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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]

Applying an interaction framework, we examined whether climate change and combined land use and disturbance changes were synergistic, antagonistic, or neutral for forest issues of wildfires, tree growth, tree species distributions, species invasions and outbreaks, and deer herbivory, focused on the eastern United States generally since the 1800s and the development of instrumental records (1895). Climate largely has not warmed during 1981–2020 compared to 1895–1980, but precipitation has increased. Increased precipitation and land use (encompassing fire exclusion and forestation, with coarse fuel accumulation due to increased tree densities) have interacted synergistically to dampen wildfire frequency in the humid eastern U.S. For overall tree growth, increased precipitation, carbon fertilization, and land use (i.e., young, fast-growing dense stands) likely have been positive, generating a synergistic interaction. Human activities created conditions for expanding native tree species distributions, non-native species invasions, and damaging native species outbreaks. No strong evidence appears to exist for recent climate change or land use influences on deer populations and associated herbivory levels. In the future, a warmer and effectively drier climate may reverse synergistic and neutral interactions with land use, although effects of climate interactions with land use will vary by species. Management can help correct non-climate stressors due to land use and support resilient structures and species against climate change. [ABSTRACT FROM AUTHOR]

While many tree species occur across the Coastal Plain of the southeastern United States, longleaf pine (Pinus palustris C. Lawson) savannas and woodlands once dominated this region. To quantify longleaf pine's past primacy and trends in the Coastal Plain, we combined seven studies consisting of 255,000 trees from land surveys, conducted between 1810 and 1860 with other descriptions of historical forests, including change to the present day. Our synthesis found support that Pinus palustris predominantly constituted 77% of historical Coastal Plain trees and upland oaks (Quercus) contributed another 8%. While Pinus still dominates these forests today (58% of all trees), most are now either planted loblolly (Pinus taeda L.) or slash (Pinus elliottii Engelm.) pines. Water oak (Quercus nigra L.), live oak (Quercus virginiana Mill.), sweetgum (Liquidambar styraciflua L.), and red maple (Acer rubrum L.) have increased their proportions compared to historical surveys; both longleaf pine and upland oaks have declined to ≤5% of all trees. Our work also supports previous estimates that longleaf pine originally dominated over 25–30 million ha of Coastal Plain forests. As late as the early 1900s, longleaf pine may still have covered 20 million ha, but declined to 7.1 million ha by 1935 and dropped to 4.9 million ha by 1955. Longleaf pine's regression continued into the mid-1990s, reaching a low of about 1.3 million ha; since then, restoration efforts have produced a modest recovery to 2.3 million ha. Two centuries of overcutting, land clearing, turpentining for chemicals, fire exclusion followed by forest densification by fire-sensitive species, and other silvicultural influences, including widespread loblolly and slash pine plantations, have greatly diminished the Coastal Plain's once extensive open longleaf pine forests. [ABSTRACT FROM AUTHOR]

Climate classifications supply climate visualization with inference about general vegetation types. The Köppen classification system of thermal classes and an arid class is widely used, but options are available to strengthen climate change detection. For this study, I incorporated temperature and aridity information into all climate classes to isolate climate change, added a hypertropical class to better detect warming and drying in tropical zones, and developed a consistent ruleset of thermal classes with one temperature variable for streamlined application, yet maintained primary Köppen thermal classes. I compared climate currently to 6000 years ago (ka; Mid-Holocene) and 22 ka (Last Glacial Maximum) worldwide. Growing degree days > 0 °C was the most efficient variable for modeling thermal classes. Climate classes based on growing degree days matched 86% of Köppen thermal classes. Current climate shared 80% and 23% of class assignments with the Mid-Holocene and Last Glacial Maximum, respectively, with dry conditions shifting to the tropical and hypertropical classes under current climate. Contributing to our understanding of global environmental change, this classification demonstrated that the hypertropical class experienced the greatest change in area since 6 ka and the second greatest change in area since 22 ka, and the greatest increase in percentage arid classes during both intervals. The added hypertropical class with aridity information delivered sensitive detection of warming and drying for relevant climate classes under climate change. [ABSTRACT FROM AUTHOR]

Ecologists have a long history of describing species associations including oak-hickory, one of the predominant associations in the eastern United States. But historically, oak composition did not appear particularly related to hickory composition. I assessed the relevance of the oak-hickory association and other associations using older and recent (c. 1981 and 2007) USDA Forest Service surveys. For common hickory and oak species, I determined percent composition (i.e., percent of total stems ≥12.7 cm in diameter, relative density or abundance) in ecological subsections, changes in composition throughout ranges, and compared composition of oaks and hickories and other potential associations using correlation and ordination. Oaks were among the most abundant species while hickories were minor species. Hickory composition was stable while the trajectory of oak continued to decrease during the survey intervals from presettlement dominance. Rank-order correlation between oaks and hickories throughout their ranges was about the maximum as for other species (0.55 and 0.42 during the two survey periods) and in the Oak-Hickory forest region, correlation between oaks and hickories was 0.04 (older surveys) and 0.16 (recent surveys). Oaks were not associated with hickory in the “oak-hickory” forests of Missouri during the mid-1800s, nor were oaks associated with hickory more recently beyond correlations that occur between other eastern forest species. Oak-hickory association in particular is not an informative term for either historical open oak ecosystems or current eastern broadleaf forests. Mixed mesophytic associations, perhaps not best termed as an association, are eastern broadleaf forests where many tree species dominate forested ecosystems in the absence of filtering disturbance. Associations, even if species share similar traits, generally are not strong, stable in time, or extensive in space; differences between species result in different and changing distributions in response to the environment, land use, disease, and other influential factors.