Soil quality standards and guidelines for forest sustainability in northwestern North America☆
Introduction
Forest managers are increasingly using the terms ‘sustainable forests’, ‘sustainable forestry practices’, and ‘long-term productivity’. These terms are used interchangeably, but usually have different meanings for different forest users depending on ecosystem perspective, use, and spatial and temporal scales (Amaranthus, 1998). Although there are many terms and definitions for a sustainable forest, soil productivity is a key factor to maintaining ecosystem function (Powers et al., 1998). On public lands in the USA, maintenance of soil productive capacity is a common objective, which is governed by the Multiple Use and Sustained Yield Act of 1960, the National Environmental Policy Act of 1969, the Forest and Rangeland Renewable Resources Planning Act of 1974, and the National Forest Management Act of 1976. Forest industry members of the American Forest and Paper Association must satisfy the Sustainable Forestry Initiative for soil productivity by using strategies appropriate to soil, topography, and climate (American Forest and Paper Association, 1994). In Canada, the Forest Practices Code developed by the British Columbia Ministry of Forests establishes mandatory requirements for planning forest practices, including methods to assess hazards and soil degradation; a rehabilitation guide is also provided (Forest Practices Code of British Columbia, 1995, Forest Practices Code of British Columbia, 1997).
Soil productivity monitoring protocols are being developed as indicators of sustainable forest management on broad national levels (Ramakrishna and Davidson, 1998). On federal lands in the USA, post-disturbance monitoring is used to assess changes in soil productivity or quality (Powers and Avers, 1995). Other ownerships are not under the same regulations as federal lands, but many states have, or are considering, methods to measure changes in soil quality (Powers et al., 1998). The important role of soil productivity to sustained forestry is clear, but which soil properties to monitor as indicators of forest sustainability are not (Burger and Kelting, 1998, Staddon et al., 1999).
For forests to be sustainable, soil quality — the inherent capability of soil to support vegetative growth — must be maintained (Power and Myers, 1989). Evaluating soil quality is difficult because of the diversity of soil properties to be measured, appraisal techniques, and soil uses. Proposed first-order soil physical property assessments of soil quality are infiltration, available water holding capacity, and soil depth (Papendick and Parr, 1992). Chemical indicators include pH, salinity, cation exchange capacity, organic matter, and any site-specific factors (i.e. heavy metals) (Karlen and Stott, 1994). Suggested biological indices are soil enzyme activity (Dick, 1992), soil fauna (Stork and Eggleston, 1992), or microbial activity (Powlson et al., 1987). These variables have been suggested mainly for agricultural soils. Powers et al. (1998) proposed measuring a small set of indices (soil strength, anaerobic incubation of organic matter, and soil invertebrate activity) to establish baseline soil quality conditions in more variable forest soils.
In diverse forest ecosystems, model simulations may be a more effective method for determining changes in soil quality (Burger and Kelting, 1998). For instance, water infiltration and retention can be measured directly or predicted from soil bulk density, organic matter content, or conductivity. Development of relationships among most measurable soil properties and soil functions is not widely available, but there are several models (e.g. WEPP, EPIC, NLEAP) which simulate them (Doran and Parkin, 1994). Harvesting impacts on soil organic matter characteristics and ecosystem function have been modeled using FORECAST (Morris et al., 1997). Although there are a myriad of methods used to measure soil changes associated with anthropogenic or natural disturbances, the information that relates the measured variables to soil processes, vegetative growth, or disturbance regime is limited. Process models can be constructed to substitute for soil monitoring, but to date information is lacking on vegetation-soil systems (Burger and Kelting, 1998).
Efforts to construct definitive soil quality/sustainability standards and guidelines are still in their infancy and reflect the wide diversity of soil properties and climate conditions in forest ecosystems. Most current standards and guidelines are based on easily measured soil properties and/or readily available climate and site information. For example, guidelines for surface erosion are based on topography, rainfall duration and intensity, infiltration, and vegetative cover (Wischmeier and Smith, 1978, Dissmeyer and Foster, 1985, Elliot and Hall, 1997). Soil compaction guidelines were developed to account for either an increase in bulk density or decrease in porosity. Visual criteria are used to quantify puddling, soil displacement, and sometimes compaction (Miller and Sirois, 1986, Aust et al., 1998).
