Functional Indicators

The agreement to manage forests in a scientific and sustainable manner, signed by participating countries at the Earth Summit in Rio in 1992, has led to several processes focused on developing criteria and indicators of sustainable management and conservation of world-wide forests (Ramakrishna and Davidson 1998). The Montreal Process is the largest of these efforts geographically and defines a criterion as a category of conditions or processes by which sustainable forest management may be assessed, or, as a set of related indicators which are monitored periodically to assess change. In turn, it defines indicators as a quantitative or qualitative variable that can be measured or described and that when observed periodically demonstrates trends (Montreal 1995). Functional indicators can therefore be defined as variables that assess changes and trends in forest function over time.

Forest function can also be broadly defined as ecosystem services, using Cairns and Pratt's (1995) definition of ecosystem services as any attribute of the natural system that is perceived as beneficial to human society (as cited in Rapport et al 1998). Costanza et al. (1997) listed the following ecosystem services for forests: climate regulation, disturbance regulation, water regulation, water supply, erosion control, soil formation, nutrient cycling, waste treatment, biological control, food production, raw material production, genetic resources, recreation, and cultural usage. In addition, the National Research Council (2000) identified carbon storage, stand structure, and species diversity as important functional aspects of forests.

Multiple indicators for each of these functions have been proposed (see following sections). Since the number of indicators can be overwhelming, the following are broad functional categories within which to frame these indicators: productivity, carbon storage, nutrients, soils, hydrologic cycle, and diversity. These forest functions are in turn also directly affected by several non-functional attributes, such as stand structure, land use, disturbance, neighborhood, and history. Can we use these broad categories of forest function to create the framework for an index of sustainable forestry?

Production and Carbon Storage

The concept of sustainability emerged within European forestry practices during the late 18th century with the recognition that certain human actions were detrimental to forest stands. Concerns over timber supply created a supply-oriented concept of sustainability and let to the formulation of concepts of sustained yield and maintenance of production capacity (Wiersum 1995). Productivity is not only a useful indicator of timber production and economic development, but has also been proposed as a potential indicator of soil health, soil microbial activity, and nutrient cycling, and carbon storage (Richardson et al. 1999). Forests are also considered to play an important role in the global carbon budget as carbon sinks and sources of CO2 emissions (Jepma et al. 1997, Solberg 1997).

Several indicators of production and carbon storage within forests have been proposed, including using a plant growth index, measuring timber growth and harvest (Heinz Center 2002), net ecosystem production (NEP), net primary production (NPP) (National Research Council 2000), productive capacity as measured by are of forest land available for timber production, total growing stock, and annual removal of forest products compared to a pre-determined sustainable level (Montreal Process 1995), root growth, and ability to regulate soil carbon balance, as measured by litterfall, root turnover, soil respiration, soil organic matter (Burger and Kelting 1998), biomass, volume, basal area (Richardson et al. 1999), gross national product (GNP), gross ecosystem production (GEP), and gross primary production (GPP) (Costanza et al. 1998). Odum (1985) also notes that in stressed ecosystems community respiration increases, the ratio of production to respiration becomes unbalanced, the ratios of maintenance to biomass structures increase, the importance of auxiliary energy increases, and exported or unused primary production increases.

Nutrient Cycling, Soils, and Hydrologic Cycles

While nutrient cycling, soil quality, and hydrologic regulation are all listed as separate functions that forests provide, in reality, each function is interlinked with the others, so that it is difficult to discuss one without touching upon the others. Nutrients are often limiting factors for species and ecosystem processes and increased deposition can alter both function and biodiversity. High nitrogen loads can lead to eutrophication and alteration of oxygen availability, species abundance, and diversity within aquatic ecosystems (Rennings and Wiggering 1997). In stressed ecosystems, nutrient turnover increases, horizontal transport of nutrients increase, while vertical cycling decreases, and nutrient loss increases (Odum 1985). Soils are the crucible for critical microbial processes that control rates of decomposition and nutrient cycling. Soils serve as an interface between aboveground terrestrial processes and water resources, including the capacity to hold water available for uptake by plants (Burger and Kelting 1998). Forest soils filter water entering streams and reservoirs, reducing the effects of non-point source pollution. Sediment loads and pollutant concentrations in streams remain low when good soils are in place (Johnston and Crossley, Jr. 2002). There are currently considerable efforts to define and quantify soil health and quality. Two proposed definitions of soil quality are:

The capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation. (Karlen et al. 1997).

A measure of the condition of soil relative to the requirements of one or more biological species and/or to any human purpose. (Johnson et al. 1997).

Proposed indicators of nutrient cycling include nitrogen supply, organic matter content, nitrogen mineralization, carbon content, soil respiration, active organic matter, cation exchange capacity (CEC), effective CEC, labile nutrient content, pH, extractable nutrients (Burger and Kelting 1998), nitrate in streams (Heinz Center 2002), nutrient-use efficiency, rates of nutrient runoff, and nutrient balances (National Research Council 2000). Proposed indicators of soil functioning, including nutrient availability, water holding capacity, organic matter content, water filtration, soil structure, and biological activity are porosity, water content, air content, macroporosity, redox potential, oxygen levels, biologically active organic matter content, soil temperature, soil moisture, pH, microbial biomass (Burger and Kelting 1998), soil surface protection, compaction, soil aggregate stability (Belnap 1998), forest floor chemistry (C/N ratio, lignin content, P, K, Ca, Mg contents), and rooting depth (National Research Council 2000).

Direct Effects on Function

There are attributes of ecosystems that directly impact function and thus can serve as indirect indicators or predictors of ecosystem function. Among these are stand structure, or the physical composition and condition of the stand, the disturbance regime, the land use or management of the ecosystem or stand, and the neighborhood matrix within which the stand is situated, including the community sense of place (National Research Council 2000, Stedman 1999).

Proposed indicators of stand structure include light penetration, crown condition, extent and trends of physical damage to trees (recorded by species and age class), including insects, pathogens, weather stress, air pollution, storms, fire, poor management, logging, foliage-height profiles (National Research Council 2000), and mapping and monitoring of the abundance, density, size, and decay class of key structural features, including CWD, gaps, snags, and layers (Noss 1999). Vitality of the ecosystem and resilience to or recovery from disturbance are considered to be important criteria for sustaining ecosystems (Kijazi and Kant 2003) and can be indicated by rotation period and predicatability, invasive species (Noss 1999), and presences of heavy metals (Azar et al. 1996). Land use and the percent of land per management type and the ecosystem services provided therein contribute to the definition of within stand sustainability and overall ecosystem or global sustainability (Montreal Process 1995, Rapport 1998).