Concurrent with increased tree densities after surface fire exclusion due to Euro-American settlement, the historical dominance of fire-tolerant oak and pine species was replaced by a diverse array of fire-sensitive, early- and mid-successional tree species in the eastern United States (Hanberry et al., 2019; Hanberry et al., 2023). A comparison between tree composition in a historical assessment (1907 to 1909; Holmes and Bradfield, 1908; Hall, 1909; Holmes and Hall, 1909) and modern surveys (2010 to 2014) in Kentucky revealed similar trends of oak declines relative to gains in red maple. Historically dominant, fire-tolerant oaks decreased from 52% of all trees to 21%, while maples increased from 5 to 19%, with similar trends observed in each region. The historically dominant maple species was primarily the shade-tolerant, late-successional sugar maple, which was evaluated as a species in the assessment (i.e., the species description of maple referred only to sugar maple), whereas red maple was included in the appendix list of trees and shrubs due to its historical limitation to swamps and other wetlands (Holmes and Bradfield, 1908; Hall, 1909; Holmes and Hall, 1909). Currently, red maple is the dominant species in Kentucky, comprising 9.1% of all trees, albeit just slightly more abundant than yellow-poplar and sugar maple, which constitute 8.9 and 8.6% of all trees, respectively. The state transition from grasslands and open oak or pine forests to denser forests of diverse species, including red maple, is well-documented for the eastern U. S. (e.g., Shaler, 1888; Maxwell, 1910; Harrod et al., 1998; Frost and Harrold, 2013; Tulowiecki et al., 2025). Shaler (1888:27) described the vegetation of Kentucky as primarily continuous old-growth forests that were ‘singularly open, so that the early track-ways and wagon roads were easily made through them.’ The departure in tree composition aligns with changes based on historical land surveys compared to modern tree surveys in part of the Appalachians in Kentucky (Frost and Harrold, 2013), and bordering Kentucky in West Virginia (Thomas-Van Gundy and Morin, 2021), southern Ohio (Deines et al., 2016), Indiana (Hanberry, 2019), and Missouri (Hanberry et al., 2012a, 2012b) within the historically oak-dominated central eastern United States.

In Kentucky, the three ecological regions influenced species distributions. Yellow-poplar increased from 3.5 to 9% of all trees in Kentucky, but it was concentrated in the Appalachian Mountains region, where its proportion rose from 5 to 12% of all species, similar to southern Ohio (Deines et al., 2016) and Indiana (Hanberry, 2019). In the Appalachians, American chestnut and shortleaf pine decreased, along with oaks. Eastern redcedar increased from <1 to 6% of all trees state-wide, but this change occurred in the Interior region, where eastern redcedar increased from a trace presence (0.3% of all trees) to the most abundant tree species at 11.7% of all trees, similar to Missouri (Hanberry et al., 2012a) and Indiana (Hanberry, 2019). Eastern redcedar has been the most successful tree species at colonizing non-forested areas such as the Interior region, where grasslands have historically been converted to pastures and crops (Hanberry et al., 2012a; Hanberry, 2019). Although sweetgum has increased in the uplands of the southeastern U. S., south of Kentucky (Hanberry, 2019; Hanberry et al., 2019; Hanberry et al., 2023), it has decreased in the historical floodplains of the Lower Mississippi Alluvial Valley (Hanberry et al., 2012b), which also occurred in the Loess Hills region of Kentucky, bordering the Mississippi River. Instead, elms, ashes, and hickories increased, similar to changes in Missouri (Hanberry et al., 2012a, 2012b), along with the addition of planted loblolly pine in the Loess region.

These changes in tree species resulted in divergent historical and current forests, as measured by the squared-chord distance, in Kentucky. Following Euro-American settlement and the associated exclusion of fire, the greatest divergence occurred in the Interior region, with the least divergence in the Appalachian region. The Appalachian region as a whole had more divergent forests than the Daniel Boone National Forest within it, perhaps due to management efforts in the Daniel Boone National Forest aimed at maintaining and restoring oak forests. However, the Appalachians and Daniel Boone National Forest were almost as divergent as the other regions from historical forests when American chestnut was included in the divergence analysis. American chestnut was extirpated by an introduced fungal disease (chestnut blight; Cruikshank, 1943). Nonetheless, the Land Between the Lakes National Recreation Area, within the Interior region where chestnut was not as abundant as in the Appalachian region, had the lowest squared-chord distance value (0.14), within the borderline range between similarity and divergence.

