Late-glacial and Holocene climatic effects on fire and vegetation dynamics at the prairie-forest ecotone in south-central Minnesota

1 Treeline ecotones, such as the prairie - forest boundary, represent climatically sensitive regions where the relative abundance of vegetation types is controlled by complex interactions between climate and local factors. Responses of vegetation and fire to climate change may be tightly linked as a result of strong feedbacks among fuel production, vegetation structure and fire frequency/ severity, but the importance of these feedbacks for controlling the stability of this ecotone is unclear. 2 In this study, we examined the prairie - forest ecotone in south-central Minnesota using two lake sediment cores to reconstruct independent records of climate, vegetation and fire over the past 12 500 years. Using pollen, charcoal, sediment magnetic analyses and LOI properties, we investigated whether fires were controlled directly by climate or indirectly by fuel production. 3 Sediment magnetic and LOI data suggest four broad climatic periods occurring c. 11 350 - 8250 BP ( cool / humid), c. 8250 - 4250 BP (warm/dry), c. 4250 - 2450 BP (warm/ humid), and c. 2450 - 0 BP (cool/humid), indicating that, since the mid-Holocene, climate has shifted towards wetter conditions favouring greater in-lake production and fuel production on the landscape. 4 The area surrounding both lakes was characterized by boreal forest c. 12 500 10 000 BP, changing to an Ulmus-Ostrya forest c. 10 000 - 9000 BP, changing to a community dominated by prairie (Poaceae-Ambrosia-Artemisia) and deciduous forest taxa c. 8000 - 4250 BP, and finally shifting to a Quercus - dominated woodland/savanna beginning c. 4250 - 3000 BP. 5 Charcoal influx increased from an average of 0.11 - 0.62 mm(2) cm(-2) year(-1) during the early Holocene forest period (c. 11 350 - 8250 BP) to 1.71 - 3.36 mm(2) cm(-2) year(-1) during the period of prairie expansion ( c. 8250 - 4250 BP) and again increased to 4.18 - 4.90 mm 2 cm - 2 year - 1 at the start of the woodland/savanna period ( c. 4250 BP). 6 As a result of the influence of climate on community composition and fuel productivity, changes in fire severity may be the result and not the cause of shifts in vegetation.