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Carbon sequestration in the forests of the eastern United States is an important offset to the country's CO2 emissions. Much of the eastern forestland is the product of reforestation of abandoned agricultural land or recovery following clear‐cutting over a century ago. This has led to concerns that eastern forests are even‐aged and that rates of carbon sequestration will decline as forests increase in carbon. Our objective was to examine the successional dynamics of forest carbon sequestration—using live tree carbon stocks as a proxy for successional status—for the six broadly defined forest types present in the region. We used datasets from the National Forest Inventory (NFI) for the 31 US states from Minnesota south to Louisiana and eastward and analyzed live tree net carbon increment for 2007–2021, the period for which NFI plot remeasurement data were available for all 31 states. Spruce–fir and southern pines were the only forest types for which carbon increment declined even modestly over a significant fraction of the range of live tree carbon observed in the region, and southern pine–hardwood forests were the only forests in which predicted sequestration in live tree carbon declined to zero within the range of carbon stocks observed in the region. Northern hardwood–conifer forests, oak–hickory forests, and lowland forests experienced either no decline or a slight increase in sequestration in live tree carbon across the range of successional status observed in the region. Thus, the average stocks of live tree carbon per unit area increased steadily over the study period. At some point in succession, rates of mortality are expected to increase and balance gross growth, leading to zero net sequestration in live tree carbon. Mature and old‐growth stands, however, are rare in all six forest types, and mortality as a fraction of live tree carbon for all six forest types declined across the range of successional status present in the region. Our results thus provide no support for the hypothesis that the successional dynamics of forests in this region can be expected to lead to near‐term declines in net carbon sequestration. [ABSTRACT FROM AUTHOR]

Wildfires and land use play a central role in the long‐term carbon (C) dynamics of forested ecosystems of the United States. Understanding their linkages with changes in biomass, resource use, and consumption in the context of climate change mitigation is crucial. We reconstruct a long‐term C balance of forests in the contiguous U.S. using historical reports, satellite data, and other sources at multiple scales (national scale 1926–2017, regional level 1941–2017) to disentangle the drivers of biomass C stock change. The balance includes removals of forest biomass by fire, by extraction of woody biomass, by forest grazing, and by biomass stock change, their sum representing the net ecosystem productivity (NEP). Nationally, the total forest NEP increased for most of the 20th century, while fire, harvest and grazing reduced total forest stocks on average by 14%, 51%, and 6%, respectively, resulting in a net increase in C stock density of nearly 40%. Recovery from past land‐use, plus reductions in wildfires and forest grazing coincide with consistent forest regrowth in the eastern U.S. but associated C stock increases were offset by increased wood harvest. C stock changes across the western U.S. fluctuated, with fire, harvest, and other disturbances (e.g., insects, droughts) reducing stocks on average by 14%, 81%, and 7%, respectively, resulting in a net growth in C stock density of 14%. Although wildfire activities increased in recent decades, harvest was the key driver in the forest C balance in all regions for most of the observed timeframe. Plain Language Summary: We estimate past forest fires in four regions of the contiguous United States from 1926 to 2017 using historical statistics and satellite data. We compare the biomass removed from forests by wildfires, wood harvest, and forest grazing to identify which of these indicators had the strongest impact on forest biomass. Fire suppression and biomass recovery from past harvest in the twentieth century led to biomass regrowth. Forest regrowth was stronger in the East of the United States than in the West. Recently, wildfires have increased in the West. Higher tree mortality, drought, windthrow, and insects counteracted forest growth in the West. We conclude that fire suppression contributed to forest regrowth in the past, but harvest had the strongest impact in all regions. We show that the connection between wildfires, harvest, and grazing is an important factor for biomass change in forests and needs to be investigated over long time periods. Key Points: We present a new data set, reconstructing biomass removals by fire, harvest, and grazing in U.S. forests on a regional scale from 1941 to 2017Harvest and fire were the most intensive removals of biomass over the 20th century. Regional trends diverge from national trendsIn the context of natural climate solutions, long‐term and regional dynamics of interconnected drivers of forest biomass change need to be considered [ABSTRACT FROM AUTHOR]

