The North American tree‐ring fire‐scar network

Abstract

Fire regimes in North American forests are diverse and modern fire records are often too short to capture important patterns, trends, feedbacks, and drivers of variability. Tree‐ring fire scars provide valuable perspectives on fire regimes, including centuries‐long records of fire year, season, frequency, severity, and size. Here, we introduce the newly compiled North American tree‐ring fire‐scar network (NAFSN), which contains 2562 sites, >37,000 fire‐scarred trees, and covers large parts of North America. We investigate the NAFSN in terms of geography, sample depth, vegetation, topography, climate, and human land use. Fire scars are found in most ecoregions, from boreal forests in northern Alaska and Canada to subtropical forests in southern Florida and Mexico. The network includes 91 tree species, but is dominated by gymnosperms in the genus Pinus. Fire scars are found from sea level to >4000‐m elevation and across a range of topographic settings that vary by ecoregion. Multiple regions are densely sampled (e.g., >1000 fire‐scarred trees), enabling new spatial analyses such as reconstructions of area burned. To demonstrate the potential of the network, we compared the climate space of the NAFSN to those of modern fires and forests; the NAFSN spans a climate space largely representative of the forested areas in North America, with notable gaps in warmer tropical climates. Modern fires are burning in similar climate spaces as historical fires, but disproportionately in warmer regions compared to the historical record, possibly related to under‐sampling of warm subtropical forests or supporting observations of changing fire regimes. The historical influence of Indigenous and non‐Indigenous human land use on fire regimes varies in space and time. A 20th century fire deficit associated with human activities is evident in many regions, yet fire regimes characterized by frequent surface fires are still active in some areas (e.g., Mexico and the southeastern United States). These analyses provide a foundation and framework for future studies using the hundreds of thousands of annually‐ to sub‐annually‐resolved tree‐ring records of fire spanning centuries, which will further advance our understanding of the interactions among fire, climate, topography, vegetation, and humans across North America.

Indigenous fire management and cross-scale fire-climate relationships in the Southwest United States from 1500 to 1900 CE

“…All fire-scar chronologies used in the analysis are archived in the International Multiproxy Paleofire Database (IMPD) and were compiled as part of the North American Fire Scar Network (NAFSN) ( 73 , 74 ). All Superposed Epoch Analyses (SEAs) were conducted using the burnr ( 75 ) library [function sea() ] in the R programming environment ( 76 ).…”

Fire Regimes Over a 1070-m Elevational Gradient, San Francisco Peaks/Dook’o’oosłííd, Arizona USA

“…Fire regimes have been studied at many temporal and spatial scales from sedimentary analysis (Marlon et al 2009) to remote sensing (Balch et al 2020), as well as through re-adaptive traits (Keeley and Pausas 2022). While these various approaches have unique advantages, tree-ring reconstructions of past res through dating of re scars on surviving trees provide multi-century, annually resolved re records that can be compared with data on climate, ages of tree cohorts, other disturbances, and human activities at ne to coarse scales (Margolis et al 2022).…”

“…Our present results from the Peaks cannot be interpreted directly in terms of Indigenous re management practices because much of this context is not available or beyond the scope of this study. However, adding additional re regime data to the growing North American dataset(Margolis et al 2022), particularly in unique or poorly understood areas such as the Peaks, is of value for improving future analyses aimed at this question.…”

Climate forcing of regional fire years in the upper Great Lakes Region, USA

“…Comparisons of climate variables (i.e. temperature, precipitation, moisture deficit, and evapotranspiration) between historical (fire scar) and modern (North America Fire Atlas data) records, for example, find larger climate-space discrepancies in the northern forests of the upper Great Lakes Region than in any other North American ecoregion (Margolis et al 2022). Similarly, contemporary fires in the upper Great Lakes Region have occurred almost entirely in spring (Cardille and Ventura 2001).…”