New and Renewed Stewardship: Toward a Silviculture of Diversity

Dave Perry, Fred Swanson, and Jerry Franklin 2

ABSTRACT: Intensive forest management maximizes fiber yields, at least in the short run, but at the cost of aesthetics, habitat, and perhaps long-term ecosystem stability. new approaches produce an array of market and non-market values by managing to preserve natural diversity. Some elements of this include: retention of dead wood and non-crop plant species, growing trees at wide spacings, longer rotations, and creating diverse, un-fragmented landscapes. Specific techniques will vary with objectives and forest type.

INTRODUCTION
Land managers, and society as a whole, are currently grappling with an array of difficult questions regarding managed forests. What do we want forests of the future to look like? How do we balance the competing wants and needs of society; the economic demands of the present with our ethical obligations to other species and to the future of our own? What criteria do we use to decide? How do we evaluate forest values, such as aesthetics, watershed protection, and global climate amelioration, that are undeniably important to society, but carry no market value?

There are basically two choices regarding how forests should be managed. One, which has had precedence since it was originated in Germany in the 19th century, is to homogenize forests and forested landscapes, reducing natural diversity in order to concentrate on the economics of wood production. The second, advocated by a growing number of scientists and managers, is to manage forested landscapes in such a way that natural diversity is preserved. Advantages of homogenizing are mostly economic (Table 1): productivity is maximized, at least in the short-run, by shortening rotations and growing only those species and genotypes that produce highest economic yields; management is relatively simple, hence, cheap; products are homogenized, which makes for more efficient logging and milling.

On the other side of the ledger, disadvantages of homogenizing forests are broader based and more far reaching, but also less tangible and more difficult to quantify (Table 1). First, homogenization is not aesthetically appealing, a significant factor on public lands, whose owners -- the people -- are making it increasingly clear that they do not want their lands managed as tree farms (the public has not yet confronted the problem of their own demand for wood products). Second, habitat is eliminated for some species, such as spotted owls and red-cockaded woodpeckers, and it is reduced for others, making them vulnerable to local extinction. Third, because diversity serves as an important (but poorly understood) natural stabilizing mechanism within ecosystems, homogenization risks making forests more vulnerable to various stresses and, because of this, could actually decrease long-term yields (Perry 1988, Franklin et al. 1989, Perry et al. 1989). Economically, the tradeoff between homogenizing forests and reducing their stability is no different than that associated with investments in financial markets: high yields do not come free; they are paid for by higher risks. At a time when humans are facing significant uncertainties --about how climate may change, in how ecosystems work --we believe that the economic gains of the homogenized forests are no longer worth the risks. Like any prudent investor in uncertain times, foresters must become more conservative in how they manipulate the assets that they hold in trust.

Maintaining aesthetics, preserving species, and protecting natural stabilizing mechanisms are all valid arguments for changing the way we manage our forests. Fortunately, they are not mutually exclusive: all are linked through the common thread of maintaining --or restoring --natural diversity within the managed landscape. In the following we look briefly at the intensively managed forest within the context of the natural one, examine how the two differ (which tells us much about the specifics of new approaches), and highlight ways to retain important pieces of the natural system within the managed.

NATURAL DIVERSITY AND INTENSIVE FOREST MANAGEMENT
All forest ecosystems are periodically disturbed, though the scale and frequency of disturbances vary widely both among different forest types and within a given type. Species are well adapted to disturbances that have been a normal part of their history; thus, far from being a negative factor, these natural events tend to renew and diversify forested landscapes. There are two important lessons in this. First, stability in nature does not mean "no change" --things that do not change are dead --but rather means constraining change within certain bounds which include maintaining both the productive capacity of soils and populations of indigenous species. Second, forestry does not necessarily degrade ecosystems, and can even be a tool for keeping them diverse and healthy. To accomplish this, however, silviculture must do two things: (a) protect species and habitats that may have no market value; and (b) reflect, to at least some degree, natural patterns. Intensive forest management does rather poorly on both scores.

In many forest types, the natural successional sequence following disturbance includes three general stages: (a) a relatively diverse early successional forb/ shrub/ sapling stage; (b) a closed-canopy stage in which diversity usually declines; and (c) an "old-growth" stage, in which structural and biological diversity again increase. Patchiness in the occurrence and severity of disturbances, which seems to be the rule in all except the most catastrophic events, creates a complex mosaic of successional stages across the landscape. Intensive forest management focuses on the middle, least diverse, stage. Techniques aimed at hastening full site occupation by commercial species shorten the early successional stages, and harvesting when mean annual increment culminates completely eliminates the old-growth state --along with the unique and ecologically important habitats it produces (large dead wood, multiple canopy layers, late successional tree species). The results are simplified forests and simplified landscapes.

THE DIVERSE MANAGED FOREST: SOME PRINCIPLES
Following are some elements of the diverse managed forest, and known or suspected roles each plays in ecosystem processes (including the dynamic processes that underlie forest stability).

Large Dead Wood

Snags are habitat for animals that consume defoliators and bark-beetles. Old, decayed soil-logs are water reservoirs, sites of nitrogen fixation, and habitat for organisms that cycle nutrients.

Diverse Plant Species

Some non-crop species fix nitrogen or otherwise benefit nutrient cycling (e.g., Ca cyclers with conifer forests) and nutrient retention following disturbance (e.g., sprouting hardwoods). Some hardwoods are relatively inflammable and, when intermixed with conifers, may decrease fire severity. Other plants serve as habitat and food for animals that prey on defoliators (e.g., at least some wasps that parasitize moth larvae require a source of nectar --flowering plants --in their diet).

