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

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https://ir.library.oregonstate.edu/concern/articles/x059c8962

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  • 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 20 years 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 (18 Mg ha⁻¹ yr⁻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 historical records indicate these species, along with knobcone pine, dominated the site for 40 years before the current mature Douglas-fir forest started gaining dominance. In contrast, the prolific natural regeneration of Douglas-fir after recent harvest and wildfire suggests that what comes back ‘‘naturally’’ in modern times will not allow this history to be repeated.
  • Keywords: N₂ fixation, Early-seral, Long-term productivity, Biodiversity, Early succession
  • Keywords: N₂ fixation, Early-seral, Long-term productivity, Biodiversity, Early succession
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  • Bormann, B. T., Darbyshire, R. L., Homann, P. S., Morrissette, B. A., & Little, S. N. (2015). Managing early succession for biodiversity and long-term productivity of conifer forests in southwestern Oregon. Forest Ecology and Management, 340, 114-125. doi:10.1016/j.foreco.2014.12.016
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  • 340
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  • Support for pre- and post-treatment, and post-wildfire sampling and analysis came from the Research Station, the US Environmental Protection Agency, Environmental Research Laboratory, Corvallis, Oregon (Interagency Agreement DW 12936179), the Joint Fire Sciences Program (grants 03-2-3-09 and 10-1-10-18), Western Washington University, the National Commission for Science on Sustainable Forestry (grant C4), and the Rogue River Siskiyou National Forest.
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