Wildland fire effects on forest structure over an altitudinal gradient, Grand Canyon National Park, USA

摘要

1. Restoration of wildland fire to forests is a challenge when historical fire regimes have been altered. We studied four fires that burned over approximately 7865 ha on an altitudinal gradient in Grand Canyon National Park, USA, in 2003. The fires met criteria for the current USA policy allowing the restoration of fire's ecological role in forest landscapes: Wildland Fire Use for Resource Benefits. 2. After the fires burned out, we remeasured 82 permanent pre-established monitoring plots burned by the fires plus 43 additional plots on unburned companion sites. 3. The maximum height of charring of tree boles and basal area mortality increased in mean value and variability with altitude. At a low-altitude Pinus-Quercus site, tree density declined significantly but basal area was unchanged. At a mid-altitude mixed-conifer site and a high-altitude Picea/Abies/Populus site, both density and basal area declined. 4. The thinning effect of fire was concentrated on smaller, shorter, fire-susceptible trees. Small-diameter trees (< 20 cm diameter) made up 79-95% of all tree mortality. Shade-tolerant conifers, particularly true firs and spruce, experienced disproportionate mortality (31-82% basal area decline), while fire-resistant ponderosa pine and Douglas fir tended to survive (2-8% basal area decline). Delayed mortality between the first and second years following the fires accounted for only 4.2% of trees dying at the low-altitude site but 15.6% and 11.2% at the mid- and high-altitude sites, respectively. Regeneration density was highly variable but forest floor and woody debris declined in burned areas. 5. Synthesis and applications. This study shows that, even after an unusually long fire-free period (1880-2003), at the mid- and high-altitude burned sites fire effects were consistent with restoration of historical patterns, moving the ecosystems closer to historical reference conditions. Fires simultaneously reduced the living, dead and ladder fuels that made the forest vulnerable to uncharacteristically severe fire. These effects make the forests more resistant to the expected increases in fire size and severity under future climate conditions. Even at longer-than-historical fire intervals, the wildland fire use policy can benefit Grand Canyon forests.