A reconceptualization of open oak and pine ecosystems of eastern North America using a forest structure spectrum

Abstract

We present a reconceptualization of forests in eastern North America by differentiating the ecological characteristics of open oak (Quercus) and pine (Pinus) forests from closed successional and oldgrowth forests. Despite historical abundance of savannas and woodlands, the fundamental ecology of open forest ecosystems remains ill-defined when compared to either closed forests or grasslands. Open forests were characterized by simple internal stand structure consisting of a single stratum of variably spaced, often very old overstory trees and limited midstory, maintained by understory disturbance that controlled tree regeneration and allowed instead a taxonomically rich herbaceous groundlayer. In contrast, closed forests have dense woody growth throughout the vertical profile, limiting herbaceous plants. To provide further clarity about these ecosystems, we developed a canopy closure spectrum model dependent on the interactions between prevailing disturbance regimes of the historical and current eras, which affect either the tree understory (regeneration) or overstory, and tree traits of fire and shade tolerance. Recognition of different stand structures, disturbance regimes, and their interrelationships should expand understanding of open forests and limit ecological misunderstandings and restoration misapplications, thereby improving management of these once historically extensive ecosystems.

Rapid digitization to reclaim thematic maps of white-tailed deer density from 1982 and 2003 in the conterminous US

“…Since Euro-American settlement, comprehensive changes in vegetation have occurred. Generally, fire-tolerant open oak and pine forests with an herbaceous vegetation ground layer have transitioned to closed forests, comprised of diverse fire-sensitive tree species and tree layers throughout the vertical profile, typically replacing the herbaceous ground layer (Hanberry, Bragg & Hutchinson, 2018). The concurrence of increased tree recruitment and decreased deer populations during Euro-American settlement may suggest that relief from browsing pressure was influential in releasing tree growth because herbivores are potential drivers of vegetation structure.…”

“…Decreasing tree species include fire-tolerant oak and pines, some wetland species, and species that are affected by novel diseases perhaps in combination with forestry practices (Hanberry, Palik & He, 2013;Hanberry & Nowacki, 2016;Hanberry & Abrams, 2019). Although woody plants have benefitted from conditions during the past century, deer herbivory is an additional stressor on herbaceous plants that have become less abundant due to intense competition with woody species; with limited growing space, abundance of forbs and grasses necessarily decreases as tree densities increase (Hanberry, Bragg & Hutchinson, 2018).…”

Recent shifts in shade tolerance and disturbance traits in forests of the eastern United States

“…Fire exclusion enables fire-intolerant tree species to survive and recruit to sizes less susceptible to fire injury. As fire exclusion is prolonged and woody vegetation biomass accumulates, flammability decreases due to loss of fine fuels from the herbaceous layer and changing leaf litter of dominant species (while increasing the severity of future fires; Kreye et al 2013;Hanberry et al 2018a).…”

“…Succession in composition to more shade-tolerant species was rare in forests of the eastern USA before Euro-American settlement (1600s to 1800s; e.g., Hanberry et al 2018a). Excluding boreal forests, infrequent severe disturbance, generally by wind and fire, occurred on rotations of hundreds of years that were longer than tree lifespans; when stand-replacing disturbance occurred, canopy loss was diffuse (Seymour et al 2002).…”

Tree Mortality Following Mixed-Severity Prescribed Fire Dramatically Alters the Structure of a Developing Pinus taeda Forest on the Mid-Atlantic Coastal Plain

“…Fire exclusion has led to dramatic changes in the composition and structure of Eastern US forests, often with negative implications for species and habitat diversity [1][2][3]. Forest management, including the use of prescribed fire, that generates structural and compositional heterogeneity is advocated as a viable approach to promoting adaptation and resilience in the context of a changing climate [4][5][6].…”