Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests1

Environmental policies in the United States and abroad are reducing timber harvests while wood demand is mounting. Reduced harvesting on public lands means that privately owned lands will be managed with greater intensity in the United States and that wood will be imported from other nations lacking strong environmental safeguards. It is imperative, therefore, that both public and private forest lands be managed to sustain their productivity, and many nations are seeking effective monitoring methods. Central to this is our ability to estimate a site’s fundamental capacity for growing vegetation, and to detect changes in this capacity caused by management. Because soil is the factor of a site modified most easily and profoundly by management, and because soil largely is independent of the current condition of vegetation, soil-based variables offer our most effective and practicable indices of sustainable productivity. The North American Long-Term Soil Productivity cooperative research program (LTSP) is the world’s most extensive coordinated effort to address questions of sustainable productivity in managed forests. Early findings from the 12 LTSP sites in California illustrate the physical importance of organic soil cover in reducing soil erosion and maintaining favorable soil temperature and moisture relations during summer drought. Findings also show that the biological significance of soil compaction depends on soil texture. Moderate compaction degrades vegetative growth on fine-textured soils but can enhance growth on coarse-textured soils where drought is a factor. Impacts of soil compaction on tree growth often are masked by effects of competing vegetation. Measurements taken under operational conditions show that compaction associated with mechanized thinning can reduce soil rooting volume by as much as one-half. Subsoiling seems to mitigate the effect. Root damage caused by subsoiling did not adversely affect the growth of residual trees. Results are providing practicable field methods for monitoring management impacts on sustainable productivity. Forests offer many values and commodities beyond wood production. Concerned that forests have suffered from overemphasis on timber, environmentalists call for more conservative forest management practices that reduce wood harvest and preserve or restore other ecological values (Drengson and Taylor 1997). On many public lands of the western United States— historically a major source of domestic timber—harvesting continues, but at a rate less than one-third that of the last decade (USDA Committee of Scientists 1999). Pressure to de-emphasize wood production here and abroad comes while the land area formerly available for production forestry is shrinking at an annual rate of 0.4 percent (FAO 1997). Paralleling the trend on public lands of the United States, a “green advocacy” has gained momentum and has spawned an expansive international industry to certify what is, and is not, “sustainable forestry” (Hammond and Hammond 1997, Journal of Forestry 1995). In general, leading forest scientists 1 An abbreviated version of this paper was presented at the Symposium on the Kings River Sustainable Forest Ecosystems Project: Progress and Current Status, January 26, 1998, Clovis, California. 2 Senior Scientist, Leader of the Fundamental Factors of Site Productivity Science Team, and cofounder and Technical Chair of the North American LTSP Cooperative Research Program, Silviculture Laboratory, Pacific Southwest Research Station, USDA Forest Service, 2400 Washington Ave., Redding, CA 96001. USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 63 Powers Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests agree that timber harvesting, if conducted so as to preserve potential site productivity, need not compromise other ecosystem values (Attiwill 1994, Kimmins 1996). Many in North America’s private forestry sector are skeptical of third-party “green certification” where criteria may be based more on speculation than on science (Berg and Olszewski 1995). Yet, ignoring green certification could limit markets for industrial wood. Progress has been made toward developing more uniform and objective standards for green certification. The central international body is the Forest Stewardship Council with two affiliates in the United States—SmartWood and Scientific Certification Systems (Mater and others 1999). Green certification aims at protecting multiple forest values and long-term site productivity, but advocates often seem naïve and myopic. If all the world’s forests were managed under green certification standards of the Forest Stewardship Council, harvests would average 0.7 m3 ha-1 annually (Binkley 1997). Unfortunately, this average would require a one-third increase (1.1 billion ha) in global forest area just to meet the current wood demand of the world’s population. Kimmins (1996) points out that popular standards for green certification often are so stringent that harvested yields are lowered by more than if the sites had been severely degraded by exploitative management. Implications of mandating unnecessarily low harvests are serious. World population and demand for wood products are rising at similar rates (FAO 1997). This demand permeates all societies (FAO 1997) and increases by 70 to 80 million m3 annually—a volume equivalent to British Columbia’s entire allowable cut in 1993 (Kimmins 1996). Reduced wood production from green-certified nations creates a strong