Although these results are credible and align with reported ecosystem transitions, I used county assessments of standing volume for trees ≥ 30.5 cm due to the absence of accessible data sources about historical forests in Kentucky. Volume measurements from that time may not directly translate to relative tree proportions. However, the similarity to historical land surveys from the early 1800s in an overlapping part of the Appalachians (Frost and Harrold, 2013) and the Loess region (Bryant and Martin, 1988) indicates that standing volume was generally representative of historical forests. The historical assessment reflected the older trees of the forests, which may have helped in representing tree surveys from the early 1800s. Indeed, representativeness was indicated by (1) tree species/genera percentages from the assessment conducted during 1907–1909 being similar to overlapping historical records from the early 1800s, (2) the surveyors conducted the assessment to represent forests during 1907–1909 in Kentucky, not unrepresentative forests (Holmes and Bradfield, 1908; Hall, 1909), (3) older, larger diameter trees were characteristic of historical forests (Defebaugh, 1906; Holmes and Bradfield, 1908), and (4) the tree species and genera percentages from the assessment during 1907–1909 were consistent with oak-dominated historical forests from surrounding studies, with no evident discrepancies (Frost and Harrold, 2013; Hanberry et al., 2012a, 2012b; Deines et al., 2016; Hanberry, 2019; Thomas-Van Gundy and Morin, 2021). Agricultural cultivation had already occurred by the time of the assessment, affecting the abundance of American beech, sugar maple, chestnut, hickories, and the red oak group, even though these tree species had not been harvested for timber, and commercial supply remained excellent at the time of the assessment (Holmes and Bradfield, 1908:91). Harvesting had exhausted accessible sources of large diameter yellow-poplar, and the more limited black walnut and black cherry, followed by white oak and the commercially similar chestnut oak (Q. montana) by the time of this assessment (1907–1909; Holmes and Bradfield, 1908). Nonetheless, trees less than 46 cm in diameter were ‘usually too small to be cut with much profit’, resulting in relative abundance of most species (Holmes and Bradfield, 1908, p. 67). As with many tree surveys, smaller-statured trees were not measured as carefully as larger species in the historical assessment. For example, while sourwood is a component of Appalachian forests, this species was grouped in the assessment with other undistinguished species (i.e., ‘scattered’ grouping; Holmes and Bradfield, 1908; Hall, 1909; Holmes and Hall, 1909). As an additional caveat, small samples of modern survey plots were conducted in the Loess Hills and the Land Between the Lakes National Recreation Area.

In the historical assessment, the Appalachian region of eastern Kentucky was dominated by fire-tolerant species, with a significant component of late-successional species. Fire-tolerant oaks, American chestnut, and shortleaf pine comprised 65% of all trees, and when including the hickories, fire-tolerant species represented 70% of all trees. The late-successional species, primarily American beech, along with eastern hemlock and sugar maple, accounted for another 17% of all trees. Tree species were distributed along strong topographic gradients based on fire exposure and protection (Frost and Harrold, 2013; Lafon et al., 2017). The most fire-tolerant species, such as shortleaf pine, were found on the most exposed sites of south-facing upper slopes and upland flats, while the late-successional species were located in the lower elevations, in valleys along aquatic barriers to fire, and on high-elevation rocky ridges and outcrops with limited fine fuels, which reduced the chance of ignition (Frost and Harrold, 2013).

