Multiproxy, Cross-Biome Analysis Of Ecosystem Dynamics During Late-Glacial And Holocene Climatic Change In North-Central North America

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Conference Proceeding

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Vegetation ecotones and lake ecosystem dynamics have the potential to change dramatically with rapid climate warming. We present data for 15 proxies from eight well-dated lake sediment cores documenting late glacial and Holocene changes in both terrestrial and lake processes across a latitudinal gradient in central North America spanning grassland, aspen parkland, boreal, and tundra biomes. Our goal was to examine the timing and magnitude of terrestrial and aquatic ecosystem proxies across known climatic gradients in space and time. Results indicate that fire and vegetation dynamics were influenced by how climate controlled the relative abundance of arboreal vs. herbaceous taxa. Fire severity was greatest during the Holocene Thermal Maximum (HTM, 8500-5000 BP) only in forest-dominated boreal and northern parkland landscapes. At the grassland-woodland border and tundra-woodland ecotones, fire severity peaked after the HTM, presumably during more mesic conditions that supported greater landscape productivity. Lake ecosystems differed across the latitudinal gradient, with warmer grassland lakes showing a potential shift from diatoms to cyanobacteria following HTM aridity, P inputs, N:P (molar) declines to < 5-15, and N limitation, leading to poor or negative overall correlations among biogenic silica, nutrients, and organic matter. At the northernmost parkland and boreal and tundra sites, there was no indication from the pollen, magnetics, grain size, TP, or N:P data of significant mineral transport to these lakes or shifts in lake stoichiometry at or following the HTM, suggesting that aridity was less severe in higher latitudes. Unlike the grassland sites, which may have experienced a state change in the plankton community from diatoms to cyanobacteria as a result of HTM mineral inputs, cyanobacteria probably played a smaller role in the northernmost parkland, boreal, and tundra sites because the strong positive correlations between organic matter and bSi (P < 0.05) as well as low molar C:N ratios (9-12) suggest that lake organic matter is primarily derived from plankton and that diatoms have been the dominant primary producers over the Holocene. Our work shows that the magnitude of climate-driven changes in catchment processes varies latitudinally, with more southerly lakes subject to greater past aridity and nutrient-driven state changes compared to higher-latitude lakes. If precipitation and temperature increase to a greater extent at higher latitudes with climate warming, we may see differential changes in fire severity, terrestrial material export to lakes, and shifts in stoichiometry and nutrient limitation, thereby strengthening geographic differences in ecosystem function.


Presented at the 2010 Fall Meeting of the American Geophysical Union (AGU), San Francisco, CA.

Abstract Number B21A-0288.

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