Five centuries of structural development in an old-growth Douglas-fir stand in the Pacific Northwest: a reconstruction from tree-ring records
Tree-ring records were used to reconstruct the history of an old-growth Douglas-fir stand in the western Cascade Range of southern Washington. Prior to a scheduled harvest, a 3.3 ha plot was inventoried and mapped. After felling in 1992, samples were collected from stumps of all mapped trees, and from multiple additional heights of a subset of these trees. One tree was intensively dissected to locate and extract embedded branches. Data taken from crossdated samples were used to reconstruct a history that focused on stand initiation and canopy disturbances, but also included diameter, height and crown development.All sampled Douglas-fir were initial colonizers, establishing 1500--1521 under open conditions following a stand-replacing fire. A minor component of sampled western hemlock were also initial colonizers. Growing space filled as tree crowns widened, and by 1540 closed canopy conditions had developed. At this time, Douglas-fir were spaced about 3.5 m from equivalent competitors (ca. 800 trees/hectare).In the centuries following canopy closure, considerable natural thinning of the initial colonizers occurred. Although the canopy never opened enough to allow further Douglas-fir establishment, at least three disturbances thinned the canopy across areas large enough to reliably reconstruct, each affecting areas ≥0.8 ha. Surviving Douglas-fir increased in stature and developed long crowns despite the narrow initial spacing, and without epicormic branching. Most western hemlock that were canopy trees in 1992 established after 1540, originating in the understory where they grew slowly for years to decades before ascending to the canopy through multiple abrupt increases in growth.This reconstruction provides a case history, extending across centuries, that may be useful where management policies emphasize the development of old-growth structures. The quick establishment at close spacing by Douglas-fir in the study stand is similar to establishment patterns for typical young stands in the region, and is very different from the prolonged establishment at wide spacings found for Douglas-fir in other reconstructed old-growth stands. Such differences show that old-growth structures can develop by multiple pathways, and that intrusive management may not be required to allow many typical young stands to develop old-growth structures.
- Forestry