Effects of Holocene climate change on the C₄ grassland/woodland boundary in the Northern Plains, USA

To determine how grassland, woodland, and bordering forests respond to increased aridity, we used paleoecological methods to examine past responses along a transect of three sites at the eastern boundary of the Northern Plains of North America. Our study region corresponds to the confluence of three air streams that control central North American climates and, hence, should be sensitive to climate change. Sediment cores were analyzed for evidence of Holocene vegetation and fire from tall-grass prairie in eastern North Dakota (Moon Lake), from mixed forest near the prairie border in northwestern Minnesota (Deming Lake), and from mixed forest more remote from prairie in western Wisconsin (Dark Lake). Together with pollen and charcoal analysis, we present a new method for determining δ¹³C of terrestrial (charred) organic matter and, thus, the relative importance of C₃ and C₄ photosynthetic pathways in past vegetation. Paleorecords were supplemented with surface charcoal accumulation and δ¹³C from 21 North American lakes that span boreal, deciduous, pine, and mixed forest to tall- and mixed-grass prairie. Surface charcoal and δ¹³C follow vegetation and climate gradients, with high charcoal accumulation and δ¹³C (-20‰) in the Plains (Dakotas, Nebraska, and southwest Minnesota) and decreases to the east, west, and north. The δ¹³C pattern is consistent with observed patterns of C₃:C₄ dominance across the region. Sediment, pollen, charcoal, and terrestrial δ¹³C show that vegetation response to climate change varied substantially among tall-grass prairie, bordering woodland, and forest. During maximum aridity (8000-4000 yr BP) prairie vegetation in eastern North Dakota showed a demise of woody vegetation followed by a fluctuating dominance of grasses (40% C₄) and forbs. Meanwhile, prairie expanded eastward into northwestern Minnesota, where it produced a shifting dominance between mostly C₄ grasses and woody vegetation until more humid conditions and mixed forest developed after 4000 yr BP. Mixed forest in southwestern Wisconsin showed little response to mid-Holocene aridity. Elevated δ¹³C values from 5000 to 3000 yr BP suggest that composition of grasses changed (to increased C₄), although pollen data indicate that the total abundance of grasses remained constant. The increase in C₄ grasses at this time is consistent with previous studies suggesting a delayed dry interval in eastern Iowa. Reduced aridity of the last 2000 yr brought increased fire to tall-grass prairie as higher primary productivity led to increased fuel load. Meanwhile, forest expanded in northwestern Minnesota, leading to decreased ignition and fine fuels, in turn resulting in decreased fire at the woodland margin.