The influence of basin morphology and landscape position on the sensitivity of kettlehole ecosystems to peatland expansion
Recently it has been shown that hydroseral succession in kettlehole basins, particularly the initiation and lateral expansion of floating peat mats, occurs episodically in response to hydroclimate variability. Therefore, anticipated increases in hydroclimate variability during the coming decades and century will likely increase the likelihood of peatland expansion in kettlehole ecosystems, with implications for carbon accumulation and other ecosystem services. However, anticipating changes at local-to-regional scales requires an understanding of how site-specific characteristics, including basin morphology and position within a regional groundwater flow regime, influence sensitivity to climate-induced peatland expansion. For example, shallower portions of lakes should be more susceptible to peatland expansion, as should lakes higher in a watershed that receive less groundwater and experience higher water-level variability. To test the hypotheses that basin morphology and landscape position affect the timing and spatial pattern of peatland development within kettlehole ecosystems, we developed records of peatland history at five sites along a well-characterized groundwater flow gradient in northern Wisconsin. Three sites were high in the watershed and two sites were low in the watershed. Basin morphology was measured with probe rods and ground-penetrating radar, and over 40 sediment cores were collected from across basin-depth gradients. Analyses of organic content and plant macrofossils were used in combination with radiocarbon dating to identify spatiotemporal patterns of peatland inititation within each basin, and patterns were compared to underlying basin morphologies. Peatland developmental histories were then compared among sites high and low in the watershed.
Results provide additional support for the idea that hydroclimatic variability is an important driver of peatland establishment and expansion in kettlehole basins. Episodic pulses of terrestrialization were centered on 9,500 yr BP, ~5200 yr BP, ~4000 yr BP, ~3000 yr BP, and ~700 yr BP. In addition, some sites experienced paludification at 2000 yr BP. Within each basin, peatland established at similar times in areas of similar basin depth, leading to depth-dependent spatiotemporal patterns of peatland expansion that reflect millennial-scale changes in water balance. Although the timing of mid and late Holocene episodes of peatland establishment was similar at sites high and low in the watershed, only sites low in the watershed contained peatland in the early Holocene. Patterns suggest that basin depth is a good predictor of the likelihood of peatland expansion in response to future climate variability, although more work is needed to better understand if and how landscape position affects the probability of peatland establishment.