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

BASAL AREA. Trees establishing prior to 1860 represented approximately 75% (SD = 12.8) of the basal area in 2000 (Table 3). The relatively low value (60%) for the Warm Mesic Shrub Herb plant association group was due to significant mortality of historic overstory trees (see deadwood below). Trees present in the 1860 understory (

MORTALITY. As stand density increased, mortality occurred across all age-class groups including trees that had established in the 1 500s. Deadwood was a major component of the forest structure in 2000 for WMSH, WDSH, but to a lesser degree in WDTS, CDG and HDSG plant association groups (Figs. 2-6). The initial Pseudotsuga menziesii post-fire establishment peak had significant mortality in the WMSH, WDSH, and WDTS plant association groups (Figs. 2-4). This level of mortality did not occur in CDG or HDSG plant associations where initial post-fire recruitment was less abundant (Figs. 5 and 6). Similarly, mortality was high in the initial post-fire recruitment for Pinus ponderosa and Larix occidentalis in WMSH and WDTS plant associations, but less so in WDSH, CDG and HDSG plant association groups.

DEADWOOD. The vast majority of current snags and logs were from trees that died since the last fire event (1866-1898). Charred trunks of historic snags and logs were a rarity (

A large portion of the historical forest was found in the current deadwood in WMSH and WDSH plant association groups; approximately 81 and 86% of trees that established prior to 1860 were part of the deadwood component in 2000. Mortality of pre- 1860 trees was significantly less (24, 23, and 20%) in WDTS, HDSG, and CDG plant associations, respectively. However, tree densities in these latter groups were 148 to 242% above 1860 levels in 2000, and under the stress of an abundant understory.

Discussion. The fire regimes of the plant association groups were representative of dry fir-pine forests of eastern Washington. Firefree intervals reported here were bracketed by those previously reported for this forest type in eastern Washington (3-38 yrs; Agee 1994, Ohlson 1996, Harrod et al. 1999, Everett et al. 2000, 2003b, Schellhaas et al. 2002, 2003). Also, as in these earlier reports, our stands had not had a fire since the late 1800s.

The centuries-long structural history in live trees and deadwood indicated these forests were in a constant state of change. Without exception, there were numerous peaks and troughs of different magnitudes in species establishment and subsequent mortality in each of the plant association groups. No single forest condition was sustainable, but the forests have been resilient to change in the long-term. Similar lengthy recruitment histories have been reported for Pinus ponderosa forests of the Southwest (White 1985, Ful� et al. 1997, Mast et al. 1999) and Pinus ponderosa-Pseudotsuga menziesii forests in the Northwest (Arno et al. 1995, 1997). Variability in tree density (140 to 1185 trees/ha) among the plant association groups in 1860 indicated historic dry fir-pine forests were not a uniform open forest savanna, but a mix of forest structures as suggested by Arsenault and Klenner (2004).