Development and Calibration of a New Method of Reconstructing Fire History in Low-Severity Fire Regimes

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This research project will address a basic hypothesis in biogeography -- that the structure of dry pine forests like ponderosa pine is dominated by a low density of large trees and is maintained by frequent low-severity fires that limited tree regeneration. This hypothesis rests in part on the assumption that low-severity fires were historically frequent, an assumption largely dependent on a fire-history method that uses composite fire intervals. Some evidence has been presented from analysis, simulation, and an initial modern calibration that the use of composite fire intervals is biased and inaccurate. To overcome limitations of the composite-fire-interval method, the investigators have developed a new approach, the all-tree-fire-interval (ATFI) method, and showed by simulation that the use of is new method is unbiased and accurate. This project will complete a field-based modern calibration and apply the ATFI method in a historical fire-history reconstruction to bring the ATFI method to facilitate use of this method to help test the basic hypothesis. The investigators will first gather and analyze field data on scarring fraction, a central parameter needed for using the ATFI method, by sampling about 48 plots in 22 fires that burned in ponderosa pine forests in northern Arizona, a key area for current understanding of low-severity fire. This scarring-fraction analysis will facilitate development of a spatial model to predict scarring fraction from environmental setting, tree characteristics like density and diameter, and fuels. The scarring-fraction model then will be used to complete a modern calibration of the ATFI method and historical fire-history reconstructions using the ATFI method in 1000-ha areas in Grand Canyon National Park and Fort Valley Experimental Forest. The investigators will extract and cross-date about 640 increment cores to date trees and about 105 fire scars to count fires for the ATFI calibration and reconstruction. Finally, they will complete a spatial analysis of extant pre-EuroAmerican fire-scar evidence in these two areas to improve basic understanding of how fire-scar evidence occurs across landscapes. In the process of completing these three parts of the research, the investigators will test seven specific hypotheses about scarring fraction, three about fire history, and three about extant fire-scar evidence. They expect to be able to greatly improve understanding of how fire-scar evidence is left by fires and how to use this evidence to accurately reconstruct the history of low-severity fire across landscapes. This project will test and validate a new method for accurately reconstructing fire history in forests. Fire histories provide fundamental information needed in managing forests in national parks, national forests, and on other public lands. More specifically, this project will further develop a new fire-history method and help to resolve uncertainties about the rate that historical fires burned in ponderosa pine forests. Too much fire or too little fire can have adverse effects on old-growth trees, understory plants, and wildlife, so it is important to resolve this uncertainty. The research undertaken in this project will provide both field and laboratory educational opportunities for a graduate student and several undergraduate students.