By Derek Churchill
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of Fact Sheet #24
It is commonly understood that west side old growth forests in the
Pacific Northwest are highly variable (Spies & Franklin 1991)
and developed from multiple growth pathways as a result of varying
starting conditions and disturbance patterns (Spies et. al. 2002).
Underlying these different pathways is an approximate south to north
and east to west gradient of decreasing fire frequency and increasing
fire size (Morrison and Swanson 1990, Spies et. al. 2002). Localized
disturbance agents, such as wind, root diseases, insect outbreaks,
floods, and ice storms interact over time with fire regimes to create
a dynamic environment that results in the development of the complexity
inherent in many natural old forests.
|There has been increasing regulatory pressure
on public and private forestlands to provide for the ecological
benefits associated with old-growth forests. Whether in riparian
zones or habitat areas, the result has been that thousands of
acres of previously harvested forestlands are no longer being
managed. Recent attention, however, has questioned the ability
of these young forests to provide old growth functionality without
management to reduce stem densities (Muir et. al. 2002, Rapp
2002, Hunter 2001). Scientific evidence has shown that thinning
of younger forests can accelerate the development of old growth
characteristics (Acker et. al 1998, Tappeiner et. al. 1997,
Carey et al. 1999, Muir et. al. 2002, Bailey & Tappeiner
1998, Garman 2003). Scientists, environmentalists, and forest
managers are recommending more active management in young stands
(Curtis et. al. 1998, Franklin et. al. 2002, Carey et. al. 1998,
Heiken 2003, Spies et al. 2002).
In the past, small frequent fires contributed to development of
heterogeneous natural forests with wide Douglas-fir age distributions,
often in discrete age classes in the southern and central Oregon
Cascades (Morrison & Swanson 1990, Spies & Franklin 1991).
A temporal pattern of long Douglas-fir establishment periods (60-200+
years), multiple low-to-moderate severity fires, seed source deficiencies,
low initial tree densities and little competitive exclusion has
been linked to the development of old growth forest conditions in
this southern, drier part of the Pacific Northwest (Oliver &
Larson 1996). Two recent studies of 38 old growth stands in the
Oregon Cascades and Coast Range support this hypothesis (Tappeiner
et. al. 1997, Poage & Tappeiner 2002). Comparisons of the growth
rates in the first 50 years of old growth stands with growth rates
of young stands of known densities on similar site classes, suggest
that these old growth stands started at densities of 40 - 52 trees
per acre (tpa). Tree sizes at ages 100, 200, and 300 years were
found to be much more positively correlated with early growth rates
than with site or climatic factors, suggesting that widely-spaced
early stocking density, associated with a wide range of establishment
periods (100-420 years), was the principal factor in the growth
trajectory of individual trees (Poage & Tappeiner 2002).
In the wetter, northern part of the region, a history
of larger and less frequent fires may have resulted in more homogeneous
forests with narrower age distributions, which developed after large
fires 500 and 700 years ago in the Cascade and Olympic Mountains
(Agee 1991). It is hypothesized that at least some of these forests
developed at high densities with understory exclusion and growth
reduction from stocking competition (Spies et. al. 2002). Winter
(2002) found evidence of this pathway in a 500-year-old stand in
the southern Cascades of Washington. Using a similar comparison
technique to Poage & Tappeiner (2002), she estimated a density
at crown closure of 320 tpa and an establishment period of 21 years
dominated by Douglas-fir. Although anecdotal evidence of this higher
density pathway has been reported (Spies et. al 2002), no other
published reconstruction studies have found quantitative verification
of similar stand origin characteristics. While a young forest density
of 320 tpa is not dissimilar to that of some planted forests, the
establishment period, although much shorter than that found by Poage
& Tappeiner (2002), is much longer than that of a plantation.
These investigations suggest that many of today's young, previously
harvested forests may be on developmental pathways that are very
different from those that resulted in natural old growth stands.
Young planted forests, established at high densities in very short
time periods with the expectation of pre-commercial and commercial
thinnings, are typically uniform and dense with little differentiation.
