Managing early succession for biodiversity and long-term productivity of conifer forests in southwestern Oregon

• Published in: Forest ecology and management 2015-03, Vol.340, p.114-125
• Main Authors: Bormann, Bernard T., Darbyshire, Robyn L., Homann, Peter S., Morrissette, Brett A., Little, Susan N.
• Format: Article
• Language: English
• Subjects: Biodiversity; Conifers; Early succession; Early-seral; Forests; Hardwoods; Long-term productivity; N2 fixation; Pinus attenuata; Plantations; Plants (organisms); Productivity; Pseudotsuga menziesii; Supports; Trees
• ISSN: 0378-1127; 1872-7042
• DOI: 10.1016/j.foreco.2014.12.016

[Display omitted] •We experimentally compare two types of early-seral forest in SW Oregon.•Early-seral with shrubs had more understory species than did Douglas-fir alone.•Hardwood shrubs and trees accumulate biomass much faster than young Douglas-fir.•Historically hardwood shrubs and trees were more dominant than in modern succession.•Early-seral can be intentionally managed to better meet forestry objectives. Early-successional stages have been truncated and altered in many western U.S. forest landscapes by planting conifers, controlling competing vegetation, suppressing fire, and focusing on maintaining late-seral species and undisturbed riparian zones. Declining area of early-successional stages may be reducing resilience and sustainability on landscapes that experience elevated disturbance related to future climate changes. In this study, two post-harvest early-successional treatments were compared to each other and to two mature-forest treatments using 20years of evidence from replicated 7-ha experimental units in a southwestern Oregon forest dominated by Douglas-fir (Pseudotsuga menziesii Mirb. Franco). One early- successional treatment (Douglas-fir plantation) planted Douglas-fir and was followed by a brushing to reduce hardwood competition to move quickly to the conifer stem-exclusion stage; the other (Early-seral plantation) favored natural sprouting and regeneration of hardwood shrubs and trees and planted scattered knobcone pines (Pinus attenuata Lemmon) and Douglas-fir. Plant diversity in the Early-seral plantation was 56% (year 2) and 26% (year 6) higher than in the Douglas-fir plantation. Both early-successional treatments far exceeded plant diversity in Unaltered and Thinned mature stands. Fifteen years of growth of shrubs and hardwood trees in the Early-seral plantation was remarkable, resulting in total aboveground biomass increment (18Mgha−1yr−1) double that of the Douglas-fir plantations. Important process effects related to primary productivity were noted: losses of soil organic matter from the B horizon in young Douglas-fir, and, after wildfire, increases in N2-fixing plant cover in Early-seral plantation. The burl-sprouting and deep rooting of many hardwoods also created opportunities for nutrient retention and release from primary minerals as well as deep-profile water supply. Recognizing the importance of intentionally managing for shrubs and hardwood trees is particularly relevant at this site, because stand reconstruction and hist