PARENT SESSION
Poster Session #38: Landscape Ecology.
Thursday, August 9, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall
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Influence of fire exclusion on stand structure and nitrogen cycling in Ponderosa pine/Douglas-fir forests of western Montana.
MacKenzie, M. Derek1, DeLuca, Tom1, Sala, Anna1, 1
ABSTRACT- Low elevation Ponderosa pine forests of western Montana have historically been maintained by frequent low intensity wildfires resulting in open, uneven aged stands, with a grass dominated understory. Fire exclusion over the last 90 years has caused a shift in forest structure towards greater domination by Douglas-fir and shade tolerant shrubs. To date it is not clear how this long-term change in fire frequency has influenced ecosystem function in terms of water availability, allelopathy and soil nitrogen (N) availability. The objectives of this study were to determine how fire exclusion and water availability shape forest structure and understory composition, and how these changes in turn alter soil N availability. A fire chronosequence approach was adopted in which 24 paired sites of varied time since fire were selected to represent change over a 120 year time span. Each site was assessed for stand level and understory vegetation characteristics, depth of O horizon and biochemical properties (N availability, microbial biomass, phenolic and charcoal content) of the upper soil layers, on 20 replicated radial plots per site. Preliminary results demonstrate an increase in stand basal area, Ericaceae species (phenolic producers) and depth of O horizon with increasing time since fire, while relative water content, nitrification and mineralization decrease. Added charcoal significantly increased rates of mineralization. And, the inverse relationship between Ericaceae cover and nitrification suggests inhibition rather than N immobilization.
KEY WORDS: Allelopathy, N cycling, Water availability, Fire exclusion