incentive for other countries to accelerate forest harvesting beyond sustainable levels to reap the rewards of global demand (Kimmins 1996). A scarcity in domestic wood supply will raise wood prices, stimulating consumer preference for nonrenewable substitutes. Recent studies show that with each 1 percent rise in the price of softwood lumber, the use of cement rises by 0.15 percent, structural steel by 0.3 percent, and brick by 0.65 percent (Binkley 1997). Sustaining the productivity of United States forests, regardless of ownership, is in our national interest. Currently, a simple definition of “sustainable forestry” lacks international consensus (Nambiar and Brown 1997, Sullivan 1994). No one in good conscience can support management practices that degrade forest productivity. The problem is how to produce more wood from less area without impairing the land’s potential to provide other social benefits now or in the future. Clearly, the need for a closer linkage between management and research in the forest planning process has never been stronger (USDA Committee of Scientists 1999). This paper describes the genesis of the North American Long-Term Soil Productivity (LTSP) cooperative research program and summarizes results from the various component studies done to date in the Sierra Nevada and Cascades of California. Developing Indices of Sustainable Forestry An International Movement The United Nations Conference on Environment and Development of 1992 (“The Earth Summit”) led to a nonbinding agreement to establish principles for sustainable forest management (United Nations 1992). As a result, international committees have formed to develop criteria and indicators for the conservation and sustainable management of forests of the world. One such committee met informally in Montreal, Canada, in 1993. Deliberations of what has come to be called “The Montreal Process” culminated in a 1995 meeting in Santiago, Chile. There, in the “Santiago Declaration,” representatives from Argentina, Australia, 64 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests Canada, Chile, China, Japan, Republic of Korea, Mexico, New Zealand, Russian Federation, Uruguay, and the United States agreed to develop, implement, and continually update nonbinding criteria and indicators for the sustainable management of temperate and boreal forests (Canadian Forest Service 1995). A first step is to find an unambiguous, effective, and objective way to monitor the land’s health that covers all levels of management intensity. Monitoring Productivity Directly The fundamental indicator of a forest’s well-being is the rate at which atmospheric carbon is captured photosynthetically and accumulated as organic matter. This rate is termed “net primary productivity” (NPP). In turn, NPP is the common basis for most fundamental ecosystem processes that produce the characteristics of forests valued by society. Accordingly, degrading a site’s NPP potential also degrades its potential for producing flora, fauna, habitat, clean and abundant water, and recovery from disturbance. Monitoring the departures in NPP from baseline conditions would give us a sensitive measure of the health of a forest ecosystem and whether it is agrading, degrading, or stable. Unfortunately, NPP is extremely difficult to measure. Current rates are affected not only by site quality, but also by the present age, stocking, and structure of the forest. Therefore, they may not indicate the site’s true potential at full stocking, or “leaf area carrying capacity” (Grier and others 1989, Powers 1999b, Waring and Running 1998). And even at full stocking, it is almost impossible to measure NPP accurately in forests of irregular structure. An unbiased surrogate for NPP is needed that is independent of the current condition of the vegetation. The Soil Quality Approach Soil can be a strong and independent surrogate for measures of potential NPP. Together with climate and biotic potential of vegetation, soil forms the foundation for forest production. As recognized in the Montreal Process and Santiago Agreement, soil-based indicators of sustainable forestry must include measures of erosion, organic matter, compaction, nutrient cycling, and pollution (Ramakrishna and Davidson 1999). The USDA Forest Service recognized this requirement well in advance of the Montreal Process. The National Forest Management Act of 1976 mandates that the USDA Forest Service must manage public forest lands without impairing their permanent productivity. Accordingly, and in consultation with Forest Service Research, the Watershed and Air Management Staff of the USDA Forest Service adopted a program for monitoring the effects of management practices that is based on the following logic: management practices create soil disturbances; soil disturbances affect soil and site processes; and soil and site processes control forest productivity. Monitoring soil and site processes is not feasible at an operational scale. Therefore, USDA Forest Service monitoring strategy centers on measurable soil variables, which, if altered beyond a threshold, indicate that potential productivity has been degraded. These thresholds of soil quality are based partly on research, but largely on professional judgment. Threshold standards for the USDA Forest Service Regions of the United States have been summarized by Powers and others (1998). Current standards for the Pacific Southwest Region are shown in table 1. Although these standards represent a progressive step, they are not universally accepted.