History In The Margins: Ecotonal Dynamics and Abrupt Vegetation Change In Holocene Lower Michigan

Abstract

Abrupt changes in ecological systems are expected to become more frequent in the near future, as a result of on-going climate change and increases in both the frequency and intensity of disturbance and extreme climate events. Fossil pollen records from the late Quaternary have offered crucial insights into the mechanisms of abrupt ecological change, which in eastern North America has particularly focused on the mid- to late-Holocene declines of mesic tree populations such as American beech (Fagus grandifollia) and Eastern hemlock (Tsuga canadensis). Some research has suggested that ecotonal populations are more sensitive to changes in climate and disturbance, although investigation of this claim in the paleoecological record has been sparse and with mixed support. This thesis maps the shifting position of the Michigan Tension Zone (MTZ), an ecotone separating southern deciduous forest from northern mixed-pine forest, in Lower Michigan and tests whether ecotonal position is a predictor of past community variability. The data analyzed include a new pollen record from Sunrise Lake, Michigan, just north of the MTZ, and 19 extant pollen records from the Neotoma Paleoecology Database. The placement of the ecotone and its movements during the Holocene was determined through a combination of non-metric multidimensional scaling (NMDS) and empirical Bayesian kriging (EBK) was employed to visualize millennial-scale movements of the MTZ.

The pollen record at Sunrise Lake showed an expansion of Tsuga and Fagus populations at 6.7 ka BP (thousand years before present) and 6.2 ka BP, respectively. An abrupt decline in Tsuga abundance was recorded after 5.1 ka BP, but while Fagus exhibited high variability with a declining trend after 4.8 ka BP, there was no signal of collapse in Fagus populations. Both populations generally began to recover after 2.4 ka BP, before experiencing late-Holocene declines after 1 ka BP (Tsuga) and 800 years BP (Fagus). Timing of these events was determined via calibration of 16 14C dates, including a basal date of 10.9 ka BP, plus a core top (-70 years BP) and Ambrosia rise (80 years BP) indicating Euroamerican settlement-era disturbance.

NMDS Axis 2 clusters southern hardwoods from northern mixed-forest, indicating a latitudinal signal which closely mirrors MTZ position, while Axis 1 captures the decline of spruce (Picea) woodlands at the Pleistocene-Holocene transition, Axis 3 captures the rise of Ambrosia-dominated assemblages after Euro-American settlement and logging, and Axis 4 discriminates hydroclimate sensitivities of tree taxa. Regional analysis following these ordination results determined that the MTZ shifted over 150 km between its southern- and northernmost extents, from 11-10 ka BP to 7-6 ka BP respectively, before moving south again to reach its current position roughly 30 km north of its establishment after 2 ka BP. Rates of compositional change were on average highest near the MTZ, upholding the predictions of ecotonal theory. However, maximum rates of change were more likely to be observed at sites of intermediate distances (100-150 km) from the MTZ. These results thus provide the most detailed analyses yet of the Holocene shifts of the MTZ, while also confirming expectations that ecotonal communities may be particularly sensitive to past and contemporary climate-driven change. However, the interacting effects of climate change, disturbance regime, and vegetation composition may cause some non-ecotonal populations to have particularly high rates of change.