The Fragmented Forest

Island Biogeography Theory and the Preservation of Biotic Diversity

Larry D. Harris

• Publisher: University of Chicago Press
• Year: 2013
• Print ISBN: 9780226317649, 9780226317632
• eBook ISBN: 9780226219950
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

In this poineering application of island biogeography theory, Harris presents an alternative to current practices of timber harvesting.

"Harris pulls together many threads of biological thinking about islands and their effect on plant and animal survival and evolution. He weaves these threads into a model for managing forest lands in a manner that might serve both our short-term economic and social needs as well as what some people feel is our ancient charge to be steward of all parts of creation."—American Forests

Winner of the 1986 Wildlife Society Publication Award

PART 1

Problem Setting

1

Introduction

Recent changes in the forests of the world have received attention at the highest scientific and governmental levels (I.U.C.N. 1980; C.E.Q. 1980; Meyers 1980; Lanly 1982; U.S.D.A. 1982). The most notable changes fall into three principal categories: (1) reduction in total forest acreage; (2) conversion of naturally structured and regenerated forests to even-aged monoculture plantations; and (3) fragmentation of remaining natural forests into progressively smaller patches isolated by plantations or by agricultural, industrial, or urban development (Harris 1980). These processes are not independent of one another and must be considered in an interrelated fashion.

Worldwide wood consumption has increased to approximately 100 billion cubic feet per year (F.A.O. statistics in U.S.D.A. 1982, p. 81), about half of which is used for industrial products and half for fuelwood.¹ In most less developed countries, 90% of the people depend on firewood as their chief source of fuel (Eckholm 1980) and 90% of the wood harvested is used for energy (Meadows 1981). Since worldwide demand for wood is far outstripping supply, quips such as “it now costs more to heat the cooking pot than to fill it” are commonplace. Forests that covered approximately one-fourth of the earth’s land area in 1960 now cover only about one-fifth of the land area, and will likely cover only one-sixth by the year 2000. This decline is projected to stabilize at about one-seventh of the land area by the year 2020 (C.E.Q. 1980, 117).

In spite of these trends, standing timber volumes have increased from their lowest level in the 1930s in several Western countries such as the U.S., but this is principally because of aggressive coniferous reforestation, timber stand improvement programs, and conversion of understocked second-growth stands to tree plantations. Contemporary commercial forestry is both silviculturally and capital-intensive. Short rotation, even-age tree plantations are not like natural forests in that they are oriented toward maximum timber production, are not autogenic, and generally rely on energy-intensive management such as site modification, control of competing vegetation, genetic improvement, planting, and fertilization (Demoulin et al. 1976; Farnum et al. 1983). While the productivity of the U.S. commercial forest land has increased nearly threefold over the last twenty years, capital investments for processing have increased over sixfold (Bingham 1976; Clawson 1976).

Forest fragmentation results from patchwork conversion and development of the most accessible and/or more productive sites, leaving the remaining forest in stands of varying size and degrees of isolation (Burgess and Sharpe 1981). These forest fragments take on characteristics of habitat islands in proportion to their degree of and length of time since isolation.

Along with our increased awareness of these trends has come a worldwide concern for the preservation of biotic diversity (T.N.C. 1975; C.E.Q. 1980; I.U.C.N. 1980; U.S. Dept. State 1982). Strategy has shifted from a former species-by-species approach toward a planning approach. Thus the U.S. Department of Agriculture policy states that habitat management for viable populations “will be accomplished through the forest planning process . . .” (U.S.D.A. secretary’s memorandum 9500–3, 1982). This begs the question of whether ecological theory and resource management principle can contribute to and guide the planning process.

A large and significant body of information, principle, and theory falls under the heading of island biogeography theory (sensu MacArthur and Wilson 1967). Observations and generalizations about the patterns of floral and faunal communities on oceanic islands predate the voyages of Darwin and Wallace but have only been synthesized into a cohesive body in the last thirty years (Darlington 1957; MacArthur and Wilson 1967; Sauer 1969; Pielou 1979). Conservationists have been particularly receptive to these principles because islands present such compelling conservation issues. For example, although islands represent less than 7% of global land area and less than 10% of all bird species occur there, more than 90% of all bird extinctions have occurred on islands. Of the 396 bird species listed in the International Union for the Conservation of Nature and Natural Resources (I.U.C.N.) Red Data Book as threatened or endangered, 236 (60%) are endemic to islands (Gosnell 1976). If the biological principles and generalities from true islands are applicable to patches of old growth in the forest landscape, the biological foundations of a planning strategy can be established. Ecological theory might again contribute to forest management on a par with technological mechanics and economic calculus. Although analyses of conditions have been performed (e.g., Burgess and Sharpe 1981), I am not aware of any specific application of island biogeography principles to a forest management problem.

