Forest structure and organic horizon analysis along a fire chronosequence in the low elevation forests of western Montana

Although fire consumes much of the forest floor, few studies have examined the change in forest floor characteristics with increasing time since fire. Mixed forests of ponderosa pine (Pinus ponderosa Doug. Ex. laws) and Douglas-fir (Pseudotsuga mensizii (Mirb.) Franco) in the inland northwest once burned with greater frequency than today. Fire exclusion over the last 100 years is believed to have caused a shift in forest structure, forest floor decomposition and nitrogen availability. However, no research has clearly demonstrated this in forests of the dry inland northwest. The objective of this study was to determine how fire exclusion has shaped forest structure and understory composition in ponderosa pine/Douglas-fir forests and how these changes have altered forest floor characteristics. Stand level and understory vegetation characteristics, organic horizon depth and biochemistry were analyzed along a 132-year chronosequence at 25 sites in second growth forests of western Montana. Principle components analysis confirmed that time since fire was significantly correlated to most biotic variables, including nutrient availability and indicates that the disturbance regime is tightly coupled to ecosystem function. Douglas-fir basal area, total shrubs and forest floor thickness were observed to increase with time since fire. Graminoids and forbs did not change significantly with time since fire. There was a significant increase in the content of total C, total N, NH4+ and potential mineralizable N (PMN) in the forest floor, while NO3- content decreased significantly with time since fire. Total phenols increased significantly and were positively correlated with forest floor thickness, total shrub cover, PMN and NH4+, but not correlated with NO3- content, suggesting that phenolic compounds accumulate with time and may affect N transformations.