Above-Ground Net Primary Production, Leaf Area Index, and Nitrogen Dynamics in Early Post-Fire Vegetation, Yellowstone National Park

Abstract

The 1988 fires in Yellowstone National Park (YNP), Wyoming, affected >250,000 ha, creating a striking mosaic of burn severities across the landscape which is likely to influence ecological processes for decades to come (Christensen et al. 1989, Knight and Wallace 1989, Turner et al.1994). Substantial spatial heterogeneity in early post-fire succession has been observed in the decade since the fires, resulting largely from spatial variation in fire severity and in the availability of lodgepole pine (Pinus contorta var. latifolia) seeds in or near the burned area (Anderson and Romme 1991, Tinker et al. 1994, Turner et al. 1997). Post­fire vegetation now includes pine stands ranging from relatively low to extremely high pine sapling density (ca 10,000 to nearly 100,000 stems ha-1) as well as non-forest or marginally forested vegetation across the Yellowstone landscape may influence ecosystem processes related to energy flow and biogeochemisty. We also are interested in how quickly these processes may return to their pre­ disturbance characteristics. In this pilot study, we began to address these general questions by examining the variation in above-ground net primary production (ANPP), leaf area index (LAI) of tree (lodgepole pine) and herbaceous components, and rates of nitrogen mineralization and loss in successional stands 9 years after the fires. ANPP measures the cumulative new biomass generated over a given period of time, and is a fundamental ecosystem property often used to compare ecosystems (Carpenter 1998). Leaf area (typically expressed as leaf area index [LAI], i.e., leaf area per unit ground surface area) influences rates of two fundamental ecosystem processes -­ primary productivity and transpiration -- and is communities (<1000 saplings ha-1 in some areas commonly used in ecosystem models (e.g., Forest­previously characterized by coniferous forest. We are interested in how the variability and spatial pattern of early post-fire successional BGC, Running and Coughlan 1988; FIRE-BGC, Keane et al. 1996). Disturbances cause reductions in leaf area that cause simultaneous reductions in transpiration and photosynthesis and increases in stream flow (Helvey et al. 1976, Gholz et al. 1985, Davis 1987, 1993, Keane et al. 1996). This study had three specific objectives. The first was to measure ANPP and LAI in stands representing a range of early post-fire vegetation structure. Our second objective was to compare our measured ANPP and LAI with values in the literature to determine how close these 9-year old post-fire stands in YNP have come to developing ANPP and LAI characteristic of mature coniferous forests. Thirdly, we measured nitrogen mineralization and nitrogen loss in stands representing a range of early post-fire vegetation structure. This was a pilot study (Reed et al., submitted), designed in part to test methods and hone our hypotheses for a research proposal to the National Science Foundation which we submitted in December, 1997 (and which was funded in June, 1998).