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Exclosure Size Influences Rodent Damage To Blue Oak Seedlings

Oaks ‘n’ Folks – Volume 13, Issue 2 – August, 1998

Studies have demonstrated that a large number of blue oak seedling sexist in the southern Sierra foothill rangeland. However, few persist to the sapling stage (i.e., five to ten feet tall. How can we get these seedlings to become saplings? Attempting to answer this question, we are using grazing exclosures to protect seedlings until they grow above the browsing line. We compared blue oak seedling survival in cattle exclosures of various sizes:16 x 16 feet square and 5-, 4-, and 2-foot diameter. This article summarizes the results of these studies. All exclosures were made of 0.25 inch thick,54-inch- tall, welded steel cattle panels.

Compared to seedlings in an unfenced control area, dramatic increases in rodent damage to oak seedlings were observed in 16 * 16-foot exclosures (Table 1). Damage ranged from removing part of the bark at ground level to removing bark as high as 48 inches above ground. In some instances, bark was removed from half or less of the trunk of a seedling. However, 55% of the seedlings were girdled, resulting in the death of the shoot. During the next growing season, these seedlings often sent up new shoots from the roots. None of the monitored seedlings was killed by rodent damage, but,in year 5, 85% of shoots in exclosures were rodent damaged compared to 21%in controls. In contrast to rodent damage in the 16 x 16-foot exclosure,no rodent damage occurred to seedlings inside the 5-, 4-, and 2-foot diameter exclosures.

These studies indicate that managing the amount of residual plant material(primarily grasses and ground cover) can influence the amount of rodent damage to blue oak seedlings. In large areas excluded from grazing, large amounts of plant residue, which provides habitat for rodents, can accumulate.Conversely, habitat for rodents is not provided by individual tree exclosures between two and five feet in diameter and plant residue outside the exclosures can be reduced by grazing, mechanical mowing or disking, or by the use of chemicals.

Table 1. Percent of seedlings rodent damaged in fenced, 16 * 16- foot exclosures and unfenced areas.

Plot 1 2 3 4 5
Fenced Exclosure 0 25 50 75 85
Unfenced 0 0 7 0 21


 prepared and edited by Richard B. Standiford

Ralph L. Phillips, Neil K. McDougald, Range/Natural Resources and Livestock Advisors, U.C. Cooperative Extension, Kern and Madera Counties