Without density reductions, planted forests eventually evidence
suppressed growth, high height to diameter ratios, and short crowns;
conditions that have been shown to make stands susceptible to windthrow
and inhibit the development of the large trees associated with old
growth forests (Wilson & Oliver 2000).
Although some researchers theorize that young stands will eventually
develop old growth characteristics regardless of early establishment
conditions, it will take much longer. Heavy or repeated thinning
of dense young forests has been proposed as a way to silviculturally
shift these stands onto a development pathway more likely to produce
old forest structure with large diameter trees (Poage & Tappeiner
2002). Researchers, however, also stress the importance of creating
variability by using a mix of thinning densities within stands and
across the landscape (Carey et. al. 1999a, Garman 2003, Hunter 2001,
Muir et. al. 2002, Franklin et. al. 2002, Spies et. al. 2002).
Several studies have found that thinning accelerates the development
of other old growth characteristics in addition to diameter growth.
Three major research projects, the Managing for Biodiversity in
Young Forests Project in western Oregon (Muir et. al. 2002), the
Forest Ecosystem Study in western Washington (Carey et. al. 1999a),
and the Young Stand Thinning Study on the Willamette National Forest
(Hunter 2001), have undertaken comprehensive investigations into
the effects of thinning. Results of these studies show that understory
vegetation, shade tolerant tree regeneration, and the vertical distribution
of the canopy in thinned stands tend to be more similar to old growth
conditions than in un-thinned stands (Acker et. al 1998, Tappeiner
et. al. 1997, Muir et. al. 2002, Bailey & Tappeiner 1998). Wildlife
and plant diversity, including birds, macrolichens and bryophytes,
fungi, small mammals, and bats, have also been shown to be greater
in thinned stands (Carey et al. 1999, Hayes et al. 1997, Muir et.
al. 2002, Hunter 2001).
Different thinning strategies appear to produce different results.
Thinning from below that strives for regular spacing may create
a uniform light environment that leads to a thick understory of
shade tolerant species with little diversity that shades out forest
floor vegetation. Development of coarse woody debris, decadence,
and cavities may also be delayed by heavy thinning. Removing hardwood
species, wildlife trees, and snags may limit many of the habitat
gains from thinning (Muir et. al. 2002). On the other hand, thinning
that retains at least some of these structures and leaves patches
of variable densities has been shown to increase plant and wildlife
diversity even further. Under-planting shade tolerant conifers,
hardwoods, and native shrubs, as well as augmenting coarse woody
debris and snags can increase similarity to old-growth structure
(Rapp 2002, Carey et. al. 1999a). However, even a simple thin-from-below,
designed to create uniform available growing space and favor dominant
crop trees, has been shown to increase wildlife and plant diversity
when compared to a no action management alternative (Tappeiner 1997,
Muir et al. 2002).
Regulatory constraints intended to protect sensitive species and
provide riparian function, as well as the economic costs of selectively
harvesting low value trees, presently limit the potential for some
thinning activities. However, current research suggests that a significant
portion of young stands will need active management if forest habitats
suitable to old growth dependent species are to be developed in
the next 25-150 years. Replication of the complexity and variability
found in old-growth forests, thought to exist at the landscape level
prior to commercial harvest, will require intervention to diversify
the developmental pathways of young uniformly planted forests (Heiken
2003, Spies et. al. 2002). Studies have suggested that customized
harvests designed to achieve variable densities within stands and
augment snags, understory species, and coarse woody debris may be
ecologically preferable to commercial thin-from-below alternatives
(Carey 1999a, Muir et. al. 2002). Without incentives, however, the
economic costs will likely restrict such ecological thinning activities
to small areas and public forestlands. Even on public lands, the
more standardized thin-from-below approach, with the possibility
for both positive economic and environmental outcomes, has greater
likelihood of application on a broader scale given current market
conditions and government funding levels.
Whether on National Forest lands, State Forests, Tribal lands,
or private lands, an ecological paradigm shift is occurring (Heiken
2003). A growing body of scientists, environmentalists and forest
managers are recommending that in many forests with a prior history
of harvest, continued management will be necessary to avoid the
development of stagnant, overstocked stands that provide few old-growth
habitats, are more susceptible to disturbance and disease, and fail
to achieve the variability of pre-settlement forests.