One of the earliest soil quality standards and guidelines were developed by the US Forest Service (USFS) to act as a first warning of reduced forest site productivity and sustainability after harvesting and subsequent site preparation. These guidelines represent one of the first attempts at either a national or international level to quantify a threshold of soil changes which are detrimental to forest soil productivity. The general concepts and the basis for the various guideline categories are described by Griffith et al. (1992). Threshold values (quantifying change) were based on the assumption that site quality will be maintained if <15% of an area is detrimentally impacted after disturbance (Powers et al., 1998). Although soil quality guidelines vary for each Forest Service Region, they generally fall into four categories: (1) soil physical properties; (2) soil erosion; (3) soil organic matter content; and (4) fire effects. Guidelines for three Forest Service Regions in the Pacific Northwest are briefly described in Table 1 and are further discussed in Powers et al. (1998).
As currently formulated, these Forest Service soil quality sustainability guidelines are uniformly applied across each USFS Region regardless of soil or ecosystem properties. However, Burger (1997) maintains that indicators of soil quality must be site- and soil-specific. For example, soil quality indicators for a young, drought prone Entisol should be different than those used for monitoring an older, poorly-drained Alfisol (Burger and Kelting, 1998). Therefore, the purpose of this paper is to evaluate the effectiveness of applying uniform soil quality guidelines and threshold values over diverse forest landscapes in the Pacific Northwest. The guidelines selected for evaluation were soil displacement, erosion, and burned conditions. Each of these guidelines represent one or more soil function, and are monitored through either empirical measurements or visual assessments.
Soil displacement is the loss of surface horizons (forest floor and mineral topsoil) by a combination of harvesting disturbance and mechanical site preparation, and is an indicator of soil organic matter and nutrient loss. Erosion is a measure of soil movement induced by wind or water after the initial harvest/site preparation impact. Similar to soil displacement, erosion addresses changes in soil organic matter and nutrient content, but also possible sedimentation in adjacent streams and lakes. The major impact of detrimental burn conditions on soil productivity is through the loss of organic matter and volatilization of soil nitrogen (N) (DeBano et al., 1979).
Disturbance impact on soil organic matter is a factor in all three of these guidelines. Forest floor and mineral soil organic matter have important roles in nutrient availability and cycling, gas exchange, water supply, soil structure, and disease incidence (Harvey et al., 1987, Powers et al., 1990, Blake and Ruark, 1992, Henderson, 1995, Jurgensen et al., 1997). Studies from the Northwest USA and elsewhere have shown that loss of organic matter after harvesting or site preparation can have profound effects on soil physical, chemical and biological properties, and reduce soil productivity (Perry et al., 1989, Powers et al., 1990, Dyck et al., 1994, Everett et al., 1994, Harvey et al., 1994, Henderson, 1995, Jurgensen et al., 1997). Three important components of organic matter (C, N, and CEC), were used to estimate variability in Soil Quality Guidelines applied across different climates and soil types. Three representative soils from three USDA Forest Service Regions (Region 1 [Northern], Region 4 [Intermountain], and Region 6 [Pacific Northwest]) were used in this appraisal (Fig. 1). These soils cover a wide range of taxonomic and climatic conditions found in forests of the Pacific Northwest (Table 2).
Section snippets
Total soil pools
Total mineral soil C, N, and CEC pools were estimated for each soil using Natural Resource Conservation Service (NRCS) soil survey profile descriptions and analyses (Sparks, 1996). Soil horizon chemical and CEC data were extrapolated to a consistent soil depth of 1 m using horizon bulk density values and rock-fragment content. Since the soil survey descriptions did not contain any information on forest floor horizons, C and N values in the forest floor were obtained from various research studies
Total nutrient pools and distribution
As expected, the total amounts and distribution of soil C varied widely both within and among the three USFS regions (Table 4, Fig. 2A). Total C contents were highest in soils from Region 6 (146–289 Mg ha−1), and lowest in those from Region 4 (105–110 Mg ha−1). Region 4 soils also had a much smaller percentage of C in the forest floor than Region 6 soils, which reflects generally thin litter layers in dry, fire-impacted Intermountain forests. Compared to Region 6, the forest floor in both Region 4
Suitability of regional soil monitoring guidelines
It is increasingly evident that forests will be harvested under a planned sustainable forest regime (Amaranthus, 1998). Under such management plans harvest and site preparation activities require accurate monitoring of site impacts to maintain soil productivity. Forest soils are inherently variable; some are resilient to harvest activities, while others are at risk of losing their productive capacity after harvesting because of a shallow forest floor or thin mineral mantle over bedrock (Burger
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