A Management System to Address Forest Fragmentation
by G. Andrew Bartholomay - 11/12/99
Current approaches to applied conservation biology rely heavily on Island Biogeography
Theory to model and explain plant and animal population dynamics in the face of habitat
fragmentation (MacArthur and Wilson 1967, Harris 1984, Shafer 1990). The theory states that
plants and animals on fragmented forest lands are subjected to similar temporal and spatial
constraints as those found on oceanic islands that had previously been connected to larger land
masses (Terborgh 1974, Harris 1984). In other words, there had been a larger contiguous
terrestrial habitat until some event segmented the “island” into isolated habitats. In the case of
an oceanic island, the event was a rise in sea levels that eventually created an island where there
was once a peninsula. Forest fragments on the other hand, were created by events such as
urbanization and clearcutting that eliminated surrounding homogeneous habitats.
Whether isolated by water or discontinuous habitat, true islands and habitat islands show
reduced species richness and biological diversity, reduced genetic flow and diversity, and
reduced immigration and emigration of individuals (MacArthur and Wilson 1967, Terborgh
1974, Wilcox 1980). These factors are obvious regarding true islands where the surrounding
water acts as a barrier to terrestrial species, but less obvious regarding forest fragments where
dissimilar terrestrial conditions between fragments constitute the barrier.
These problems intensify with increases in adjacent habitat dissimilarities and increasing
distance between fragments (Wilcox 1980). Large forest lands managed with even-aged
silvicultural techniques are particularly susceptible to fragmentation. The Pacific Northwest
region of the United States is especially problematic where large clearcuts create a patchworklike landscape of discontinuous fragments of mature and old-growth stands (Harris 1984). When
large clearcuts surround fragments, animals requiring interior old growth habitat cannot cross the
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clearcut to use other old growth fragments and plant genetic flow and ingrowth among the
remaining fragments is prevented.
Harris (1984) has developed a landscape level approach to forest management in the
Pacific Northwest that addresses habitat fragmentation and its associated problems. His
approach is unique in conservation biology in that it fully incorporates forest product extraction
among old-growth reserves.
Harris’ (1984) system relies on numerous “Long-rotation Islands” strategically placed
across a landscape. The core of each island is an old-growth reserve whose area is determined
by a log-normal size-frequency distribution with 5% of the total managed landscape in oldgrowth preserve and 20% of the area under long-rotation management. Each old-growth core is
then surrounded by 9, 40 acre stands which are each managed on a 320 year rotation. The Longrotation Island looks like a pie whose core remains untouched and is surrounded by the 9 longrotation stands which appear like slices of the pie.
The clearcut remains the silvicultural regeneration system for each stand, but no two
adjacent stands are ever clearcut within 140 years of each other. This is accomplished by
regenerating only one of the 9 stands every 35 years and moving each consecutive clearcut two
stands away from the previous clearcut. The remaining uncut stands surrounding the core act to
increase the effective size of contiguous old-growth habitat within the Long-rotation Island.
Under this system, 66% of the stands around the core will be older than 100 years and 33% will
be in a regenerating condition. The Long-rotation Islands are surrounded by forest stands
managed on a shorter rotation, but with clearcut areas limited to 40 acres (Harris 1984).
This system elegantly addresses the problems of forest fragmentation noted earlier. Oldgrowth cores provide a store of genetic material, individual colonizers, and naturally maintained
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ecosystem components throughout the landscape. The Long-rotation Islands increase the size of
old and mature habitat while also providing a balance of early successional habitat. Even-aged
management units are less of an obstacle to immigration, emigration, and genetic flow because
their sizes are reduced and a clearcut is never placed in connection with another clearcut.
Harris (1984) suggests that this system is applicable to any forest ecosystem, however I
believe it’s requirement of contiguous ownership over large areas is a major impediment to
implementation outside of the Pacific Northwest. Ownership patterns in the Pacific Northwest
would enable this system to function on large Federal and State forests, but the NIPF ownership
mosaic in the Midwest and Eastern United States would prove problematic. Frequent ownership
turnover and conflicting ownership goals on private lands would provide formidable barriers to
the success of this approach. Even with these noted drawbacks, I believe Long-rotation Islands
could prove to be a sound ecosystem management technique.
If the US moves toward a national system of biological reserves as some conservation
biologists believe, NIPF ownership will be a problem for any plan. Harris’ system would
provide financial returns to private landowners while incorporating ecosystem management
across landscapes. The system also appears far less restrictive of individual ownership goals
than plans like the Wildlands Project (http://www.wildlandsproject.org) and may prove to be
more adaptive as well. My particular opinion is that Harris’ system better integrates human
needs within ecosystem management than does the Wildlands Project and will probably be more
acceptable to the general public.
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REFERENCES
Harris, L.D. 1984. The fragmented forest. The University of Chicago Press, Chicago. 211pp.
MacArthur, R.H. and Wilson, E.O. 1967. The theory of island biogeography. Princeton
University Press, Princeton. 203pp.
Shafer, C.L. 1990. Nature reserves: Island theory and conservation practice. Smithsonian
Institution Press, Washington. 189pp.
Terborgh, J. 1974. Preservation of natural diversity: The problem of extinction prone species.
BioScience 24: 715-722.
Wilcox, B.A. 1980. Insular ecology and conservation. In: Conservation biology: An
evolutionary-ecological perspective. Sinauer Associates, Inc., Sunderland. pp. 95-117.
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