Development of current stand structure in dry fir-pine forests of eastern Washington

Journal of the Torrey Botanical Society, Apr-Jun 2007 by Everett, Richard, Baumgartner, David, Ohlson, Peter, Schellhaas, Richard, Harrod, Richy

EVERETT, R., D. BAUMGARTNER (Department of Natural Resources Sciences, Washington State University, Pullman, WA 99164-6410), AND P. OHLSON, R. SCHELLHAAS, AND R. HARROD (Okanogan-Wenatchee National Forest, Wenatchee WA 98801). Development of current stand structure in dry fir-pine forests of eastern Washington. J. Torrey Bot. Soc. 134: 199-214. 2007.-Stand reconstruction, using live trees and deadwood, provided a four to five hundred year recruitment history for the current stand structure in dry fir-pine forests of northern Washington, USA. The lengthy structural record indicated these dry fir-pine forests were resilient to change, that all forest conditions were transient, and that forests were currently transitioning away from maximum tree density levels. Historically, frequent fires maintained low tree abundance, but fire cycles lengthened in the 1860s as Euro-settlement progressed. Average stand density had already increased to 194% (SD = 116) of 1860 levels by the start of effective fire suppression in 1915. From the 1930s to 1960s, average stand density peaked at 258% (SD = 98) of 1860 levels. By 2000, tree mortality in these overstocked stands had reduced average stand densities to 68% (SD = 19) of maximum levels. However, tree densities in 2000 still averaged 173% (SD = 64) above historical levels. Although there had been significant tree recruitment and a century without fire, the trees present in the historic stands provided 75% (SD = 13) of the basal area in 2000. Numerous understory (

Key words: age structure, basal area, deadwood, Douglas-fir, Ponderosa pine, western larch, understory.

Effective management of forests has required a basic understanding of the forest history, its current condition, and probable future pathway. Historic forest structures have been under-estimated when only current live trees were analyzed (Mast et al. 1999). We examined the relative timing and amounts of live and deadwood contributions to the current structure of dry fir-pine forests of northern Washington using stand reconstruction techniques (Johnson et al. 1994, Mast et al. 1999). Sampled Pseudotsuga menziesii (Mirbel) Franco-Pinus ponderosa Dougl. ex Laws, forest plant associations of eastern Washington were similar to those found elsewhere in the Pacific Northwest (Steele and Geir-Haynes 1995, Powell 1999), and Canada (Arsenault and Klenner 2004). These forests were a composite of forest structures with origins before, during, and after Euro-settlement (Covington et al. 1994a). Pre-Euro-settlement has been a common "historical" reference point in retrospective forest studies in the Inland West (Covington and Moore 1994b, Arno et al. 1997, Ful� et al. 1997, Mast et al. 1999). Livestock grazing, reduced use of fire by indigenous peoples, roading, timber harvest, and fire suppression were all associated with Euro-settlement and linked to extended fire-free intervals (Arno 1988, Ag�e 1994, Covington et al. 1994a, Wright and Agee 2004).

Previous retrospective studies described increased tree densities and basal area since Euro-settlement in fir-pine forests of the Inland West (Covington and Moore 1994b, Arno et al. 1995, 1997, Ful� et al. 1997, Harrod et al. 1999, Mast et al.1999, Youngblood et al. 2004). However, most of these studies compared only two time frames (historical and current) and did not describe the relative contributions of historical overstory, historical understory, and subsequent post-fire recruitment to current stand structure.

Pollock and Suckling (1997) suggest previous surveys of historic Pinus ponderosa stands have under-sampled small trees, and that their inclusion would have shown a well represented tree understory. Although understory trees were repeatedly thinned by fire in dry fir-pine forests of eastern Washington, Ohlson (1996) reported abundant tree recruitment immediately following low severity fires. Also, the lengthening fire cycles during Euro-settlement allowed for continued growth of understory trees, and their increased resistance to the last low severity fire events (Everett et al. 2000). Johnson et al. (1994) reported that an established understory/overstory had a competitive edge over subsequent tree recruitment, was somewhat resilient to increased tree competition, and would exert long-term site domination.

The age-class structure present in historic stands changed in response to tree recruitment, self-thinning mortality, and the aging of stands (Mast et al. 1 999). Frequent fires thinned the tree understory, reduced long-term recruitment, and created a relatively flat age distribution (White 1985, Mast et al. 1999). In the absence of fire, new recruitment and the development of older age-classes broadened the age-class structure (Fuie et al. 1997, Mast et al. 1999).

Although Morgan et al. (1994) suggested using historical range of variability as a guide to managing forest lands, Schmidt (1996) indicated that restoration of a frequent fire regime was unlikely for most of the dry firpine forests of the western United States. If restoration was unlikely, we needed to understand the adaptations of these forests to longer fire cycles, and how to work with these natural processes for reduced fire hazard and increased forest resiliency. This reconstruction study examined dry fir-pine forest resiliency (without human intervention) to altered fire regimes, and pre-and post-fire forest structures. We tested the hypotheses: 1) dry fir-pine forests have declined from previous maximum tree densities, 2) historical (1860) overstory and understory trees dominated forest structure after a century without fire, 3) deadwood has increased from historical levels, and 4) since 1860 the age-class structure has broadened and evenness in abundance has declined among age-classes.