Changes in canopy fuels and potential fire behavior 1880?2040: Grand Canyon, Arizona

Abstract

We applied detailed forest reconstructions measured on broad-scale plot grids to initialize forest simulation modeling in 1880 and modeled spatially explicit changes in canopy fuels (canopy biomass, canopy bulk density, species composition) and potential fire behavior (crowning index) through 2040, a 160-year period. The study sites spanned a 500-m elevational gradient from ponderosa pine forest through higher-elevation mixed conifer, aspen, and spruce-fir forests on the North Rim of Grand Canyon National Park in northern Arizona. The simulations were relatively accurate, as assessed by comparing the simulation output in the year 2000 with field data collected in 1997–2001, because a regionally calibrated simulator was used (Central Rockies variant of the Forest Vegetation Simulator) and because we added regeneration by species and density in the correct historical sequence. Canopy biomass increased at all sites, rising an average of 122% at the low-elevation sites and 279% at the high-elevation sites. The intermediate-elevation site, where mixed conifer vegetation predominated, began with the highest canopy biomass in 1880 but had the lowest increase through 2040 (39%). Canopy bulk density increased roughly in parallel with canopy biomass; however, density values were considered less accurate in non-contemporary dates because they were based on assumptions about canopy volume. Species composition of canopy fuels was consistent at low elevation (ponderosa pine) but shifted strongly toward mesic species at higher elevations, where ponderosa pine declined in absolute as well as relative terms. Potential crown fire behavior was assessed with the Nexus model in terms of crowning index (CI), the windspeed required to sustain active canopy burning. CI values decreased 23–80% over the modeled period. Canopy fuel and CI values were mapped across the entire North Rim landscape. At a threshold windspeed of 45 km/h, only 6% of the landscape was susceptible to active crown fire in 1880 but 33% was susceptible by 2000. Implications of the changes over time and space include altered contemporary habitats and the high likelihood of rapid, broad-scale disturbance by fire. If managers choose to intervene to reduce canopy fuel mass and continuity, actions should be guided by the distinct ecological attributes of the different forest types.