The Appalachians were frequently exposed to surface fires (Ayres and Ashe, 1905; Lafon et al., 2017). The Appalachian region was bordered on the west side by a large extent of relatively flat topography, creating a compartment ideal for the spread of fire, resulting in grasslands (Figure 3; see the Interior region below). Historical accounts and models of fire compartments described grasslands extending into the southeastern counties of the Appalachian region (Frost and Harrold, 2013; Hanberry and Noss, 2022). In addition, fire occurred within the smaller compartments of the Appalachian Mountains, with evidence of surface fires covering 80% of the 2.2 million ha examined during 1900 and 1901 (Ayres and Ashe, 1905; Lafon et al., 2017). Fire compartments encompassed entire mountainsides, and only the most protected sites avoided fire exposure, particularly given fire as a management tool used by both native humans and Euro-American settlers (Ayres and Ashe, 1905; Lafon et al., 2017; Colenbaugh and Hagan, 2023). Frequent surface fire removed small-diameter tree and shrub species, historically producing savannas and woodlands in the Appalachians (Frost and Harrold, 2013). Although historical photographs may show continuous or nearly continuous tree cover, fire histories and descriptions of the understory indicated that beneath the overstory canopy, small-diameter tree species were removed, resulting in closed woodlands with closed overstories and open under- and midstories, relatively free of tree growth (Ayres and Ashe, 1905). The forage available in the open understory of woodlands supported livestock until fire protection led to dense tree growth (Cruikshank, 1943).

Fire exclusion has occurred slowly over time in the Appalachian region, with greater tree establishment beginning during the 1930s–1960s (Harrod et al., 1998; Hutchinson et al., 2019; Stambaugh et al., 2020). Typically during fire exclusion, the historically dominant species first establish more densely, followed by a gradual expansion of fire-sensitive species and a loss of spatial differentiation in species distributions, which is then followed by the replacement of fire-tolerant tree species by fire-sensitive species in the uplands (Harrod et al., 1998; Frost and Harrold, 2013; Flatley et al., 2015). Specifically, species such as beech, sugar maple, hemlock, and eastern white pine (Pinus strobus) expanded upslope from fire-sheltered lowlands or downslope from ridges, intermixing into upper slopes and upland flats, thereby blurring the sharp zonation by elevation (Frost and Harrold, 2013). After the initial increase in establishment by historically abundant species, invasion by colonizing, early- and mid-successional species, such as red maple, yellow-poplar, and Virginia pine, was accelerated by the clearing of historical trees. Eventually, oaks and shortleaf pine diminished in abundance relative to fire-sensitive species.

Large but disrupted fire compartments with conditions suitable for frequent surface fires, potentially supporting grasslands, occurred throughout the Interior region (Frost and Harrold, 2013). The relatively flat Interior region of Kentucky was documented as grasslands and oak savannas (‘barrens’), specifically with tree development where hills broke flat topography, and contained large native herbivores such as bison (Bison bison) and elk (Cervus canadensis), which interacted with surface fire regimes and the open ecosystems of herbaceous vegetation (Michaux, 1805; Engelmann, 1863; Shaler, 1888; Frost and Harrold, 2013). Grassland fire compartments experienced limited topographic disruption of fires spread by strong westerly winds and were fueled by herbaceous vegetation, which thrived due to precipitation in the humid eastern U. S. (Figure 3; Hanberry and Noss, 2022). Frequent surface fires in grassland fire compartments exposed surrounding areas, which had greater topographic resistance, to fire. Ignitions were encouraged by Native Americans, who burned for various purposes, including to support foodways and travelways by removing woody undergrowth and promoting pasturage (Michaux, 1805; Davidson, 1840; Shaler, 1888; Frost and Harrold, 2013). The ‘Great Trail’, a network of trails, including those used during bison migration, was burned annually from Indiana to Tennessee (Shaler, 1888; Jakle, 1969). Grasslands and open forest ecosystems of the eastern U. S. contained a significant component of cane (Arundinaria gigantea), a perennial bamboo favored by bison (and livestock; Campbell, 2015). Fires could extend for several kilometers, moving rapidly due to westerly winds; however, for surface fires spreading through herbaceous fuels, safe spaces for humans could be created by removing herbaceous vegetation, such as by burning it in advance of the main fire or encouraging tree growth (Michaux, 1805). Similarly, vegetation discontinuities protected trees. Grasslands included a tree component, although sometimes stunted to a brushy, shrubby form due to repeated damage to their aboveground parts. Land management through fire played an integral role in maintaining the historical and paleo-ecosystems of grasslands and woodlands (Frost and Harrold, 2013).