Forests of the Douglas fir² region of western North America represent an ideal opportunity to apply such a strategy. As the only remaining expanse of virgin forest in the conterminous United States, the old-growth Douglas fir represents a unique challenge to our planning ability and commitment to conservation. The National Forest Management Act (NFMA) and related legislation mandates that annual sales must be less than or equal to the quantity that can be removed in perpetuity. This concept, referred to as “nondeclining even flow,” is at the root of the current challenge and public debate. Since old-growth forests have a low to negligible mean annual increment of usable wood (MAI), almost any harvest is above the level sustainable by that particular stand or forest type. Because present regional harvest rates are well above annual production in the Pacific Northwest, some argue that the rate of old-growth cutting must be drastically reduced to achieve a balance with production. On the other hand, young managed stands exhibit a very high mean annual increment and thus the “supply-side” view is that in order to bring production into balance with present cutting levels, the conversion of decadent old growth to young, well-stocked stands should be accelerated. One school wishes to reduce cutting down to a level equal to present production rates; the other wishes to continue or perhaps increase the conversion rate of old growth and thereby greatly increase the annual sustained yield.

Economics are also involved. Old-growth forests that have low and sometimes even negative timber growth rates represent large amounts of nearly idle capital investment. Clawson (1976) calculates that at modest interest rates, a $12 billion excess inventory in standing timber means an annual cost of $600 million in total or about $3.00 for each citizen. He asks what would result if every citizen of the United States were asked to contribute $3 annually toward the maintenance of this “excess inventory” of old trees. Thus, while we possess a rare opportunity to maintain significant acreages of a unique North American forest type, we also incur significant pressure to liquidate it.

To the extent that old-growth forests represent more than wood fiber and timber, we should question the trade of old growth for young growth. To the extent that remaining old-growth Douglas fir ecosystems possess unique structural and functional characteristics distinct from surrounding managed forests, the analogy between forest habitat islands and oceanic islands applies. Forest planning decision variables such as total acreage to be maintained, patch size frequency distribution, spatial distribution of patches, specific locations, and protective measures all need to be addressed. Island biogeography theory and the lessons learned from true-island biogeography provide a basis for developing a management strategy and addressing these specific decision variables.

A recent publication (Franklin et al. 1981) has synthesized what information was known of the structure and function of old-growth Douglas fir ecosystems in the western Cascades. A related volume (Thomas 1979) has detailed certain wildlife habitat implications of forest management and laid groundwork for stand scheduling decisions. Building on that information, this work describes important characteristics of the animal communities in Douglas fir–western hemlock forests of the western Cascades and develops guidelines for an old-growth management strategy.

At present, island biogeography theory represents little more than a set of interwoven ideas. The following specific application to old-growth management in the Douglas fir region will establish that it also represents a viable tool. The degree to which the approach is applied to forest management problems worldwide is dependent upon its demonstrated utility in specific regions such as the western Cascades. Although the approach outlined here will gain power in proportion to the validity of the analogy between old-growth patches and true islands, the overall strategy is sufficiently robust to be useful whether or not island biogeography theory is valid.

2

The Approach

To address the needs of forest management planning, this work attempts to (1) refine, articulate, and extend certain ecological principles in applied terms; (2) address the issue of diversity from the community and ecosystem level rather than the species-by-species approach; (3) describe and demonstrate the utility of a specific approach; and (4) generalize potential applications to other regions and forest types.

Both inductive and deductive approaches to science have been used to advance ecological knowledge and establish principles of use to resource management. The inductive approach (from observation of specifics to formulation of generalities) has dominated until recently. The strength of the approach is its reliance on empirical field data, but it is very time-consuming and has rarely resulted in theoretical formulations of wide-ranging power. The quicker, more powerful deductive approach involves the testing of hypotheses under specific conditions. The strength of this approach derives from the ability to test repetitively cause-and-effect relations between nonconfounded variables (e.g., bird species diversity ∝ foliage height diversity). The weakness is that “principles” all too often become encoded as truth before being tested and verified.

Since the theory of island biogeography has led to many falsifiable hypotheses and the existing data and natural history wisdom of scientists such as Chris Maser could be used to evaluate certain of these, this study draws heavily on the hypothetico-deductive approach (Hempel 1966). Field work was conducted during a short period in 1979, six months of 1980, and three months during 1981. I have used simple logic and algebraic models to piece together existing data with ecological principles and island biogeography theory in an attempt to develop a cohesive whole.

