Thinning interior live oak in California's Southern Sierra Nevada

This study describes a thinning and resprout control study in Madera County. The study site was a dense, 40-year old interior live oak stand (Quercus wislizeni) that originated from resprouting, with 100 percent canopy cover. Tree thinning was initiated in 1998 in cooperation with the local Resource Conservation District to evaluate thinning treatment to reduce first risk and to increase forage production. Three thinning treatments were evaluated: (1) thin one-third of the standing tree basal area (resulting in a post-thin basal area of 46 square feet per acre); (2) thin two-thirds of the standing tree basal area (resulting in a post-thin basal area of 27 square feet per acre); and (3) control/no thin (basal area of 73 square feet per acre). Because interior live oak is such a prolific sprouter, half of the thinned plots were treated with herbicides to prevent resprouting and to retain the open canopy structure. Canopy cover, diameter and height growth, and acorn production were monitored over a 13-year period. Periodic annual increment for basal area and volume was not significantly affected by the thinning treatment. Individual tree DBH growth was significantly increased with the thinning. 13 years after the thinning, the 1/3 treatment had virtually identical volume to the control, although the 2/3 thinning treatment had significantly less volume per acre. The sprout control treatment had no significant effect on the overstory tree growth, although a more open understory was maintained by controlling resprouting. Individual tree acorn production was significantly increased as a result of the thinning treatments. Thinning appears to be a promising management tool to diversify stand structure in dense live oak stands in the Southern Sierra, increasing economic value for livestock use and reducing wildfire risk.