Deer Herbivory and Riparian Restoration

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Riparian corridors are systems of high biotic, structural, and functionaldiversity. They serve as critical links between terrestrial and freshwaterecosystems and play an integral role in maintaining healthy streams. Riparianvegetation provides much of a stream's energy and nutrient budget in theform of leaf and wood litter. Larger branches and tree trunks that enterthe channel provide habitat and cover for fish, and play a major role inthe formation of pools. Overhanging foliage shades streams, helping to maintainlower water temperatures that are critical to the survival of cold-waterfish species. Riparian vegetation contributes to bank stability throughroot systems that anchor the soil, and by increasing channel roughness toreduce 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, particularlyin arid or semiarid regions, provide critical habitat components that cannotbe found elsewhere.

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

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

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

Vegetation along Feliz, Parsons, and Robinson Creeks was surveyed inthe summer of 1997. Regeneration within exclosures was compared to controlreaches where livestock grazing did not occur. Six exclosures were includedin 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 measurementto allow comparisons between streams.

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

These results suggest that deer may play a large role in reducing therate of natural regeneration of degraded riparian corridors. Thus, thispossibility should be considered when developing restoration strategiesfor sites with high deer densities. The response of vegetation within deerexclosures on Feliz and Robinson Creeks indicate that eliminating deer herbivorywas the key factor in the recovery of the riparian corridors. Recovery occurredwithout planting, irrigation, or channel modification. Therefore, removingthe stressor of deer herbivory resulted in dramatic improvement withouthaving to use more expensive, and often risky, active restoration techniques.Deer herbivory may not play as large a role in other regions, or along othersizes or types of stream. However, it may be possible to identify otherstressors that act as ecological constraints to system recovery. Reducingor eliminating these stressors may be the first step toward successful restoration.Where elimination of stressors is not effective, or not feasible, activetechniques for restoration can then be considered.


prepared and edited by Richard B. Standiford and Pamela Tinnin