Belsky, Joy, and Dana Blumenthal. 1997. Effects of livestock grazing on stand dynamics and soils in upland forests of the interior West. Conservation Biology 11(2):315-327.
RELEVANT TO: |
LIVESTOCK GRAZING FORESTS/RESTORATION MONITORING/ADAPTIVE MANAGEMENT |
DESCRIPTION OF DOCUMENT
This review of published Forest Service, BLM, and peer-reviewed literature examines the relationship of livestock grazing to the increase during the last hundred years in the West of dense forest stands of fire-sensitive and disease-susceptible trees. These have replaced ponderosa pine and mixed-conifer stands that were once more open and fire-tolerant. These changes have generally been attributed to fire suppression (which allowed fire-sensitive, shade-tolerant trees to increase) and "high-grading," i.e., selective logging of larger, more fire-resistant trees.
This review examines the evidence that livestock grazing is a third factor contributing to dense, fire- and insect- and disease-susceptible forest stands. Livestock (1) reduce the biomass and density of understory grasses and sedges, which can outcompete conifer seedlings; and (2) reduce fine fuels, which formerly carried low-intensity fires through the forests.
.
MAJOR FINDINGS
A. This paper describes three case studies that have compared grazed and nearby ungrazed forest stands:
1. WASHINGTON: Ponderosa pine and mixed-conifer forests grazed for 40 years prior to the 1951 study on Devils Table of central Washington; and nearby, similar forests never grazed on Meeks Table. Neither table had burned in 125 years.
At the time of the study, forests on ungrazed Meeks Table were open, park-like ponderosa pine and mixed-conifer, and "luxuriantly thick" grasses. Grazed Devils Table had only a sparse herbaceous understory and approximately 8000 ponderosa pine, Douglas fir, and larch seedlings/hectare.
2. IDAHO: Grazed and ungrazed ponderosa pine and Douglas fir forests, foothills of Bitterroot Mountains. The forests were selectively logged in 1925 and grazed from the turn of the century through the 1960s. In 1941 a 600 hectare livestock exclosure was established; elk and deer could enter the exclosure.
Forty years later, the grazed ponderosa pine had twice as many trees of <5cm diameter; these had been established after the exclosure was established. There was a greater density of Douglas fir outside the exclosure.
3. UTAH: Grazed ponderosa pine forests on Horse Pasture Plateau (HPP) of Utah and nearby, compositionally similar forests on Church and Greatheart Mesas, which had been protected from grazing livestock and fire by steep cliffs. Neither the mesas nor HPP had burned between 1892 and 1962.
In the 100 years prior to this study, tree recruitment on the grazed HPP had increased by a factor of 10 or more; whereas recruitment on the two nearby ungrazed mesas was unchanged. Mature-to-young tree ratio on HPP was 1:598; on Church and Greatheart Mesas it was 1:0.8. The researchers concluded that the vigorous understory vegetation inhibited tree recruitment on the ungrazed mesas.
The mechanisms by which livestock grazing contributes to dense, fire-, insect- and disease-susceptible forests are referenced and include: reduced competition to tree seedlings and fewer surface fires leading to dense tree regeneration, leading to suppressed tree growth, leading to increased tree vulnerability to insects and pathogens leading to increased tree mortality and fuel accumulation, leading to stand-replacing fires (see Figure 2, p. 319)
Studies are cited (see Table 1, pp. 321-322) for each of several mechanisms by which forest understory composition and soils are changed:
- Direct and selective consumption of grasses (uplands) and shrubs and trees (riparian areas), reduce and alter understory cover and consumption.
- Reduction of biomass to be converted into litter, and therefore increase of bare ground.
- Compaction of ground
- Increase in runoff and erosion
QUESTIONS THIS RAISES FOR THE THREE FORESTS
- What relevance might these livestock case studies and mechanisms have for forest stands experiencing the following conditions on these Three Forests?
- Low or absent aspen recruitment
- Dense conifer presence within aspen stands
- Encroachment of pinyon-juniper into sagebrush
- Dense, fire-susceptible forest stands
- Increase in invasive exotic species within forests
- Without large exclosures, how can the Three Forests separate forest health effects of turn-of-the century livestock grazing from current livestock grazing, fire suppression/logging from livestock grazing, or drought from livestock grazing?
- While the case studies in this article focus on livestock grazing versus no grazing rather than various intensities of livestock grazing, does any evidence exist that the current levels of livestock grazing on these Three Forests are not contributing to the forest conditions described in this study?
FOREST MANAGEMENT SIGNIFICANCE
- Large blocks of each of the Three Forests need to be livestock-free in order to understand the consequences and costs of livestock grazing in terms of forest health and/or degraded soil conditions and ecosystem roles. Small exclosures are insufficient in part because they are subject to elk/deer attraction; in part because they are unable to support landscape-scale restoration.
- It is a waste of public money to support fire suppression, prescribed fire, logging, and/or native shrub/forb/grass plantings if livestock grazing negates the effects.