Response of C ₃ and C ₄ plants to middle-Holocene climatic variation near the prairie–forest ecotone of Minnesota

Abstract

Paleorecords of the middle Holocene (MH) from the North American mid-continent can offer insights into ecological responses to pervasive drought that may accompany future climatic warming. We analyzed MH sediments from West Olaf Lake (WOL) and Steel Lake (SL) in Minnesota to examine the effects of warm͞dry climatic conditions on prairie-woodland ecosystems. Mineral composition and carbonate ␦ 18 O were used to determine climatic variations, whereas pollen assemblages, charcoal ␦ 13 C, and charcoal accumulation rates were used to reconstruct vegetation composition, C3 and C4 plant abundance, and fire. The ratio of aragonite͞calcite at WOL and ␦ 18 O at SL suggest that pronounced droughts occurred during the MH but that drought severity decreased with time. From charcoal ␦ 13 C data we estimated that the MH abundance of C4 plants averaged 50% at WOL and 43% at SL. At WOL C4 abundance was negatively correlated with aragonite͞calcite, suggesting that severe moisture deficits suppressed C4 plants in favor of weedy C3 plants (e.g., Ambrosia). As climate ameliorated C4 abundance increased (from Ϸ33 to 66%) at the expense of weedy species, enhancing fuel availability and fire occurrence. In contrast, farther east at SL where climate was cooler and wetter, C 4 abundance showed no correlation with ␦ 18 O-inferred aridity. Woody C 3 plants (e.g., Quercus) were more abundant, biomass flammability was lower, and fires were less important at SL than at WOL. Our results suggest that C 4 plants are adapted to warm͞dry climatic conditions, but not to extreme droughts, and that the fire regime is controlled by biomass-climate interactions. P ervasive drought, expected to be associated with climatic change in continental interiors, will have profound ecological, economic, and societal repercussions (1-4). Drought conditions are likely to alter ecosystem function by changing the relative abundance of plant functional groups (e.g., C 3 vs. C 4 ) in natural systems (5-7). For example, episodic droughts that occurred throughout the 20th century in the midwestern United States killed woody C 3 genera, such as Quercus (8-10), and favored better adapted C 3 and C 4 herbaceous species (9). General circulation models coupled with dynamic crop-growth models project that, in agricultural systems, such climatic conditions could significantly reduce both C 3 (e.g., wheat, Triticum aestivum L.) and C 4 (e.g., corn, Zea mays L.) cereal crop yields (11,12), resulting in billions of dollars of economic loss (13,14). Empirical evidence of the response of plant functional groups to climatic conditions characteristic of drought is therefore important for evaluating predictions of future change. However, such evidence is mostly limited to historical records (15, 16), which lack the full range of past drought variability (3), and to short-term experimental manipulations (17, 18), which lack a sufficient temporal dimension for understanding future vegetational response.During the middle Holocene (MH), Ϸ8.0-4.0 thousand years (ka) B.P. (19), the midweste...

Fire cycles in North American interior grasslands and their relation to prairie drought

“…Similarly, grass pollen and fuel loads were also positively correlated with moisture availability at Kettle Lake in the early Holocene, synchronously tracking short-term moisture availability (16). A comparable climate-fuel-fire relationship has also been observed at West Olaf Lake in Minnesota (14), where in the mid-Holocene, both charcoal and grass pollen increased concomitant with a long-term decline in moisture deficit. Reduced LOI carbonate content is contemporaneous with drought, signifying decreased groundwater input of dissolved Ca and alkalinity to the lake as well as increased clastic input associated with the loss of grass biomass and vegetation cover.…”

“…In North America, evidence of long-term climate-fire interaction is primarily from forested ecosystems such as eastern hardwoods (2,3), boreal forests (4,5), coastal interior and Rocky Mountain communities (6,7), and coastal temperate rainforests (8,9). Nonforested ecosystems have generally received less attention, and only a few long-term fire studies are available from chaparral scrublands (10) and continental grasslands (11)(12)(13)(14).…”

Linking abundances of the dung fungus Sporormiella to the density of bison: implications for assessing grazing by megaherbivores in palaeorecords

“…As one of the few megafaunal survivors of the late Quaternary extinctions in North America (Koch & Barnosky 2006), bison provide a rare opportunity to continuously study the interactions between vegetation, megaherbivores and climate variability since the last deglaciation. Several well-dated, high-resolution Great Plains lake sediment records show 100-160 year oscillations of drought variability during the early to middle Holocene (9.8-2.8 ka BP); these are recorded in the pollen record by alternating high Poaceae abundances during wet intervals and high Ambrosia abundances during dry intervals (Clark et al 2002;Nelson et al 2004;Grimm, Donovan & Brown 2011). When interpreted climatically, this pattern has been called the 'Ambrosia paradox' (Grimm 2001) because Ambrosia and other forbs are less able to compete with grasses for moisture (Fahnestock & Knapp 1994) and so would be expected to decline during dry periods.…”

Buried paleoindian-age landscapes in stream valleys of the central plains, USA

“…Strong evidence also exists for Holocene "megadroughts" of unprecedented severity and duration (hundreds of years), unlike any experienced by modern societies in North America (Laird et al, 1996;Woodhouse and Overpeck, 1998;Cook et al, 2004Cook et al, , 2007Miao et al, 2007). These protracted droughts caused major perturbations among plant communities in the midcontinent (Grimm, 2001;Clark et al, 2002;Nelson et al, 2004;Brown et al, 2005) and probably had considerable impact on prehistoric people in the Central Plains.…”