Century to Millennial Timescale—Synthesis

At longer timescales, the interaction among climate, ecosystems, and the abiotic components of the environment become increasingly important. These relationships are apparent in the three chapters in part IV. Fountain and Lyons (chapter 16), examining the McMurdo Dry Valleys (MCM) ecosystem in Antarctic, provide an excellent example of a case where past climatic variations truly dictate current ecosystem status. The relatively large climate variations at MCM have concentrated nutrients that could not have been attained without this climate variability. Fountain and Lyons infer climate change from geomorphic evidence of past glacier positions and lake level heights as well as more recent isotopic results from ice cores and temperature measurements from boreholes. They focus on evidence from the most recent 60,000 years. Monger (chapter 17) provides an analysis of millennial-scale climate and ecosystem variability at the Jornada LTER site in southern New Mexico. Monger notes the difficulty of untangling prehistoric climate/ecosystem interactions, where researchers must rely on indirect proxy indicators in lieu of measured data. Monger analyzes a number of proxy data sources, including paleolake levels, plant remnants preserved in packrat middens, fossil pollens, carbon isotope ratios in paleosols, and erosion rates. Although noting the danger of circular reasoning in using proxy data (i.e., ecosystem response used to infer information about climatic change, which is in turn inferred from ecosystem response) Monger uses these data to construct a cogent picture of climate change at the Jornada site (JRN) since the Last Glacial Maximum (LGM) about 18,000–20,000 years b.p. Using remains of beetles, Elias (chapter 18) constructs a temperature history of the Colorado Alpine since the LGM. These late Holocene insect records show a progression from warmer-than-modern to coolerthan- modern summers, and back to warm again. All the authors in this section provide examples to show that it is at century to millennial timescales that ecosystems form, are broken apart and imprinted by the past, and reformed in new configurations. The McMurdo Dry Valleys is the most poleward-terrestrial ecosystem where streams, lakes, and soil are interconnected. In this polar desert, the biotic system must adopt a strategy to survive the winter in isolation, and the disturbance and formation of the landscape has been primarily dictated by climate and associated abiotic processes. During the last glacial period, the Ross Ice shelf entered Taylor Valley, damming the valley and forming a 200-m-deep lake (23.8 kyrs).