Canopy structure on forest lands in western Oregon: differences among forest types and stand ages

Abstract

Canopy structure is an important attribute affecting economic and ecological values of forests in the Pacific Northwest. However, canopy cover and vertical layering are rarely measured directly; they are usually inferred from other forest measurements. In this study, we quantified and compared vertical and horizontal patterns of tree canopy structure and understory cover along a successional gradient of forests and among stands with different thinning histories on nonfederal lands in western Oregon. Analyses focused on three dominant forest type groups: wet conifer, wet hardwood, and dry hardwood. We used data from 917 systematically located, forested Forest Inventory and Analysis plots measured between 1995 and 1997. On each plot, canopy cover by layer and species was measured online-intercept transects, and cover of understory species was measured on five subplots. Trends in canopy structure with stand age did not always follow the patterns predicted by common successional models. Most of the cover in moist stands was in the upper tree layer, but cover in dry hardwood stands was more evenly distributed among layers. Contrary to expectations of canopy closure, mean canopy cover by age class rarely exceeded 85 percent, even in unthinned productive young conifer forests. Possibly as a result, effects of stand age on understory vegetation were minimal, except for low levels of forbs found in 20-to 40-year-old wet conifer stands. Shade-tolerant tree species rarely made up more than 20 percent of canopy cover, even in the lower canopy layers and in stands >100 years old. Although heavily thinned stands had lower total cover, canopy structure did not differ dramatically between thinned and unthinned stands. Our findings suggest potential limitations of simple stand succession models that may not account for the range of forest types, site conditions, and developmental mechanisms found across western Oregon.