Hanberry et al., 2020 (Frontiers in Forests and Global Change)
RESEARCH BRIEF #39
A summary based on the following publication:
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MANAGEMENT IMPLICATIONS
Eastern US historical fire regimes were dominated by frequent, low-severity fires.
Regionwide fire exclusion implemented circa 1900 as a primary driver of changes in eastern US forest ecosystems provides a unifying explanation for increased tree density, species composition shifts, and tree encroachment.
Misplaced focus on the incorrect drivers of ecosystem change may lead to management inefficiencies and loss of biodiversity.
In this review, authors assessed four hypothesized drivers of widespread forest community changes (fire exclusion, precipitation, white-tailed deer densities, and loss of American chestnut (Castanea dentata)) which have been observed across the eastern US over the last century. Across this region, historically fire-maintained open oak and pine dominated woodlands and savannas with robust and diverse herbaceous groundlayers have transitioned to closed-canopy forests comprised of increased numbers and species of fire-sensitive trees with groundlayers depauperate of herbaceous vegetation. In addition to increased woody stem density and species composition changes in former woodlands and savannas, former grasslands and shrublands have been invaded by trees and transitioned to forests.
Fire exclusion
This hypothesis suggests that in frequent-fire regimes, fire-tolerant species preferentially survive relative to fire-sensitive species, tree density is limited by the top-killing of small trees, and a continuous bed of fine and flammable fuels (e.g., herbaceous vegetation, upland oak litter) would be promoted and create a positive fire-feedback loop. For this to be supported, it is expected that following fire suppression, fire-tolerant oaks and pines should decrease in composition and fire-sensitive species composition and tree densities should increase.
Increased precipitation
This hypothesis proposes that increased precipitation since the late 17th century is responsible for overall increased tree survival. In particular, that increased survival of drought-sensitive trees led to changes in structure and species composition of eastern US forests, and facilitated tree encroachment into formerly open areas. To be supported, it is expected that wetter landscapes would contain more drought-sensitive tree species, and drier landscapes would contain more drought-tolerant species. Additionally, it is expected that drought would kill drought-sensitive species and reduce overall tree densities. Conversely, periods of increased precipitation (pluvial) and decreased drought would cause increased survival of drought-sensitive species and increase tree densities.
Increased white-tailed deer densities
This hypothesis suggests that the near elimination of white-tailed deer in the eastern US in the late 1800s, and reductions of free-ranging pigs and cows, allowed for increased tree densities due to lack of browsing pressure. For this hypothesis to be supported, it is expected that in areas and times of reduced browsing pressure, increases in tree densities, in particular preferred browse species, would occur, and tree density would decrease with increasing deer density.
Loss of American chestnut
This hypothesis suggests that the sudden, widespread loss of American chestnut (caused by non-native chestnut blight (Cryphonectria parasitica)) led to major transitions to eastern US forests due to an abrupt increase in available growing space, which was then occupied by fire-sensitive tree species. For this hypothesis to be supported, it would be expected that the historical distribution and level of dominance of A. chestnut, would be similar to the spatial extent and degree of observed forest community transition.
Review findings
While the changes observed in eastern US forests initially appear to be connected to coincident changes in precipitation and deer density, the study authors concluded that fire and fire-exclusion offer the most parsimonious and consistent explanation to the shifts in forest density and composition observed since the early 1900s.
Climate has long been considered an explanatory factor for vegetation community patterns (e.g., ‘plant hardiness zones’). Though reconstructions of drought conditions for the past 500 years have detected increased occurrence of pluvials and decreased drought in the eastern US since the 1870s, analyses considering an expanded record of drought (1000 years) do not show drought conditions to have been statistically different during the 1900s. Additional contradicting evidence for this proposed driver includes that the shifts in tree species have followed along lines of fire-tolerance rather drought-tolerance traits (e.g., increased prevalence of drought-tolerant, fire-sensitive eastern red cedar (Juniperus virginiana)), and that tree density has increased, and coverage expanded, during periods of drought and increased precipitation.
While the last major pulse of oak recruitment and the first pulse in fire-sensitive species coincided with the lowest point of browsing pressure, the authors’ review casts doubt on its suitability as an explaining factor. Contradicting evidence includes that the densities of nearly all tree species increased regardless of browsing preference of tree species, that the current high levels of deer across the eastern US are not reducing forest densities or promoting increased levels of fire-tolerant tree species, and also that modeling studies have shown deer densities to not be related to tree stocking levels (amount of growing space occupied by trees).
Similarly, the loss of A. chestnut does not sufficiently or consistently explain the observed ecosystem changes. Primary among contradicting evidence is that the changes have occurred across the entire eastern US, while the range of A. chestnut was mostly along the Appalachian Mountain ranges. Additionally, oak species (especially Quercus alba and Q. montana) were prominent associates of A. chestnut and comprised a larger proportion of the forests, and therefore were better poised to occupy the growing space made available rather than fire-sensitive tree species.
Frequent and widespread wildland fire provides an efficient explanation for the open conditions of pre-Euro-settlement forests in the eastern US, which were dominated by fire-adapted tree species. Supporting evidence for the fire-exclusion driver hypothesis includes the continuous dominance of fire-tolerant vegetation communities (e.g., oak/pine woodlands) for thousands of years, including periods of different climate conditions (e.g., Medieval Warm Period (~950-1250 AD) and Little Ice Age (~1300-1850 AD)). Multiple lines of evidence (fire scar and charcoal records, written accounts) indicate that historical fire regimes across much of the eastern US were dominated by frequent, low-severity surface fires until fire-exclusion practices were implemented in the early 1900s. Similarly, the changes in tree density and species composition observed since the early 1900s are consistent with fire exclusion being a primary driver of increased tree density and shifts in species composition.
