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Since Euro-American settlement and associated fire exclusion, grasslands and open forests have converted to forests throughout the United States. Contributing to the weight of evidence, we determined if forestation also occurred in forests and grasslands of Colorado. Our study extent encompassed landscapes of the 0.5 million ha Arapaho and Roosevelt National Forests in the northern Front Range (eastern side) of the southern Rocky Mountains and the 1 million ha Weld County, which contains Pawnee National Grassland, in the Great Plains grasslands. We quantified tree composition, cover, and densities from historical (years 1863 to 1886) tree surveys, current surveys (2002 to 2011), and land cover (2016) to identify departures. In the Arapaho and Roosevelt, historical lack of tree presence and overall low tree densities suggested an open landscape, due to about 70% of 7134 survey points without two trees within 60 m. The treed landscape, which was not continuously forested, had density estimates of about 153 trees/ha. In contrast, the current landscape was 68% forested with high tree densities; fire-dependent pines decreased relative to subalpine fir (Abies lasiocarpa) increases. In Weld County, seven trees were surveyed historically, whereas currently, woody cover totaled 2555 ha. Uniquely applying historical surveys at landscape scales, we documented an open landscape in the northern Front Range, unlike previous research, and rare tree presence in the relatively understudied grasslands of Colorado. Forestation corresponded with changes in U.S. grasslands and forests following Euro-American settlement and associated fire exclusion. [ABSTRACT FROM AUTHOR]

Tree distributions and densities have been dynamic since Euro-American settlement in North America. Historically dominant fire-tolerant tree species have decreased, and fire-sensitive, successional species have increased, and tree species have expended westward since the 1800s into the central Great Plains grasslands. Divergent compositional trajectories and the westward expansion of tree species may be explained by climate change. To establish patterns expected by climate change, I predicted climate envelopes in eastern North America during 7 intervals, from the 1500s to 1961–1990, of 16 wide-ranging fire-tolerant and fire-sensitive species. The climate envelopes demonstrated that suitable climate area has remained relatively stable for all species: compared with the 1500s, areal extents during the 1900s increased 104% for fire-sensitive species and 106% for fire-tolerant species. Additionally, a pattern of northeastern shifts (i.e., following the North American land mass) resulted from climate change. Climate envelopes demonstrated northeastern shifts with slight expansion for all species, which did not accord with realized dynamics of westward tree expansion or increases in fire-sensitive species. In accordance with other lines of evidence, land use disturbance change, incorporating fire exclusion, likely has caused the divergent trajectories of fire-tolerant and fire-sensitive species and westward expansion into the Great Plains grasslands. [ABSTRACT FROM AUTHOR]

The Great Plains is the grasslands of the central United States, but precise delineation of this region has evaded agreement due to the transition between Great Plains grasslands and forests of the eastern United States. After comparing Great Plains delineations in readily available geographic information system layers, I established a northeastern boundary using evidence from historical tree surveys during the 1800s. Additionally, I described tree presence within the Great Plains over time and potential drivers of change. The definition documented here diverges from others by including most of Iowa and part of Illinois, and even small sections in Indiana and Wisconsin, but excluding Missouri due to regular tree presence overall at landscape scales of ecological subsections. In this definition, the Great Plains had an extent of 2.29 million km2 in fifteen states, including Indiana and Wisconsin, and 2.19 million km2 after removal of open forests in Oklahoma and Texas. Tree presence occurs within the Great Plains and tree encroachment is not a new process; however, tree encroachment is increasing in the Great Plains, which causes a decrease in the grasslands region. Nevertheless, clarifying the historical eastern boundary provides a solution to the problem of competing versions of the Great Plains region. Key Words: ecotone, prairie, savanna, tree encroachment. [ABSTRACT FROM AUTHOR]

• The drivers that separate grassland ecoregions are unknown. • Wind speeds of 4.5 m s−1 bounded central North American grasslands. • Wind was the most influential model variable. • Wind speeds may be an index for fire and grassland maintenance. Climate may separate grasslands from surrounding forests and shrublands, but an alternative option is that grasslands are suitable for trees except in the presence of frequent fires, which are spread great distances by strong winds. To develop a ruleset that demarcated the central North American grasslands, I modeled grasslands using precipitation and its coefficient of variation, ratio of potential evapotranspiration to precipitation, climatic moisture index, July vapor pressure, and wind speed (i.e., a fire indicator) during 5000 (4900–5100) years before present, 1600–1699, 1800–1849, and 1850–1899. Wind speeds of 4.5 m s−1 at 10 m above ground height clearly bounded grasslands. For all time intervals, the random forests and extreme gradient boosting classifiers had accuracies of 0.96 to 0.97 in classification of reserved samples; wind speed was ranked as the most influential model variable (i.e., value of 100 out of 100 scale, with the next most influential variables at ≤ 25). The simulated wind patterns matched recent wind models from atmospheric reanalysis and the Global Wind Atlas downscaled from an atmospheric reanalysis. Wind boundaries were stable over time, shifting slightly northward during 11,000 years before present to recent decades. Although wind is related to precipitation, I directly mapped and modeled precipitation and derivatives. Therefore, I propose that wind is an index of fire spread in grassland biomes, where conditions are appropriate for both fire and woody vegetation. More research on wind models is required to confirm these results and determine how winds affect grassland formation and maintenance globally. [ABSTRACT FROM AUTHOR]