Document: JAM-3-66-8

Drought cycles and prairie responses with past aridity in the northern Plains.

CLARK, J.S.* ¹, E.GRIMM ², J.DONOVAN ³, S.FRITZ ⁴ and D.ENGSTROM ⁵

Duke University, Durham, NC 27708 USA ¹
Illinois State University, Normal, IL 61790 USA ²
University of West Virginia, WV USA ³
University of Nebraska, NE USA ⁴
University of Minnesota, MN USA ⁵

Abstract:
Global warming may increase aridity in continental interiors, such as the Great Plains in North America. Because ecosystem response depends on the uncertain seasonal distribution of rainfall and fire, we have limited ability to anticipate ecosystem responses to future global warming. To determine potential sensitivity of mixed-grass prairie to conditions more arid than today we used paleoecological methods to analyze responses to arid conditions 7000 yr ago. Stratified sediments from Kettle Lake, ND showed well-defined 80 to 100 yr cycles of alternately arid and humid conditions that affected all ecosystem indicators we analyzed from sediment geochemisty, fossil pollen, and charcoal. As opposed to forested communities, the herbaceous grassland vegetation of the Great Plains is responsive to high frequency variations in precipitation, at annual to decadal scales, both in terms of biomass and species composition. Prairie fires are biomass dependent, and fossil charcoal is a proxy for biomass. During each arid phase, increased erosion caused quartz to increase from 10 to 40%. Grass pollen and charcoal were low during arid phases (< 10% and < 1 mm2cm^-2yr^-1, respectively) and then increased with declining quartz and gypsum as a result of increased production of grasses and, thus, fuels. ¹³C of terrestrial organic matter showed that, although both C₃ and C₄ grasses responded to drought cycles, a lasting shift to increased C₃ grasses occurred during one drought cycle. The diatom data indicate that changes in lake level and lakewater salinity are not always coupled to the changes in the terrestrial record and thus suggest that the hydrologic system has different thresholds and response times. Taken together, our results indicate that increased aridity need not produce uniformly dry conditions, but rather can produce periodic variation at time scales (102 yr) not apparent from contemporary climate ecords. Increased erosion and great reduction of grass pollen and charcoal during each arid phase suggests high ecosystem sensitivity to climate variation.

Keywords: climate change,prairie,fossil pollen,diatoms,geochemistry,drought cycles

This abstract is being presented at: 1:45 PM in session:
Oral Session #66: Large Scale Climate Change.