Wildland fuel and vegetation responses after reburns

Wildland fuel and vegetation responses after reburns in US Northern Rocky Mountain lodgepole pine forests

Two or more successive wildfires within a short time span (hereafter, reburns) can create substantive shifts in forest and wildland fuel succession and subsequent ecosystem resistance and resilience to future fires. This is especially important considering predicted increases in fire frequency and area burned under future climates. Here, we study reburns in the extensive lodgepole pine forests of the Bob Marshall Wilderness Complex, USA. We first developed conceptual pathways of vegetation and fuel succession based on three ecological criteria: the reproductive maturity of lodgepole pine, the timing and amount of snag fall, and the intensity of a second fire. We then sampled 21 sites that had burned twice within the last 50 years. Eight plots were established at each site; four in the first burn and four in the second burn. Tree, fuel, and undergrowth vegetation characteristics were sampled at each plot. We summarized the plot data for response variables representing tree conditions, understory vegetation, and fuelbeds, and compared the once and twice burned plots to detect changes resulting from a second fire. Following this, we post-stratified our sample by our conceptual pathways to characterize vegetation and fuel changes for sampled pathways. We found that where snags had fallen and lodgepole pine trees had no cones, reburns were usually high intensity, and the second fire converted the burned area to a grass-shrub community, with limited woody fuels and rare tree seedlings. However, when snags were still standing, the second fire caused these snags to fall, resulting in elevated fuel loadings and patchy or open lodgepole pine communities. We found that when snags had fallen, and lodgepole pine trees had cones, the reburn was usually of moderate to high intensity, and the second fire resulted in open or closed pine communities with limited surface fuels.