Chapter 12: The variable-density thinning study at Stanislaus-Tuolumne Experimental Forest
| Authors: | Eric E. Knapp, Malcolm P. North, M. Benech, Becky Estes |
| Year: | 2012 |
| Type: | General Technical Report |
| Station: | Pacific Southwest Research Station |
| Source: | In: North, Malcolm, ed. 2012. Managing Sierra Nevada forests. Gen. Tech. Rep. PSW-GTR-237. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 127-139 |
Abstract
Prior to historical logging and fire suppression, forests of the Sierra Nevada were extremely heterogeneous. Frequent low- to moderate-intensity fire was partly responsible for this heterogeneity, which in turn helped make forests resilient to high-severity stand-replacing events. Early observers of forests on the west slope of the Sierra Nevada noted the arrangement of large trees as grouped or clustered (Dunning 1923, Show and Kotok 1924) (fig. 12-1). Show and Kotok (1924) described the mixed-conifer forest as "uneven aged, or at best even-aged by small groups, and is patchy and broken; hence it is fairly immune from extensive devastating crown fire."
A major emphasis in forest management today is improving the resilience of stands to large-scale crown fires. To put forests on the path toward resilience after a long period of fire exclusion, stands are often first mechanically thinned, typically using some variation of thinning from below, which targets the smaller trees and retains the larger and more fire-resistant dominant and codominant individuals. With thinning from below, crowns of individual trees are typically separated from each other, which can lead to a relatively even forest structure. This evenness has sometimes been perceived to be in conflict with management of habitat for wildlife and other forest species. Thinning that produces a more grouped arrangement of trees may be one means of creating heterogeneity at a scale beneficial for wildlife species that prefer different forest structures for nesting, roosting, and foraging, and understory plant species that thrive in different light environments, while simultaneously increasing resilience to wildfire.
The high-variability thinning prescription described in this chapter is part of a new variable-density thinning study on the Stanislaus-Tuolumne Experimental Forest (STEF) designed to investigate the ecological effects of structural variability retained during forest thinning operations (Stanislaus National Forest 2010). Three forest structure treatments (high variability, low variability, and an unthinned control), all with or without prescribed burning as a followup treatment, are being compared. The objective of the high-variability thinning treatment is to produce an arrangement of trees and degree of spatial complexity similar to what was once found in historical forests prior to logging and fire suppression. The study planning predates publication of U.S. Forest Service General Technical Report GTR 220 (North et al. 2009) and is therefore not among the projects designed specifically to implement principles therein. We include it here because the objective of the highvariability treatment is similar to a core concept in GTR 220 of increasing spatial heterogeneity and thus provides a useful illustration.