Vegetative History of the Southeast from White Pond, SC

For the past month or so, I have been collecting plants around my home in Virginia. The collection mostly consists of herbaceous angiosperms, but I recently started to focus more on the trees nearby. I live in the western part of Virginia that goes into the Appalachian Mountains, so I mostly see a lot of the Pinus (pine), Quercus (oak), and Carya (hickory) taxa. This cool mixed forest community is common in the Appalachian range, but a recent paper by Krause et al. (2018) shows how areas in the Inner Coastal Plain of South Carolina (Fig. 1) once acted as a refugium for these communities in the late Pleistocene and early Holocene.

Because of this history of providing refuge for northern plant communities during glacial maximums, the Southeast United States is an interesting location for paleoecological studies. Additionally, the eastern US in general is known for the historic presence of no-analog communities and vegetative changes linked to larger climatic events in the North Atlantic. Looking at the changes in ecology over the past 30,000 years and using data from White Pond in South Carolina (Fig. 1), Krause et al. (2018) hoped to update and clarify information from previous studies of the Southeast that show a transition from boreal conifers like Pinus (such as P. strobus and P. rigida) and Picea (spruce) to temperate and subtropical hardwoods and conifers like Pinus (such as P. palustris and P, taeda), Quercus, and Carya.

Vegetative History from White Pond
Modern and fossilized pollen samples showed that the vegetation around White Pond consisted of three major zones, based on transitions between conifer (Pinus and Picea) and hardwood (Quercus and Carya) vegetation (Fig. 2). A no-analog community of high Picea and Fagus (beech) was also reported at 14,300 yr BP, but other than that no-analog events were rather weak at White Pond (Krause et al., 2018).

The authors used these reconstructions to determine how major climatic events during the full-glacial and subsequent deglacial period (21,000-11,700 yr BP) of the North Atlantic affected conditions in the Southeast (Fig. 3). Two of the major events of that time include the warm and wet Bølling-Allerød (14,700-12,900 yr BP) and the cold Younger Dryas (12,900-11,700 yr BP). Similar to reports from other locations in the Southeast, White Pond was anti-phased against these climate events, with cooling during the Bølling-Allerød and warming preceding and persisting well past the Younger Dryas (Krause et al., 2018). This temperature change coincided with the replacement of Pinus and Picea populations with Quercus and Carya, along with smaller populations of Ostrya/Carpinus (birch), Ulmus (elm), Fraxinus (ash), and Fagus between 19,700 and 12,000 yr BP.

In addition to pollen analysis, Krause et al. (2018) used macroscopic charcoal to determine the impact of fire on vegetation, especially at the onset of the Holocene at White Pond. Compared to the late Pleistocene, charcoal accumulation rates and the occurrence of high-severity fire events sharply increased during the Holocene, most likely due to warmer summers. Beyond fire activity, Krause et al. (2018) showed that the onset of the Holocene at White Pond was characterized by more drought-tolerant forests of Pinus and Quercus, and shallower, more eutrophic aquatic environments of Brasenia (Fig. 2)

Why does this matter?
Essentially, Krause et al. (2018) provided a much-needed update to previous data, giving us a better picture of the driving forces behind changes in vegetation in the Southeast and how the current temperate and subtropical forests developed. They used modern methods of pollen and charcoal analysis to show how climate and fire contributed to this development, with a focus on the vegetation around White Pond, which now consists primarily of Pinus in addition to Quercus and other mesic trees and shrubs (Fig. 1). 

What is interesting to me, however, is how their results differed from recent studies in other locations around the Southeast. Compared to records from Cupola Pond, Missouri (see Laruen Ellingsberg’s post on Cupola Pond), no-analogs were much weaker during the Bølling-Allerød and Younger Dryas (Fig. 3). Additionally, fire did not help in the development of no-analogs during deglaciation, which is in contrast to conditions near the Great Lakes. It is also interesting that, while climate conditions did change rather dramatically from the late Pleistocene to modern day, the Pinus taxa shows up dominantly in both contexts (Fig. 2). The presence of different pines over time is explained through evidence that northern and mid-Atlantic vegetation, such as the taxa I see in my plant collections, used the Southeast as a refugium during glacial maximums.

Reference

Krause, T. R., Russell, J. M., Zhang, R., Williams, J. W., & Jackson, S. T. (2018). Late Quaternary vegetation, climate, and fire history of the Southeast Atlantic Coastal Plain based on a 30,000-yr multi-proxy record from White, Pond, South Carolina, USA. Quaternary Research, 91(2), 861-880. https://doi.org/10.1017/qua.2018.95