Highlights
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- We experimentally compare two types of early-seral forest in SW Oregon.
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- Early-seral with shrubs had more understory species than did Douglas-fir alone.
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- Hardwood shrubs and trees accumulate biomass much faster than young Douglas-fir.
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- Historically hardwood shrubs and trees were more dominant than in modern succession.
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- Early-seral can be intentionally managed to better meet forestry objectives.
Abstract
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.
Graphical abstract
Keywords
- Early succession;
- Early-seral;
- Biodiversity;
- Long-term productivity;
- N2 fixation
Abbreviations
- dbh, diameter at breast height;
- DSLR, digital single-lens reflex;
- FEMAT, forest ecosystem management assessment team (assessment that the Northwest Forest Plan was based);
- LTEP, long-term ecosystem productivity;
- NFMA, National Forest Management Act of 1976;
- O&C, Oregon and California Revested Lands Act of 1937;
- USDA, United States Department of Agriculture
Published by Elsevier B.V.
