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.
Soil CO 2 efflux (R s ) is a significant source of carbon dioxide from soils to the atmosphere and is a critical component of total ecosystem carbon budgets. Prescribed fire is one of the most prevalent forest management tools employed in the southeastern USA. This study investigated the influence of prescribed fire on R s rates in old-field pine-grassland forests in north Florida, USA, that had been managed with prescribed fire annually and biennially for over 40 years, or left unburned for approximately the same period. Monthly measurements were taken of R s , soil temperature (T s ), and soil moisture from August 2009 to May 2011. Results showed that sites managed with annual and biennial dormant season prescribed fire had significantly lower monthly mean R s rates and estimated annual soil carbon fluxes than sites where fire had been excluded. While T s explained a significant amount of the temporal variations in R s , it did not explain the differences in R s among prescribed fire treatments. Our results provide new insight into the effects of prescribed fire and fire exclusion on soil carbon fluxes, and suggest that future methods to model ecosystem carbon budgets should incorporate not only current vegetative conditions, but also prescribed fire management activities.
Frequently burned old field shortleaf pine (Pinus echinata)–loblolly pine (Pinus taeda) woodlands in the southeastern US provide important wildlife habitat and multiple ecosystem services. Because these communities differ in composition of dominant plant species and have different land use legacies than native pine savannas, the ability to prevent encroachment by off-site broadleaf woody tree species using fire alone is in question. We use a long-term fire experiment to demonstrate that old field pine communities have been prevented from transitioning to hardwood forests for over 50 years through judicious application of prescribed fire applied at 1–2 year intervals, whereas communities with three-year fire intervals show signs of transitioning to hardwood forest. We emphasize tailoring fire regimes to particular contexts of land use history to achieve the most historic and sustainable ecosystem structure and function possible for conservation of native flora and fauna. Study Implications: Demonstrating the ability to maintain natural forest structure of old field loblolly pine–shortleaf pine communities in the southeastern US using frequent prescribed fire has implications for the future sustainability of hundreds of thousands of hectares of such land used to provide critical habitat for many species of imperiled and culturally valued wildlife. It also provides insight into restoration of longleaf pine communities on postagricultural land as promoted by multiple highly funded government initiatives. Frequently burned pine savannas and woodlands are resilient to wildfire and sustain natural hydrological cycles, both important for mitigating the effects of global climate change.
Aristida beyrichiana (wiregrass) is a foundation bunchgrass species in many southeastern U.S. native pine communities, but it has been dramatically reduced in extent. The potential for reintroduced wiregrass to reproduce and spread is not well studied because of its slow growth and limited conditions for successful reproduction. We present a case study where tussocks of wiregrass were transplanted and recensused 18 and 37 years later to study their population dynamics. We remeasured a subset of tussocks to estimate diameter growth over 2 years. With frequent prescribed fires (1-3 year intervals, about half in April-July when flowering is induced), the initial population of 160 tussocks increased to 1,199 through seed dispersal and clonal fragmentation, and the total basal area approximately tripled. Relationships among tussock density, diameter, and basal area per m 2 and their changes over time suggest density-dependent regulation of population structure, possibly from intraspecific competition and competitive exclusion. Tussock diameter growth averaged 0.9 cm per year over a 2-year period and was independent of initial diameter. This study, the longest of a wiregrass population to date, suggests that a low-density population established in native soil types has a slow but robust tendency to reproduce by seed and expand if provided frequent fire, including April-July burns, in a high light environment without soil disturbance. Wiregrass can be characterized as a competitive, late-successional, dominant species in stable, climax-like native savannas, promising long-term success under appropriate conditions as part of restoration efforts.
Using imaging spectroscopy (hyperspectral imaging), we sought to assess the effects of image pixel resolution, size of mapping windows composed of pixels, and number of spectral species assigned to pixels on the capacity to map plant beta diversity using the biodivMapR algorithm, in support of the planned NASA Surface Biology and Geology (SBG) satellite remote sensing mission. BiodivMapR classifies pixels as spectral species, then calculates beta diversity as dissimilarity of spectral species among mapping windows each composed of multiple pixels. We used NEON airborne 1 m resolution hyperspectral images collected at three sites representing native longleaf pine ecosystems in the southeastern U.S. and aggregated pixels to sizes ranging from 1-90 m for comparative analyses. Plant community composition was groundtruthed. Results show that the capacity to detect plant beta diversity decreases with fewer pixels per mapping window, such that pixel resolution limits the size of mapping windows effective for representing beta diversity. Mapping window size in turn limits the spatial resolution of beta diversity maps composed of mapping windows. Assigning too few pixels per window, as well as assigning too many spectral species per image, results in overestimation of dissimilarity among locations that have plant species in common. This overestimation undermines the capacity to contrast mapping window dissimilarity within versus among community types and reduces the information content of beta diversity maps. These results demonstrate the advantage of maximizing spatial resolution of hyperspectral imaging instruments on the anticipated NASA SBG satellite mission and similar remote sensing projects.
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