Nonetheless, grasslands in Kentucky were not monolithic like the adjacent Prairie Peninsula extension of the Great Plains grasslands in Illinois, northwest of Kentucky, but were more similar to the grasslands and savannas of Indiana and Ohio, north of Kentucky (Sears, 1926; Hanberry and Noss, 2022). The Interior region of Kentucky did not consist solely of grassland fire compartments; particularly, rivers and other areas that offered protection from fire, such as ravines, valleys, and rugged hills, contained fire-sensitive species and denser forests intermixed with grasslands and oak-dominated savannas and woodlands (Hussey, 1876; Shaler, 1888). Grassland landscapes may contain a smaller oak component than woodland landscapes because uplands in grasslands burn too frequently (i.e., nearly annually) for fire-tolerant tree species to establish, and by definition, only at low densities in grasslands. In protected locations, a range of conditions may support a variety of species. Historical assessments noted a lack of trees and the presence of limited tree species, specifically oaks, but accounts described diverse species and a mosaic of ecosystems, often depicting protected locations near watercourses or along steep valleys for forests of fire-sensitive species (Owen, 1856; Hussey, 1876; Sargent, 1884; Shaler, 1888; Campbell, 1989; Campbell, 2014, 2015). Grasslands and grassland compartments in the eastern U. S., including the eastern Prairie Peninsula merging into eastern woodlands and forests, were intermixed with a range of tree densities and species in fine-textured landscapes (Hanberry et al., 2025).

However, oaks constituted 52% of all trees, and fire-tolerant species reached 63% of all trees when including relatively fire-tolerant chestnut and hickories, in the Interior region during the 1907–1909 assessment, which occurred about a century in some counties after fire exclusion and the transition to increased tree establishment, including fire-sensitive tree species. The fire exclusion process was evidenced by diminished oak percentages in current surveys. Fire-tolerant oak species were more dominant before decades of fire exclusion allowed encroachment by other tree species into uplands, which historically were exposed to fire. Beech, at 9.4% of all trees, ranked second in abundance after oaks in the Interior for the historical assessment, which was similar to Indiana and Ohio, where beech was intermingled with oaks historically, suggesting some tolerance to fire by beech (Sears, 1926; Hanberry, 2019). The relative magnitude of fire-tolerant species to fire-sensitive species indicated that historically, more area than not was exposed to frequent fires and in open ecosystem states, rather than closed forests (Hanberry et al., 2025).

Rapid tree growth occurred after fire exclusion. The initiation of fire exclusion was dated as early as 1790 (Shaler, 1888), extending to 1850 or later, corresponding to Euro-American settlement and land use expansion across the Interior region (Frost and Harrold, 2013). By the first agricultural census in 1850, agricultural area exceeded 95% in three counties within the grasslands centered in the northeastern Interior region, or ‘Bluegrass’ area, indicating the location of the earliest fire exclusion. In contrast, agricultural area remained less than 15% in most counties in the northwestern Interior region during 1850 (Maizel et al., 1998). Grasslands were plowed into fields that were harvested annually, road density increased, native large herbivores were extirpated, and domestic livestock reached saturation to oversaturation by 1840 to 1850, eliminating grass fuel connectivity, while deliberate annual burning ceased (Shaler, 1888; Frost and Harrold, 2013). In addition, overgrazing of herbaceous plants and the direct clearing of herbaceous vegetation for agricultural use, followed by field abandonment, opened up growing space for trees. Oaks were observed to increase in density, extent, and diameter during the initial few decades of release from fire exclusion (Davidson, 1840; Hussey, 1876; Sargent, 1884). Late-successional sugar maple also increased, likely in locations protected from land use, while American beech decreased. Shaler (1888, p. 30) wrote, ‘The germs of the small-seeded trees, maples, etc., were rapidly transported by the wind from the nearest remaining trees which clung about the entrances to the canons that abound in this district and other damp places, and quickly repossessed the ground in forest’. Following increases in the historically dominant species, eastern redcedar expanded throughout the uplands, along with ashes, elms, and red maple, ultimately increasing relative to oak species. Hussey (1876, p. 37) documented, ‘the very numerous ravines, valleys, and hillsides become covered with tree growth first. The large tulip trees, hemlocks, sugar maples, beeches, and chestnuts found in these less exposed localities, prove that generations of tree growth have passed since their seeds were scattered here; but the uplands show, that long since the deep valleys and hillsides were covered with forest growth, these were almost or entirely bare.’ Land Between the Lakes National Recreation Area contained the greatest oak percentage during the historical assessment, at 74% of all trees, and while oaks have decreased by 24 percentage points in current surveys, oak composition remains significant relative to encroaching tree species. Land Between the Lakes National Recreation Area may represent changes that occurred in parts of the Interior region between Euro-American settlement and the historical assessment during the early 1900s.

