Effects of holocene climate change on the C-4 grassland/woodland boundary in the Northern Plains, USA

To determine how grassland, woodland, and bordering forests respond to incr eased aridity, we used paleoecological methods to examine past responses al ong a transect of three sites at the eastern boundary of the Northern Plain s of North America. Our study region corresponds to the confluence of three air streams that control central North American climates and, hence, shoul d be sensitive to climate change. Sediment cores were analyzed for evidence of Holocene vegetation and fire from tall-grass prairie in eastern North D akota (Moon Lake), from mixed forest near the prairie border in northwester n Minnesota (Deming Lake), and from mixed forest more remote from prairie i n western Wisconsin (Dark Lake). Together with pollen and charcoal analysis , we present a new method for determining delta C-13 of terrestrial (charre d) organic matter and, thus, the relative importance of C-3 and C-4 photosy nthetic pathways in past vegetation. Paleorecords were supplemented with su rface charcoal accumulation and delta C-13 from 21 North American lakes tha t span boreal, deciduous, pine, and mixed forest to tall- and mixed-grass p rairie. Surface charcoal and delta C-13 follow vegetation and climate gradients, wi th high charcoal accumulation and delta C-13 (-20 parts per thousand) in th e Plains (Dakotas, Nebraska, and southwest Minnesota) and decreases to the east, west, and north. The delta C-13 pattern is consistent with observed p atterns of C-3:C-4 dominance across the region. Sediment, pollen, charcoal, and terrestrial delta C-13 show that vegetation response to climate change varied substantially among tall-grass prairie, bordering woodland, and for est. During maximum aridity (8000-4000 yr BP) prairie vegetation in eastern North Dakota showed a demise of woody vegetation followed by a dfluctuatin g dominance of grasses (40% C-4) and forbs. Meanwhile, prairie expanded eas tward into northwestern Minnesota, where it produced a shifting dominance b etween mostly C-4 grasses and woody vegetation until more humid conditions and mixed forest developed after 4000 yr BP Mixed forest in southwestern Wi sconsin showed little response to mid-Holocene aridity. Elevated delta C-13 values from 5000 to 3000 yr BP suggest that composition of grasses changed (to increased C-4), although pollen data indicate that the total abundance of grasses remained constant. The increase in C-4 grasses at this time is consistent with previous studies suggesting a delayed dry interval in easte rn Iowa. Reduced aridity of the last 2000 yr brought increased fire to tall -grass prairie as higher primary productivity led to increased fuel load. M eanwhile, forest expanded in northwestern Minnesota, leading to decreased i gnition and fine fuels, in turn resulting in decreased fire at the woodland margin.