Reviewing Fire, Climate, Deer, and Foundation Species as Drivers of Historically Open Oak and Pine Forests and Transition to Closed Forests

Abstract

Historically open oak and pine savannas and woodlands have transitioned to closed forests comprised of increased numbers of tree species throughout the eastern United States. We reviewed evidence for and against a suite of previously postulated drivers of forest transition focused on (1) change in fire regimes, (2) increased precipitation, (3) increased white-tailed deer densities, and (4) loss of American chestnut. We found that fire and fire exclusion provide a parsimonious mechanism for historical dominance by open forests of fire-tolerant oak and/or pine species and subsequent transition to closed forests with fire-sensitive tree species that fill the vertical profile. Based on statistical tests, increased precipitation during the past century was within historical ranges and thus fails to provide an explanation for forest change; additionally, precipitation variability is incongruent with tree traits (i.e., both drought-tolerant and drought-intolerant species have increased and decreased) and patterns of tree establishment. Similarly, current deer densities fail to provide a statistical relationship to explain tree densities at regional scales, species trends are unrelated to deer browse preferences, and both historically open forests and contemporary closed forests contained high deer densities. Functional extinction of the American chestnut had localized impacts but chestnut was not abundant compared to oak or widespread enough in distribution to match forest transitions throughout the eastern United States. Although Euro-American settlement affected many processes, not all changes were consistent enough to cause transitions in forest composition and structure that generally trailed westward expansion by Euro-American settlers. Evidence about these drivers continues to mount and we recognize the need for further research and continual re-evaluation of drivers of historical forests and forest change due to importance for understanding and management of these ecosystems.

Revealing historical fire regimes of the Cumberland Plateau, USA, through remnant fire-scarred shortleaf pines (Pinus echinata Mill.)

“…At the bottom of the graph, a composite record of all fire-event years recorded at the site is shown. At the top, the box graph shows the number of trees in the record (blue line) and the percentages of trees scarred in fire years (vertical bars) understory (Hanberry et al 2020). Future fire-scar records that predate those reported here will be of particular value as they will help to better understand the causes and effects of fires in a period of lessened European influence and less-known ecosystem conditions.…”

Forest Disturbance Types and Current Analogs for Historical Disturbance-Independent Forests

“…The current disturbance regime of forestry favors species that reproduce vegetatively or produce prolific seeds for rapid establishment after overstory disturbance, and grow rapidly, including planted species such as red pine in Michigan and the Northern US and loblolly pine in the Southeastern US. Multiple eastern broadleaf species from the Central-Eastern US have expanded both north and south, not apparently due to climate change [15,62,63]. For example, in the Northern Lower Peninsula Michigan, severe, repetitive fires after logging at the turn to the 20th century that removed organic soil layers and cones first benefitted aspen, an established boreal species, resulting in a pulpwood industry, in which aspen stands are clear-cut on a rotation cycle of 30-60 years [2,4].…”

“…Future rates of change of forests in the Eastern US may not be likely to exceed rates of change that occurred during the exploitation era of Euro-American settlement and subsequent disequilibrium as species have been re-sorted by disturbance change. Moreover, tree dynamics respond slowly to climate change [65,66], which is why climate may not have a very apparent effect on tree distributions relative to direct disturbance [15,67]. Climate change is expected to shift forests in a poleward direction and expand southern forests, but replacement of established trees by more heat-adapted species will occur slowly due to requirements for propagule dispersal and successful competition for growing space after natural tree mortality or tree removal.…”

“…Insect herbivores and various pathogens are well-known for killing large trees. Although wind and herbivory alone do not appear to maintain unique forests at landscape scales in the Eastern United States [15], wind may be the primary disturbance of pine forests of tropical islands [16], and megaherbivores (Elephantidae) may maintain tropical savannas.…”

Reconsidering the fire ecology of the iconic American chestnut

“…While fire is an important process in many eastern North American forest ecosystems, debate on the role of fire in some deciduous forests persists (Matlack 2013, Stambaugh et al 2015, Hanberry et al 2020 a ). This uncertainty is largely due to the limited extent of existing studies and numerous gaps in our understanding, a circumstance particularly evident for forests containing the once widespread and iconic American chestnut, Castanea dentata .…”