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

Materials and Methods. Study Area. In this study we sampled five Pseudotsuga menziesii-Pinus ponderosa plant association groups: Warm Mesic Shrub Herb (WMSH), Warm Dry Tall Shrub (WDTS), Warm Dry Shrub Herb (WDSH), Cold Dry Grass (CDG), and Hot Dry Shrub Grass (HDSG) on the Okanogan National Forest in northeastern Washington, USA (Fig. 1). Plant association groups, broad "potential" vegetation units defined by vegetation and associated environmental factors, have been used to classify (Lillybridge et al. 1995) and manage (Harrod et al. 1999) National Forest lands. Sampling by plant association groups captured structural variability within groups and provided a broad view of current forest structure across these dry fir-pine forests. The Lillybridge et al. (1995) classification of these plant associations used Flora of the Pacific Northwest (Hitchcock and Cronquist 1973), but species scientific nomenclature discussed in this paper was updated using the USDA (2006) National Plant Data Center PLANTS database.

The plant association groups occurred along an elevation-aspect gradient that included lower elevation (900 m) hot dry forests on southwest facing slopes, warm mesic forests at low elevations (600 m) on north slopes, warm dry forests at 900 m elevation on northeast and west aspects, and higher elevation (1200 m) cold dry forests on all aspects. Tree cover in the plant association groups ranged from 10% to greater than 70% and was predominantly Pinus ponderosa, Pseudotsuga menziesii and Larix occidentalis Nutt. (Table 1). Shrub cover was either dominated by Purshia tridentata (Pursh) DC. - bitterbrush, Symphoricarpos albus (L.) Blake - snowberry, Arctostaphylos uva-ursi (L.) Spreng. - bearberry, Spiraea betulifolia var. lucida (Dougl. ex Greene) C. L. Hitchc. - shinny-leaf spiraea, or Physocarpus malvaceus (Greene) Kartesz - ninebark. Sub-shrub/Grass/Forb cover was predominately: Agropyron spicatum Pursh - bluebunch wheatgrass, Calamagrostis rubescens Buckl. - pinegrass, or Linnaea borealis var. longiflora (Torr.) Hulten - twinflower. Other tree species present (Pinus contorta Dougl. ex Loud - lodgepole pine, Picea engelmannii Parry ex Engelm. - Engelmann Spruce, and Abies grandis (Dougl. ex D. Don) Lindl. - grand fir) contribute less than 1 tree per ha (historically and currently) and were not included in the stand analysis.

EXPERIMENTAl DESIGN. This study used a completely randomized sampling design. Selection of individual stands (n = 26) was made at random on aerial photographs from a mapped population of stands with little or no evidence of timber harvest. Stands were grouped after field analysis into one of the five plant association groups. Each sampled stand represented a sub-plot within a plant association group, and each plant association group represented a "composite" replicate plot of forest structure in the dry fir-pine forest of eastern Washington.

FIELD METHODS. Stand structure was sampled with four to fourteen 0.04 ha plots, and twelve to seventy-six 0.4 ha plots for a total of 16-90 plots per plant association group (Table 1). This sampling level and subsequent analysis was not expected to fully describe current stand structure and historical changes within each plant association group, but should have been sufficient to discover broad consistencies in current stand structure among the groups.