Acker, S.A., T.E Sabin, L.M Ganio, & W.A. McKee. (1998). Development
of old-growth structure and timber volume growth trends in maturing
Douglas-fir stands. Forest Ecology and Management 104 (1/3):
Agee, J. (1991). Fire history of Douglas-fir forest in the Pacific
Northwest. In: Ruggerio, L.F., K.B. Aubry, A.B. Carey & M.H.
Huff (Eds.), Wildlife and vegetation of unmanaged Douglas-fir
forests. USDA For. Serv. Gen. Tech. Rep. PNW-GTR-285.
Bailey, J.D., & J.C. Tappeiner. (1998). Effects of thinning
on structural development in 40- to 100-year-old Douglas-fir stands
in western Oregon. Forest Ecology & Management. 108:
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(1999). Ecological scale and forest development: Squirrels, dietary
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to: The Journal of Wildlife Management, Vol. 63 No. 1: Wildlife
Monographs, No 142, January 1999.
Carey, A.B., D.R. Thysell, & A.G. Brodie. (1999a). The Forest
ecosystem study: Background, rationale, implementation, baseline
conditions, and silvicultural assessment. (PNW-GTR-457). USDA Forest
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lessons from natural and managed forests of the Pacific Northwest.
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Curtis, R.O., D.S. DeBell, C.A. Harrington, D.P. Lavender, J.C.
Tappeiner, & J.D. Walstad. (1998). Silviculture for multiple
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Service, Pacific Northwest Research Station: Portland, OR. 123 pp.
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development of natural forest ecosystems with silvicultural implications,
using Douglas Fir forests as an example. Forest Ecology and Management
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D. Kellogg, & J. D. Bailey. (1997). Wildlife response to thinning
young forests on the Pacific Northwest. Journal of Forestry 95(8):
Heiken, D. (2003). A synthesis of published articles on young stand
management. Oregon Natural Resource Council: Eugene, OR. http://www.efn.org/~onrcdoug/THINNING_SCIENCE.htm
Hunter, M. G. (2001). Communiqué No. 3: Management in young
forests. Cascade Center for Ecosystem Management: Corvallis, OR.
Garman, S. L.; J.H. Cissel, & J.H. Mayo. (2003). Accelerating
development of late-successional conditions in young managed Douglas-fir
stands: A simulation study. (PNW GTR 557). USDA Forest Service,
Pacific Northwest Research Station: Portland, OR. 57pp.
Muir P.S. et. al. (2002). Managing for biodiversity in young Douglas-fir
forests of Western Oregon. Biological Science Report. (USGS/BRD/BSR
2002 -0006). US Geological Survey, Forest and Rangeland Ecosystem
Science Center: Corvallis, OR.
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John Wiley & Sons, Inc.: New York, NY.
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diameter and basal area growth of old-growth Douglas-fir trees in
Western Oregon. Canadian Journal of Forest Research 32 (7):
Rapp, V. (2002). Science update- Restoring complexity: Second growth
forests and biodiversity. USDA Forest Service, PNW Research: Olympia,
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of old-growth Douglas-fir forests along the Pacific Coast of North
America: A Regional Perspective. Nov. 7-9, 2001. H.J. Andrews Experimental
Forest: Blue River, OR.
Spies, T.A., & Franklin, J.F. (1991). The structure of natural
young, mature, and old-growth Douglas-fir forests in Oregon and
Washington. In: Ruggeri, L.F., K.B. Aubry, A.B. Carey, & M.H.
Huff (Eds.), Wildlife and vegetation of unmanaged Douglas-fir
forests. (PNW-GTR-285). USDA Forest Service. Pp. 90-109.
Tappeiner, J.C., D. Huffman, D. Marshall, T.A. Spies, & J.D.
Bailey, (1997). Density, ages, and growth rates in old-growth and
young-growth forests in coastal Oregon. Canadian Journal of Forest
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management in Douglas-fir plantations. Canadian Journal of Forest
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Winter, L.E. (2002). Initiation of an old-growth Douglas-fir stand
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Contacts: For more information contact Derek Churchill,
Rural Technology Initiative, University of Washington (206) 543-0827