One of the challenges researchers face in reconstructing vegetation history is understanding why vegetation changes occur. Climate changes can be inferred from evidence of vegetation changes such as fossil pollen records, but these records do not prove that climate changes alone cause vegetation to change and cannot serve as independent climate proxies (Calcote et al. 2020). Finding alternative reference points for climate would be especially useful in the Upper Midwest, where changes in forest structure are well documented but drivers of vegetative change are not entirely understood (Calcote et al. 2020). In their study, Calcote et al. compare lake level data from sediment cores against fossil pollen records to determine if lake sediment can serve as a valid independent proxy for climate and moisture levels in the Northern Wisconsin Sand Plain.
Figure 1a. Mesic and xeric tree species found at Cheney Lake. Images courtesy Peter M. Dziuk.
Figure 1b. Cheney Lake (the white lines represent past shorelines) (Calcote et al. 2020).
Figure 2. Fossil pollen data from one of the sediment cores (from Calcote et al. 2020).
Figure 3. LOI and magnetic susceptibility data from one of the sediment cores (from Calcote et al. 2020).
The Northern Wisconsin Sand Plain (NWSP) is an area in Wisconsin that features forests with a mixture of coniferous and broadleaf trees growing in poor, sandy soils. The vegetation makeup of the NWSP responds to changes in moisture, with xeric species such as jack pine (Pinus banksiana) and red pine (Pinus resinosa) increasing during drier conditions and mesic species such as white pine (Pinus strobus) and oak (Quercus) increasing during wetter conditions (Figure 1a). Vegetation varies with soil richness and abundance of fire according to location, with fire having an important stabilizing effect on jack pine populations, as Bjorn Larson writes in this blog post.
The authors took sediment cores from the research site, Cheney Lake, and analyzed fossil pollen of mesic and xeric species along charcoal data (Figure 2). To determine the suitability of Cheney Lake for climate records, they estimated the depth of the lake and its response to climate changes in the 20th century from photographs and past shorelines (Figure 1b). They then estimated lake level over time based on percent organic matter (through LOI), magnetic susceptibility, and 14C dating of sediment cores (Figure 3). Sediments dating from 7200 cal. years BPA to 2010 CE were studied; sections of sediment core were divided into zones based on composition and appearance, as shown in Figure 3.
The authors found that the lake level and pollen records did not match as closely as expected. Between 3300 and 1500 cal. yr BP, for example, the lake levels (and therefore moisture levels) at Cheney Lake were high, but the percentage of fossil pollen from mesic species did not increase, and fire frequency did not decrease as expected; there was also not as much organic matter in the sediment as expected. A drought beginning around 1500 cal. yr BP led to decreased lake levels, but xeric species’ pollen percentages did not increase and fire frequency did not increase. The authors theorize that a shift from mesic species to jack pine did not occur during the 1500 cal. yr B.P. drought at Cheney Lake because jack pines were already established there due to a lack of fire breaks. The most significant correlation at Cheney lake was a slight increase in white pine pollen at 500 cal. yr BP, when moisture increased.
Interestingly, at other locations such as Ferry Lake (the lake in Bjorn Larson’s blog post), vegetation followed the climate trends at Cheney Lake, with jack and red pine pollen percentages increasing and oak pollen percentages decreasing during drier periods. These differences between the locations could be due to differing soil composition or presence of fire breaks. Here, oak pollen percentages did not recover even after moisture levels increased after the 1500 cal. yr BP drought, which also suggests that jack pine populations remain stable after they have been established. Records of lake level in sediment were not reliable predictors of vegetation change at Cheney Lake, but they matched regional trends in climate, growing drier during the Medieval Climate Anomaly and growing wetter during the Little Ice Age. The drought at 1500 cal. yr B.P. was previously undocumented.
The study may have not been able to construct a cohesive narrative about vegetation history out of sediment records, but it did something just as valuable: it showed that lake level changes do not consistently correlate with vegetation changes, and that local variations can greatly affect vegetation composition. It also brought up new questions—the authors introduce theories, but it is not clear why the vegetation at Cheney Lake did not match the vegetation at other sites in the NWSP. As our understanding of vegetation history develops, we will learn how to better use proxies for climate to understand the drivers of vegetative change.
Works Cited
Calcote, R., Nevala-Plagemann, C., Lynch, E. A., & Hotchkiss, S. C. (2021). Late-Holocene climate changes linked to ecosystem shifts in the Northwest Wisconsin Sand Plain, USA. The Holocene, 31(3), 409-420.
Image sources (for Figure 1a)
https://www.minnesotawildflowers.info/tree/jack-pine
https://www.minnesotawildflowers.info/tree/northern-pin-oak
https://www.minnesotawildflowers.info/tree/red-pine
https://www.minnesotawildflowers.info/tree/white-pine
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