Changes in stand structure and composition after restoration treatments in low elevation dry forests of northeastern Oregon

Abstract

In many fire-dependent forests in the United States, changes occurring in the last century have resulted in overstory structures, conifer densities, down woody structure and understory plant communities that deviate from those described historically. With these changes, many forests are presumed to be unsustainable. Broad-scale treatments are proposed to promote stand development on trajectories toward more sustainable structures. Yet little research to date has identified the effects of these restoration treatments, especially in low elevation dry ponderosa pine ( Pinus ponderosa) and Douglas-fir ( Pseudotsuga menziesii) forests of northeastern Oregon. We report changes in tree structure, coarse woody debris (logs), and understory composition from an operational-scale, replicated ( N = 4), completely randomized experiment. Treatments included a single entry thin from below conducted in 1998, a late season burn conducted in 2000, a thin followed by burning (thin + burn), and a no action treatment which served as a control. Changes in live and dead tree structure and understory vascular plant community composition were compared between pre-treatment and 2004. Tree seedling density and composition and coarse woody debris structure were evaluated in 2004. Thin, burn, and thin + burn treatments reduced the density but not the basal area of live overstory trees. Thinning reduced the number of medium-diameter trees, and tended to decrease the abundance of shade tolerant, moist-site understory species yet increased the dominance of several rhizomatous species such as Calamagrostis rubescens, Symphoricarpos albus, and Spiraea betulifolia. Burning alone had little effect on large trees but reduced the number of small Douglas-fir and logs. Shade tolerant perennial species associated with fine textured soils such as S. albus, Spiraea betulifolia, C. rubescens, Carex geyeri, and Arnica cordifolia increased in frequency and average cover with burning. Conversely, cover of the bunch grass Festuca idahoensis was reduced while non-native invasive species establishment was little affected. Ordination scores suggested that burning increased the abundance of species representing greater shade tolerance and finer-textured soils. The thin + burn treatment left both ponderosa pine and Douglas-fir with modal or normal diameter distributions, and increased the abundance of understory species representing shallow, coarse texture soils. These results are discussed in the context of management options for restoration of ecosystem health in similar low elevation dry ponderosa pine and Douglas-fir forests.