Transition from Fire-Dependent Open Forests: Alternative Ecosystem States in the Southeastern United States

Abstract

Land use and fire exclusion have influenced ecosystems worldwide, resulting in alternative ecosystem states. Here, I provide two examples from the southeastern United States of fire-dependent open pine and pine-oak forest loss and examine dynamics of the replacement forests, given continued long-term declines in foundation longleaf (Pinus palustris) and shortleaf (Pinus echinata) pines and recent increases in commercial loblolly (Pinus taeda) and slash (Pinus elliottii var. elliottii) pines. Shortleaf pine-oak forest historically may have been dominant on about 32 to 38 million ha, a provisional estimate based on historical composition of 75% of all trees, and has decreased to about 2.5 million ha currently; shortleaf pine now is 3% of all trees in the northern province. Longleaf pine forest decreased from about 30 million ha, totaling 75% of all trees, to 1.3 million ha and 3% of all trees in contemporary forests of the southern province. The initial transition from open pine ecosystems to closed forests, primarily comprised of broadleaf species, was countered by conversion to loblolly and slash pine plantations. Loblolly pine now accounts for 37% of all trees. Loss of fire-dependent ecosystems and their foundation tree species affect associated biodiversity, or the species that succeed under fire disturbance.

Westward Expansion by Juniperus virginiana of the Eastern United States and Intersection with Western Juniperus Species in a Novel Assemblage

“…Intersection of eastern and western junipers may represent the first new, viable assemblage during recent millennia in the United States. Although native tree species have changed in relative composition and become less spatially differentiated within regions during the past century or two due to land use change, including fire exclusion, native tree species shared at least limited spatial overlap within regions [46]. However, association of an eastern tree species with western species is novel, given that fires in the Great Plains grasslands historically prevented tree expansion and intermixing during the past 10,000 or more years [47,48].…”

“…Otherwise, eastern redcedar already would have expanded, rather than starting during the late 1800s [12]. In the eastern and central U.S., fire exclusion may be the main factor driving tree expansion and densification [2,6,7,42,43,46,48,49,[54][55][56]. Fire exclusion, as opposed to fire suppression, incorporates a land use change component through reduction in herbaceous fine fuels by agriculture and intensive herbivory or high tree densities and reduction in fire spread by landscape fragmentation, particularly linear barriers of roads and trails.…”

Documenting Two Centuries of Change in Longleaf Pine (Pinus palustris) Forests of the Coastal Plain Province, Southeastern USA

“…Due to widespread implementation of effective fire exclusion across much of longleaf pine range by the 1930s and 1940s [102], open longleaf pine forests became vulnerable to replacement by high densities of more fire-sensitive tree species, which emerged from fire-protected sites to compete for growing space [9,32,55,102,114]. Not surprisingly, loblolly and some broadleaf species, such as sweetgum, rapidly colonized abandoned agricultural lands across the Coastal Plain that had once been longleaf pine [16,115,116], resulting in extensive pine, broadleaf, and mixedwood forests with very few, if any, longleaf forests [114,117].…”

Comparison of seed bank composition over a gradient of pyrophilic vegetation1

“…In eastern forests of the US, this shift from pyrophilic to pyrophobic vegetation is also called mesophication, a positive feedback cycle in which the removal of fire has resulted in a landscape that is increasingly fireproof and less amenable to either the restoration of fire regimes or the maintenance of historically pyrophilic, often Pinus or Quercus-dominated, forests (Nowacki and Abrams 2008;Caldwell et al 2016;Varner et al 2016;Palus et al 2018;Alexander et al 2021;Hanberry 2021). The impacts of mesophication on soil seed banks in US eastern forests are underinvestigated, with a few studies that focus on the return of fire to the landscape to restore dominant tree species (Brose et al 2008;Iverson et al 2008).…”

“…2018; Alexander et al . 2021; Hanberry 2021). The impacts of mesophication on soil seed banks in US eastern forests are under-investigated, with a few studies that focus on the return of fire to the landscape to restore dominant tree species (Brose et al .…”