Deer Herbivory and Riparian Restoration

Oaks ‘n’ Folks – Volume 13, Issue 1 – February, 1998

Riparian corridors are systems of high biotic, structural, and functional diversity. They serve as critical links between terrestrial and freshwater ecosystems and play an integral role in maintaining healthy streams. Riparian vegetation provides much of a stream’s energy and nutrient budget in the form of leaf and wood litter. Larger branches and tree trunks that enter he channel provide habitat and cover for fish, and play a major role in the formation of pools. Overhanging foliage shades streams, helping to maintain lower water temperatures that are critical to the survival of cold-waterfish species. Riparian vegetation contributes to bank stability through root systems that anchor the soil, and by increasing channel roughness to reduce the energy of high flows. In addition to influencing stream conditions,riparian vegetation is an important ecological habitat in its own right. Riparian corridors frequently support high biodiversity, and, particularly in arid or semiarid regions, provide critical habitat components that cannot be found elsewhere.

A study was conducted to investigate the influence of deer herbivory on rates of natural revegetation within degraded riparian corridors in MendocinoCounty. The vegetation along three streams with deer exclosures in the upper Russian River watershed was surveyed and compared with unfenced control reaches to determine the effects of deer browsing on natural system recovery.

The three streams included in this study are located in the Coast Range of Mendocino County and flow directly into the Russian River. Many riparian corridors in this region have been degraded due to clearing for agriculture,overgrazing, and gravel mining. As described above, riparian vegetation plays a significant role in maintaining healthy stream habitats. Because the Steelhead trout and Coho salmon fisheries in the Russian River were greatly reduced (and recently Federally listed as threatened), the restoration of riparian corridors along spawning streams became a priority.

Restoration of riparian corridors can be implemented through active or passive approaches. Active strategies for restoration include planting riparian trees, providing irrigation, or reshaping the channel or banks. Passive restoration focuses on the removal of a stressor that has contributed to system decline. In riparian corridors affected by overgrazing, reducing or eliminating grazing frequently has been found to have dramatic results of system recovery. Livestock grazing historically occurred along the streams in this study, but had not been conducted for many years. Therefore, herbivory of riparian vegetation along these streams primarily was attributed to deer. To address this possibility, deer exclosures were erected 17 years ago on Feliz Creek, and six years ago on Robinson Creek. In 1993, deer exclosures were constructed on several sections of Parsons Creek that cross the Hopland Research and Extension Center (HREC).

Vegetation along Feliz, Parsons, and Robinson Creeks was surveyed inthe summer of 1997. Regeneration within exclosures was compared to control reaches where livestock grazing did not occur. Six exclosures were included in the study: two on Parsons, one on Robinson, and one on each of the North,South, and Middle forks of Feliz Creek. Exclosures were of varying size, therefore numbers of trees and saplings were converted to a density measurement to allow comparisons between streams.

The difference between the regeneration within fenced plots and unfenced controls was dramatic. Regeneration included several willow species, cottonwood,alder, and Oregon ash. Combining the data from the six paired plots, the mean regeneration within exclosures was 45 trees per 10 square meters, compared with controls, where mean regeneration was 4 trees per 10 square meters(Table 1). Regeneration always was greater in fenced plots.

Table 1. Density of riparian regeneration vegetation (saplings per 100 square meters) within and without fenced exclosures along three stream corridors in Mendocino County.

Stream Fenced Unfenced
Robinson 85 10
Feliz South 49 2
Feliz Middle 45 1
Feliz North 66 2
Parsons 1 11 1
Parsons 2 13 7

These results suggest that deer may play a large role in reducing the rate of natural regeneration of degraded riparian corridors. Thus, this possibility should be considered when developing restoration strategies for sites with high deer densities. The response of vegetation within deer exclosures on Feliz and Robinson Creeks indicate that eliminating deer herbivory was the key factor in the recovery of the riparian corridors. Recovery occurred without planting, irrigation, or channel modification. Therefore, removing the stressor of deer herbivory resulted in dramatic improvement without having to use more expensive, and often risky, active restoration techniques. Deer herbivory may not play as large a role in other regions, or along other sizes or types of stream. However, it may be possible to identify other stressors that act as ecological constraints to system recovery. Reducing or eliminating these stressors may be the first step toward successful restoration. Where elimination of stressors is not effective, or not feasible, active techniques for restoration can then be considered.

prepared and edited by Richard B. Standiford and Pamela Tinnin