Available Fuel Dynamics in Nine Contrasting Forest Ecosystems in North America

Available fuel and its dynamics, both of which affect fire behavior in forest ecosystems, are direct products of ecosystem production, decomposition, and disturbances. Using published ecosystem models and equations, we developed a simulation model to evaluate the effects of dynamics of aboveground net primary production (ANPP), carbon allocation, residual slash, decomposition, and disturbances (harvesting, tree mortality, and fire frequency) on available fuel (AF; megagrams per hectare). Both the magnitude and the time of maximum ANPP as well as the duration of high productivity condition had a large influence on AF. Productivity and decomposition were two dominant driving factors determining AF. The amount of AF in arid or cold regions would be affected more by climate change than that in other ecosystems. Frequent fire was an effective tool to control the AF, and medium frequency fire produced the most AF. Disturbances increased AF very rapidly in a short period. The results can be used as a basic knowledge to develop a fire management plan under various climate conditions. Fire is a common disturbance that greatly influences species composition, forest structure, carbon cycling, and nutrient cycling in many forest ecosystems (Dumonte and others 1996, Whittle and others 1997, Thompson and others 2000, Wang and others 2001). Fuel, climate, and topography have been proposed as the three major components to predict fire behavior and ignition (Whelan 1998). Under fire-favorable weather, the role of fuel is likely to be the most important factor determining fire behavior (Bessie and Johnson 1995). The amount of fuel is controlled by vegetation type, decomposition rate, ecosystem productivity, and their interrelationships (Brown and others 1999, Flannigan and others 2000, Cumming 2001, Wang and others 2001, Mickler and others 2002). The combined effects of fire prevention, fire suppression, timber harvesting, and pest management have altered the patterns of fuel loading (Thompson and others 2000), and climate change will significantly affect the intensity and frequency of fire due to changes in fuel quality and quantity (Stocks and others 1998, Franklin and others 2001). In the United States, fire burned 2.6 million ha from January to September in the year 2002, more than double the 10-year average area (http://www.nifc.gov/fireinfo/nfn.html ). The various changes in fire regime are important not only because of the significant effect of fire on management and vegetation but also because of changes in the pattern of the Earth’s carbon sequestration (Clark 1990, Overpeck and others 1990, Johnson and Larsen 1991, Stocks and others 1998, Flannigan and others 2000, Franklin and others 2001, He and others 2002). The current forest management paradigm is shifting from a strict focus on fire prevention to accommodation and emulation of the historic fire regime. The primary approach of landscape management is to maintain states of fuel loading similar to those that existed prior to European settlement to achieve sustainable ecosystem management (Boychuk and others 1997). However, a substantial gap remains between the principles of fire accommodation and emulation and their application. A clear understanding of the relationships among fire, weather, fuel, and disturbance across scales is essential. Available fuel (AF) can be defined as the total dry weight of ground-level fuel per unit area, while potential fuel (PF) is the total biomass per unit area in the system (Whelan 1998). Disturbances can be divided into three types based on the change in available and potential fuel after a disturbance. The first type of disturbance causes an increase in AF without a signifi