The impact of COVID-19 across the United States (US) has been heterogeneous, with rapid spread and greater mortality in some areas compared with others. We used geographically-linked data to test the hypothesis that the risk for COVID-19 was defined by location and sought to define which demographic features were most closely associated with elevated COVID-19 spread and mortality. We leveraged geographically-restricted social, economic, political, and demographic information from US counties to develop a computational framework using structured Gaussian process to predict county-level case and death counts during the pandemic's initial and nationwide phases. After identifying the most predictive information sources by location, we applied an unsupervised clustering algorithm and topic modeling to identify groups of features most closely associated with COVID-19 spread. Our model successfully predicted COVID-19 case counts of unseen locations after examining case counts and demographic information of neighboring locations, with overall Pearson's correlation coefficient and the proportion of variance explained as 0.96 and 0.84 during the initial phase and 0.95 and 0.87 during the nationwide phase, respectively. Aside from population metrics, presidential vote margin was the most consistently selected spatial feature in our COVID-19 prediction models. Urbanicity and 2020 presidential vote margins were more predictive than other demographic features. Models trained using death counts showed similar performance metrics. Topic modeling showed that counties with similar socioeconomic and demographic features tended to group together, and some of these feature sets were associated with COVID-19 dynamics. Clustering of counties based on these feature groups found by topic modeling revealed groups of counties that experienced markedly different COVID-19 spread. We conclude that topic modeling can be used to group similar features and identify counties with similar features in epidemiologic research. [ABSTRACT FROM AUTHOR]

This study develops mathematical tools and approaches to investigate spatiotemporal patterns of firearm acquisition in the U.S. complemented by hypothesis testing and statistical analysis. First, state-level and nation-level instant background check (BC) data are employed as proxy of firearm acquisition corresponding to 1999–2021. The relative-phase time-series of BC in each U.S. state is recovered and utilized to calculate the time-series of the U.S. states' synchronization degree. We reveal that U.S. states present a high-level degree of synchronization except in 2010–2011 and after 2018. Comparing these results with respect to a sitting U.S. president provides additional information: specifically, any two presidential terms are characterized by statistically different synchronization degrees except G. W. Bush's first term and B. H. Obama's second term. Next, to detail variations of BC, short-time Fourier transform, dimensionality reduction techniques, and diffusion maps are implemented within a time-frequency representation. Firearm acquisition in the high frequency band is described by a low-dimensional embedding, in the form of a plane with two embedding coordinates. Data points on the embedding plane identify separate clusters that signify state transitions in the original BC data with respect to different time windows. Through this analysis, we reveal that the frequency content of the BC data has a time-dependent characteristic. By comparing the diffusion map at hand with respect to a presidential term, we find that at least one of the embedding coordinates presents statistically significant variations between any two presidential terms except B. H. Obama's first term and D. J. Trump's pre-COVID term. The results point at a possible interplay between firearm acquisition in the U.S. and a presidential term. [ABSTRACT FROM AUTHOR]

Land use by Indigenous people (Native Americans) and climate are primary factors affecting the dynamics of oak (Quercus) forests and woodlands in the eastern United States. Prior to Euro-American settlement, much of the eastern deciduous forest was dominated by oak species. The role of periodic surface burning, agriculture, and other forms of land management by Indigenous peoples is frequently noted by cultural anthropologists and historical ecologists. However, these points are often debated by paleoecologists and climate scientists. Here we present a literature review, synthesis, and data summary to investigate the role of altered land use, season of burning, and climate change in relation to pre– and post–Euro-American changes in forest composition. Human-based ignitions, as reflected by dormant-season fires, prevailed over the oak- and pine-dominated forests, with intermediate fire frequency during Indigenous periods. From the 18th century on, Euro-American populations rapidly expanded, impacting much of the eastern United States through extensive timber harvesting, land clearing, and severe fires. Starting in the 20th century, a variety of ecological influences, including agricultural land abandonment, chestnut blight, fire suppression, mesophication, and urbanization, resulted in dramatic vegetation changes in eastern landscapes. These trends have culminated in recruitment failures of most oak species on all but the most xeric sites and an increase in mid- to late-successional mesic hardwoods, most notably red maple (Acer rubrum), a species with very low density in our analysis of the witness tree record. We conclude that prescribed burning, agriculture, and other land uses by Indigenous peoples created a mosaicked landscape of expansive oak savannas, woodlands, and forests. A warming world over the past century should have promoted warm-adapted, fire-tolerant, xerophytic genera such as oak, hickory (Carya), and pine (Pinus) and grassland communities but instead have promoted the invasion by cool-adapted, fire-sensitive, mesophytic trees due to the absence of burning, much to the detriment of major vegetation biomes. Understanding that eastern oak and other pyrogenic ecosystems represent an Indigenous landscape strengthens our ability to best manage vegetation against the expansion of less desirable species and restore historic fire cycles through prescribed burning. [ABSTRACT FROM AUTHOR]