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

Abstract

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.

Climatic pacing of Mediterranean fire histories from lake sedimentary microcharcoal

“…In this biome, wildfires appear to have been fuel-limited throughout the period since the LGM. This was also true in Iberia during the Last Glacial period (Daniau et al, 2007) and in the North American prairie where clear links have been identified between phases of greater humidity promoting vegetation growth, increasing fuel load and fire occurrence (Brown et al, 2005;Camill et al, 2003). This relationship stands in contrast to that in better watered and more wooded environments in Mediterranean regions, such as Italy, where microcharcoal records indicate that landscape burning was most frequent during climatically arid phases of the Holocene, showing that here wildfires were not fuel-limited (Sadori and Giardini, 2007;Vannière et al, 2008).…”

Wildfire responses to abrupt climate change in North America

“…In BC (i.e., at sites 5, 6, 7, 8, and 10), an inverse relationship in fire and fuels is apparent because biomass burning increased as closed mixed conifer forests were replaced by more open forests (44). Charcoal influx is often opposite to AP in the Midwest as well, where grass abundance (low woody biomass) is a good predictor of biomass burning (45). Important changes in woody fuel levels in AK are obscured in the AP trends, because AP does not show changes in the relative importance of shrubs versus trees.…”

Floodplain sedimentation in the Upper Mississippi Valley: Natural versus human accelerated

“…Although modest shifts in the Holocene mean location of the ecotone have been inferred, studies of Holocene pollen from area lakes and bogs imply little net change in the vegetation cover of the local study region in southwestern Wisconsin and northwestern Illinois (Davis, 1977;Maher, 1982;Bartlein et al, 1984;Baker et al, 1992). Somewhat more extensive changes in local vegetation cover have been reported for lower relief, glaciated landscapes on the western margin of the ecotone in northeast Iowa and southeastern Minnesota (Baker et al, 2002;Camill et al, 2003), but hydrologic responses to Holocene environmental changes in the study area were mainly driven by direct effects of climate on precipitation and snowmelt rather than to changes in vegetation cover (Knox, 1985). Detailed descriptions of the mosaic of grassland and forest that covered the Zinc-Lead District and adjacent areas at the time of mining and agriculture settlement are presented in field maps and notes of the Federal Land Survey of the early 1830s and in other early maps and landscape sketches (Schockel, 1917;Owen, 1844Owen, , 1852Friis, 1969).…”