Douglas G Goodin Raymond C Smith

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 iso-topic 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 cooler-than-modern summers, and back to warm again. All the authors in this section pro vide 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). Rapid warming occurred about 15 kyrs ago at the termination of the glacial period, and the lake remained until about 8.3 kyrs ago when the ice shelf retreated and the lake drained, leaving smaller lakes in low spots along the valley floor. The former large lake provided nutrients to the soils, and the drawdown of the smaller lakes 1000 years ago concentrated nutrients into pools on which the current ecosystem depends. It is because of the nutrient-poor and energy-limited environment of this polar desert that the past concentrations of nutrients play such a dominant role in the current structure and function of the ecosystem. In addition, the sensitivity of this system to the presence or absence of liquid water and the nonlinear response to changes in temperature near the melting point of water create a system where small changes in climate produce large variations in ecosystem response.

Based on paleoclimatic reconstruction, Monger identifies 9 intervals of climate variability at the Jornada LTER site over the past 20,000 years. His inferences are couched in terms of relative abundance of vegetation by life-form type (i.e., C4 grassland, C3 woodland, C3 shrubland). In general, his reconstruction shows a trend toward increased abundance of C3 shrubs, displacing C4 grasses and C3 woodlands. This trend represents a general increase in aridity consistent with regional changes in climate from the close of the Pleistocene. Although the trend is toward increased aridity, the changes are not monotonic. Particularly evident is the hot, dry Altither-mal period (see figure 17.7). This climatic period, culminating about 6000 years b.p., was characterized by an expansion of grasslands in North America well east of their current ranges. Causes for the Altithermal warming are not entirely clear, but probably represent the additive effects of return to preglacial atmospheric CO2 levels and solar radiation fluxes higher than their modern values (Kutzbach et al. 1996). Forcing by Milankovitch mechanisms has also been suggested as a cause for Altithermal warming (Kutzbach and Street-Perrott 1985; Gillespie et al. 1983). These changes probably weakened the North American monsoon circulation, causing reduction in precipitation and a shift toward predominant zonal westerly flows. Decreased precipitation and increased temperature signals show clearly in the Monger's reconstruction as a decline in lake level (or lake disappearance) and a marked increase in shrubland. Climate intervals 8 and 9 show a recovery of conditions from their mid-Holocene state, but they are still periods of aridity. Historical records for period 9 (since 1850) show a progressive increase of shrubland and a loss of grassland, consistent with continued postglacial aridity. This interval is strongly influenced by human activities, thus complicating the determination of cause and effect.

Using fossil insect records, Elias (table 18.1) provides a temperature reconstruction and vegetation history since the LGM. During the Holocene the Colorado

Front Range (NWT) has experienced a series of climatic fluctuations that have shifted glacial margins and biotic communities. Elias's predictions based on fossil beetle data agree well with a reconstruction of solar radiation based on Mi-lankovitch insolation models (Berger 1978) at millennial scales. However, conflicting interpretations of insect, pollen, and archaeological data during the mid-Holocene suggest the need for additional regional studies. Elias notes that glacial ice has been the dominant force in shaping alpine landscapes, with postglacial communities limited to those able to survive and become reestablished after deglaciation. Elias (chapter 18, p. 466) suggests that "the current group of species in the alpine ecosystem may not be the best fit for the environment, they are simply the best fit among those species able to persist regionally through the last glacial cycle." Elias also notes that the response of major components of vegetation in high-altitude ecosystems may lag behind major climatic changes.

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