Variable Density Thinning Study
Stanislaus-Tuolumne Experimental Forest (STEF)

The Stanislaus-Tuolumne Experimental Forest covers 690 ha in the central Sierra Nevada, near Pinecrest, California. It was selected as typical of mixed conifer stands of the Sierra Nevada, specifically, those of high site-quality on mid-elevation west slopes. The forest consists of two tracts: the 156 ha Stanislaus Tract on the South Fork of the Stanislaus River and the 534 ha Tuolumne Tract on the lower slopes of Dodge Ridge, just south of the North Fork of the Tuolumne River. Elevations range from 1,590 to 1,950 m.

Background

Forests of the western U.S. that experienced frequent fire were once extremely variable. Large fire resistant trees grew individually or in groups of different ages and sizes and contained a diversity of species. Trees were interspersed with gaps of various size filled with shrubs, grasses, herbs and tree regeneration. The mosaic of frequent burns consumed surface fuel, maintaining low levels of litter, duff, and downed logs on the forest floor. The tree group and gap combination provided habitat for a wide range of plant and animals species that prefer a mix of open and closed forest conditions. A diversity of tree species and much lower tree density made these forests resilient to fire, drought and other disturbances. Fire has been part of these ecosystems for thousands of years. Before the Spanish came to California, fires were ignited by lightning and by indigenous people who used fire to promote plants for food and fiber, and to attract game. Indigenous burning declined with Euro-American settlement and fire became even less frequent with the onset of fire suppression. Due to absence of fire and past cutting, today’s forests looks nothing like they once did. Tree density is much higher, gaps are now filled with trees, and trees have shaded out grass, forbs, and shrubs in the understory. In many areas, over a century of accumulated needles, twigs, duff, and downed wood cover the forest floor. These changes have made forests much more susceptible to wildfire and drought-related tree mortality.

The ‘Variable Density Thinning’ study was initiated in 2009 to test a new approach to thinning designed to restore a more variable forest structure. A ‘High Variability’ thinning treatment produced numerous small gaps and left groups of trees of various densities and sizes, similar to what historical forests shaped by frequent fire once looked like. This was compared against a standard thinning where crowns of individual trees were spaced relatively evenly (‘Low Variability’), and an untreated control. Half of the units were followed with a prescribed fire and the other half left unburned.

Click this link for more detailed information about the ‘Variable Density Thinning’ study.