Thinning – a tool for restoration of California's Southern Sierra Nevada blue oak woodlands

Fire frequency on a blue oak (Quercus douglasii) dominated rangeland in California's southern Sierra Nevada foothills was approximately every 10 years until 1965, followed by a 30 year period of fire exclusion. This resulted in a dense tree overstory with small diameters, high crown cover, poor acorn and forage production, and limited complexity in vertical habitat structure. Tree thinning was evaluated as a treatment to restore pre-fire stand structure to these rangeland areas. Three thinning treatments were evaluated: (1) thin one-third of the standing tree basal area (resulting in a post-thin crown cover of 33 percent, 66 square feet per acre of tree basal area, and 160 trees per acre); (2) thin two-thirds of the standing tree basal area (resulting in a post-thin crown cover of 12 percent, 35 square feet per acre of basal area, and 100 trees per acre); and (3) control/no thin (crown cover of 55 percent, 97 square feet per acre of basal area, and 247 trees per acre). Acorn production was monitored annually since 1995, and showed that both thinning levels significantly increased the proportion of trees with acorns, although per acre acorn yields were not affected. Significant increases in forage production were documented with the heavier thinning intensity. Over 50 percent of the stumps in the thinned areas resprouted. There was significantly higher height growth in the two-thirds thinned area than the one-third thinned area. Periodic annual increment for the stands was not affected by thinning, although individual tree diameter growth was positively affected by the heaviest thinning. Crown cover of the heaviest thinning had grown back to the same level as the moderately thinned treatment after thirteen years. Thinning results in more open stands, with larger average tree size, larger individual crowns, and a larger proportion with acorns compared to unthinned controls.