disproportionately affected across the landscape because of their high palatability and limited distribution" (p.565).
"Riparian management that would result in high densities of wild ungulates while maintaining livestock grazing may conflict with the restoration of riparian vegetation and the high degree of biological diversity inherent to these ecosystems" (p. 565).
QUESTIONS RAISED FOR THE THREE FORESTS
- What proportion of willow habitat on the Three Forests is experiencing recruitment of willow sprouts to heights above browsing, and flowering?
- What data do the Three Forests have from livestock and/or ungulate exclosures in riparian willow habitat?
RELEVANCE TO FOREST MANAGEMENT
- Salix boothii and S. geyeriana are both dominant willows in a number of Utah riparian community types, indicating the direct relevance to the Three Forests of this article's findings regarding impacts of even light domestic livestock grazing in conjunction with wild ungulate browsing.
- Where willows are rarely escaping herbivory through vertical growth or are not flowering, livestock should be regarded as incompatible with functional willow persistence, riparian integrity, and biodiversity.
- Livestock should not be grazed in riparian areas.
- Ungulate exclosures in livestock-free riparian areas are important for determining whether elk are preventing vertical growth or flowering of riparian willows.
- If willows protected from livestock grazing are not growing vertically and flowering due to wild elk herbivory, the number of elk need to be reduced (e.g., by wolves, hunters) to numbers demonstrated to be sustainable for the long-term functional presence of willows and riparian biodiversity.