The Loess Hills region is the smallest of the regions, located in western Kentucky. Historical vegetation comprised localized floodplain ecosystems on the very western margin and upland forested ecosystems, dominated by grasslands, oak savannas, and woodlands, with extensive areas of cane (Arundinaria gigantea; Bryant and Martin, 1988; Bryant, 1993). Monotypic stands of cane, a perennial grass, were known as canebrakes, and the Loess Hills region was historically called the Cane Hills (Shull, 1921). Loess, or wind-blown soil, hills of 30 to 60 m in height stretch along the east side of the Mississippi River floodplain from Louisiana to Kentucky, confined to the western quarter of the region (Bryant, 1993). The loess hills thin and flatten on the eastern side of the Loess Hills region (Bryant, 1993). Therefore, the majority of the region had conditions that favored fire spread and the maintenance of grasslands and savannas, similar to the adjacent Interior region (Bryant, 1993).

By the time of the historical assessment during 1907–1909, oaks represented 49% of all trees, and oaks and hickories together accounted for 55% of all trees, indicating the initiation of fire exclusion and the replacement of fire-tolerant oaks by fire-sensitive tree species, compared to the land survey of 1820 (Bryant and Martin, 1988). Oaks made up 61% of all trees, and oaks and hickories collectively comprised 72% of all trees during the Kentucky land survey of 1820. Agricultural area ranged from 7 to 19% in all counties of the Loess region during 1850, but by 1880, it ranged from 25 to 50% in all counties. Over the 30 years between approximately 1850 and 1880, grasslands and savannas maintained by annual fires became covered by the dominant oak species as burning ceased with Euro-American settlement (DeFriese, 1880; Loughridge, 1888). Oaks were likely released throughout the region, but with disproportionate increases by colonizing sweetgum and elms, which were a more moderate component of forests during the 1820 survey. The eponymous canes of the Cane Hills were replaced by other vegetation, as occurred throughout the southeastern U. S., accelerated by excessive disturbances from livestock (Shull, 1921).

Current forests were reminiscent of floodplain and upland forests and agriculturally disturbed areas of the adjacent Lower Mississippi River Alluvial Valley, rather than the historically dominant oak open ecosystems of this region. The current forests are comprised of abundant elms, ashes, and maples, many of which were species that historically displayed spatial separation in protected lowlands and then expanded into upland old fields after fire exclusion, due to their colonizing traits of rapid early growth and abundant seed production (Defebaugh, 1906; Hanberry et al., 2012b). Specifically, the most abundant current species are green ash (a colonizing species associated with wetlands), sweetgum (a colonizing species historically associated with coastal and floodplain forests, particularly in the Lower Mississippi Alluvial Valley), winged elm (Ulmus alata; a colonizing species in old fields and agricultural areas), southern red oak (Quercus falcata), red maple (a colonizing species from wetlands into disturbed areas), silver maple (Acer saccharinum; a colonizing species in floodplains), shagbark hickory (Carya ovata; a colonizing species in old fields), baldcypress (found in floodplains), and American elm (U. americana; found in floodplains). Sweetgum, combined with blackgum, was most abundant during the 1907–1909 assessment compared to the 1820 survey or modern surveys. Given about a century between each survey, sweetgum may have increased first, particularly due to colonization from the Lower Mississippi River Alluvial Valley and other rivers bordering the Loess region (Hanberry et al., 2012b), and then, in turn, sweetgum was disproportionately displaced by ashes, elms, and maples. Loblolly pine was a new species in current surveys, arising from plantations; however, loblolly pine is also a colonizing species, historically confined to wetlands, that expanded into upland old fields following fire exclusion (Hanberry et al., 2023). Although all the current forests in the three regions differ from historical forests in different ways, the change from fire-tolerant species and ecosystems to floodplain species in upland forests for this region may exhibit the greatest departure in terms of turnover in species traits, which historically were restricted to zones encompassing a gradient from fire exposure to flood exposure.