Summary 1Treeline 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.

The mid-Holocene (ca. 8000–4000 cal yr BP) was a time of marked aridity throughout much of Minnesota, and the changes due to mid-Holocene aridity are seen as an analog for future responses to global warming. In this study, we compare the transition into (ca. 9000–7000 yr ago) and out of (ca. 5000–2500 yr ago) the mid-Holocene (MH) period at Kimble Pond and Sharkey Lake, located along the prairie forest ecotone in south-central Minnesota, using high resolution (∼ 5–36 yr) sampling of pollen, charcoal, sediment magnetic and loss-on-ignition properties. Changes in vegetation were asymmetrical with increasing aridity being marked by a pronounced shift from woodland/forest-dominated landscape to a more open mix of grassland and woodland/savanna. In contrast, at the end of the MH, grassland remained an important component of the landscape despite increasing effective moisture, and high charcoal influxes (median 2.7–4.0 vs. 0.6–1.7 mm2 cm− 2 yr− 1 at start of MH) suggest the role of fire in limiting woodland expansion. Asymmetric vegetation responses, variation among and within proxies, and the near-absence of fire today suggest caution in using changes associated with mid-Holocene aridity at the prairie forest boundary as an analog for future responses to global warming.