Diverse Landscapes

Landscape patterns determine the rate at which virtually every type of forest disturbance propagates, and the intensities that it attains. Insects, pathogens, fire, wind -- forces that are relatively innocuous, even beneficial, when at certain levels or intensities -- become highly destructive when given a homogenous food source or a landscape with no natural barriers to their spread. Considering the variety of potential pests and pathogens in nature, virtually any uniform landscape pattern is likely to set the lunch table for something (Perry 1988). The best -- perhaps the only -- approach to containing potential catastrophes and epidemics is what we frequently see in natural landscapes --complex, even chaotic patterns of community structure, species composition, and age distribution.

Silviculturally, at least some of the elements can be managed for rather easily.

"Green retention": Leaving a cover of large trees within harvested areas --green retention -- allows two or more canopy layers to be developed and provides a future source of snags and soil-logs. The numbers of trees retained will vary with objectives and forest-type; prescriptions in the Pacific Northwest vary from less than 20% of the basal area up to two-thirds. Creating only small openings within forests that are densely stocked in the upper canopy layers (as in single tree selection) will eventually reduce forest diversity by excluding all but the shade tolerant species.

Wide spacings: Growing stands at wide spacing allows noncommercial plant species to coexist with crop trees, and produces large crop trees more quickly.

Long rotations: Growing at least a portion of the stand on long rotation is essential to produce sufficient old-growth habitat within the managed landscape, and to avoid creating too much early successional habitat for animals such as deer that can become serious pests when too abundant (as has happened in Great Britain). How long "Long" is will, again, vary with forest type.

Landscape focus: Maintaining migration corridors and avoiding fragmentation begins to shape a more diverse landscape. Landscapes are probably the greatest challenge, particularly since much of the diversity that occurred in the natural landscape no longer exists.

Specific approaches to all of the above will vary with forest type -- there is no one way. The heart of the "new" approach is not technique, but knowledge and understanding; not a specific act, but a process.

The New and the Renewed

In many respects, "New Forestry" is not new at all, but a renewal of a very old tradition: the forester who seeks to understand the natural rhythms of forested landscapes and to manage in harmony with those.

What is new, however, should spur us to reevaluate our management approach. The magnitude and rapidity of predicted climate change, and its potential to stress all ecosystems, are unprecedented. Also unprecedented in modern times is the degree to which humans, confronted with the growing homogeneity of virtually all aspects of society and nature, are rediscovering their need for diversity and beauty, as well as material goods -- a fact that has led to the strong environmental movements that are springing up throughout the world and that cut across all classes (Sheppard 1990). Much knowledge is new; every day adds to our understanding of forest ecosystems: their complexity, the vital role they play within the global ecosystem. And, as is typically the case in the study of complex systems, the more we learn, the more we realize remains to be learned, which recalls Aldo Leopold's first rule of intelligent tinkering -- to keep all the pieces.

ACKNOWLEDGEMENTS
The concepts expressed in this paper were developed through the joint efforts of numerous scientists and managers. We owe a particular debt to members of the Oregon State University Ecosystems Studies Group; to Steve Eubanks, Mike Amaranthus, Lynn Burdette, and Miles Hemstrom of the U.S. Forest Service; and to Bob Lewis of the Bureau of Land Management. Thanks also to Carol Perry for editing and Mayvin Sinclair for typing the manuscript. Research contributing to these ideas was funded by the National Science Foundation's Long-Term Ecological Research Program, the U.S. Forest Service, and Oregon State University's Department of Forest Science.

NOTES
1 Paper presented at the joint Silviculture/Forest Ecology working group session at the SAF National Convention held in Washington, D.C., on July 29-August 1, 1990.

2 Dave Perry and Fred Swanson, Department of Forest Science, Oregon State University, Corvallis, OR 97331; Jerry Franklin, College of Forest Resources, University of Washington, Seattle, WA 98195.

Bibliography
Franklin, J.F. 1989. Toward a new forestry. American Forests. Nov/Dec 1989; pp 37-44.

Franklin, J.F. 1990. Thoughts on applications of silvicultural systems under new forestry. Forest Watch. Jan/Feb 1990. pp 8-11.

Franklin, J.F.; Perry, D.A.; Schowalter, T.D.; Harmon, M.E.; McKee, A.; Spies, T.A. 1989. Importance of ecological diversity in maintaining long-term site productivity. p. 82-97 in D.A. Perry, R. Meurisse, B. Thomas, R.Miller, J. Boyle, J. Means, C.R. Perry, and R.F. Powers (eds.). Maintaining the Long-term Productivity of Pacific Northwest Forest Ecosystems. Timber Press. Portland, Oregon.

Perry, D.A. 1988a. An overview of sustainable forestry. Journal of Pesticide Reform 8, 8-12.

Perry, D.A. 1988b. Landscape patterns and forest pests. Northwest Environmental Journal 4, 213-228.

Perry, D.A.; Maghembe, J.. 1989. Ecosystem concepts and current trends in forest management: time for reappraisal. Forest Ecology and Management. 26, 123-140.

Sheppard, W. Bruce. 1990. Political motivations for the "new forestry." Coos Chapter, SAF. April 19, 1990. Coos Bay, Oregon.