The principles used are generally independent of the validity of MacArthur and Wilson’s equilibrium theory of island biogeography (1963, 1967). It must also be borne in mind that the thrust of this work is not directed at a system of nature preserves, but rather a forest management strategy that depends heavily upon, and in turn conserves, the unique characteristics of old-growth ecosystems. Indeed, the habitat island approach to the maintenance of biotic diversity assumes, and is somewhat hinged upon, the existence of the preserve and wilderness area system.

Much of island biogeography, much of contemporary ecology, and much of the criticism of forest management deal with changing distributions, patterns, and interactions of floral and faunal communities. Until recently most references to diversity involved species diversity and the integrity of biological communities. Resource management has increasingly focused on the community level, and this is generally the emphasis herein. These analyses and principles are not directed merely at saving endangered species, but rather at keeping the full complement of species from becoming endangered. Once a species is identified as endangered we have little recourse but to drop to the species level and manage specifically at that level. The two approaches are highly complementary.

Present policies governing forest management planning must move quickly from general guidelines for development of forest plans to the detail of timber harvest scheduling on specific forests. It is at the level of the forest that standards must be converted into action plans, and situation analyses and proposals must involve specific forests. Several considerations led to the choice of the Willamette National Forest in western Oregon as an area of focus (see fig. 3.1, p. 12). As the Willamette is the top timber producer of the 155 national forests, harvest scheduling there is no small task. The H. J. Andrews Experimental Ecological Reserve is contained within the Willamette and thus the large data base from this site was available. In addition to being representative of the western Cascades, the administrative climate is ideal. However, to preclude an overly narrow focus, data were also obtained from other areas (e.g., the coast range), other national forests (e.g., the Siuslaw), and lands managed by other agencies (e.g., Bureau of Land Management).

PART 2

Current States of Nature

3

The Natural Forest Community

The Douglas fir–western hemlock forest west of the Cascade mountains in Washington and Oregon originally spanned a north-south distance of 500 miles (800 km) and occupied approximately 28 million acres (11,336,00 ha) (fig. 3.1, Sargent 1884; Andrews and Cowlin 1940). Approximately 90% of the forest consisted of Douglas fir with western hemlock and western red cedar occurring as subdominants. Old-growth Douglas fir, or trees in excess of 300 years old, probably occupied 50% of this area (14 × 10⁶ acres) and constituted 75% of the volume (Andrews and Cowlin 1940; Kirkland 1946). Densest stocking and highest volumes occurred in the lower elevations and river valleys between 2,500 and 4,000 feet (762 and 1,219 m) (fig. 3.2). Although volumes on some sites were as great as 150,000–200,000 board feet (mbf) per acre (875–1165 m³/ha), a reasonable average for old-growth stands was 65 mbf/acre (375 m³/ha) (table 3.1; Gannett 1902; Plummer 1902; Langille et al. 1903; Andrews and Cowlin 1940; Groner 1949).

Detailed vegetation descriptions are given by Franklin (1979) and Dyrness and his associates (1974). Distinctive features are highlighted by Waring and Franklin (1979) and Franklin and Waring (1980). Specific attributes of old-growth Douglas fir ecosystems are reviewed by Franklin and his associates (1981). Ecosystem characteristics of particular relevance to the vertebrate fauna are discussed in six categories below.

High Latitude and Mediterranean Climate

The combination of high latitude (from 42° N to 49° N) and generally high altitude in the western Cascades creates environmental conditions ideally suited for conifers. For example, a site at 10,000 feet (3,050 m) elevation and 45° N latitude has a temperature regime approximately equivalent to that of 16,000 feet (4,900 m) elevation at 25° N latitude (fig. 3.3). A rainfall pattern that yields 90% of the annual precipitation during the winter and 10% during the summer (fig. 3.4) combines with the latitude and elevation to produce an environmental regime strikingly different from any other in North America. The combined effects favor conifers over hardwoods except at low elevations or on wet sites near permanent water. With little precipitation occurring during the summer, the forest quickly becomes dry and flammable, accentuating the threat and impact of fire. Forests near lakes and perennial streams take on additional...

Table of contents

• Cover
• Copyright
• Title Page
• Dedication
• Contents
• Foreword
• Preface
• Part 1: Problem Setting
• Part 2: Current States of Nature
• Part 3: Analysis of Alternatives
• Part 4: A Planning Strategy
• Appendix 1: Scientific Names of Species Cited in Text
• Appendix 2: Volume of Timber Cut from the Willamette National Forest, 1905–81
• Appendix 3: Approximate Acreage Cut from Different Elevational Zones
• Appendix 4: Forty-five Terrestrial Vertebrate Species of the Western Cascades That Require Cavities, Snags, and Fallen Logs
• Appendix 5: List of Research Natural Areas and Comparable Preserves in Western Oregon and Western Washington
• Notes
• Literature Cited
• Author Index
• Species Index
• Subject Index