Relationship to Framework Questions

The results discussed in this section clearly show the presence of climate variability at millennial timescales, although (as pointed out previously) they must be interpreted cautiously to avoid circular reasoning. Monger's results coincide with those of other paleoclimate analyses both in the U.S. Southwest (e.g., Hall and Scurlock 1991) and elsewhere (Gillespie et al. 1983). Elias's temperature reconstruction is consistent with Milankovitch forcing, but it differs in details from some other reconstructions. This may be, in part, because of the regional specificity of the Colorado Front Range. Evaluation of some of the other framework questions is complicated by both the nature and timescale of the changes considered here. Use of proxy data always involves inferences about the relationship between the proxies and the climate data they represent; the certainty of these relationships decreases as the inferences extend further into prehistoric time. Nevertheless, results in this section do fit into some of the framework questions. At the millennial timescale, the LGM is an important defining preexisting condition.

Fountain and Lyons show the dominant influence of preexisting conditions, in this case a paleolake and its subsequent contribution of nutrients and organic carbon to the structure and function of the current ecosystem. There are cascades at shorter timescales through the aquatic part of this polar desert ecosystem (Welsh et al., chapter 10) that are driven by factors influencing the presence of liquid water, but the legacy effects in this environment are on the order of thousands of years. Superimposed on this legacy is the nonlinear response at the melting point of ice, which is "at the heart of all observed changes." This melting transition point is critical to discussion of the flow of material and energy through, and the direction of evolution of, this system. A consideration of cycles within this context must take note of this critical transition point.

Monger's results showing changes in vegetation life-form accompanying climate change represents a cascade effect. Monger notes that climate changes can re sult in vertical reallocation of water by runoff, resulting in the increased availability of moisture downslope. Another cascade can be inferred in the activities of rock glaciers during the late Pleistocene that form new geomorphic surfaces on which new ecosystems develop. These results may be cyclical, but if so the cycle occurs over very long time periods, extending even beyond the millennial timescales considered in this section. The climate events in this case show little evidence of reversal, at least at the timescales considered here. Monger's analysis also hints at the importance of preexisting conditions in the dynamics of the arid ecosystem. Much of his analysis is presented in terms of effects relative to the local topography (i.e., piedmont vs. basin floor, see figure 17.5), suggesting that the "lie of the land" is a crucial influence in this climate/ecosystem. Similar climate change might result in a different outcome given some other geomorphic surface.

Elias draws particular attention to the possible lags between climate variability and the response of trees growing near tree line to changing temperature regimes. This is an important observation, particularly in light of the current rate of climate variability and efforts to understand and predict the response of forests to this relatively rapid change. He notes that the current group of species in the alpine zone consist of those able to survive glaciation and become reestablished in the alpine zone. These species are not necessarily the best "fit" among all possibilities; instead, they are the best fit among those species persisting through the last glacial cycle. Elias further states that present-day ecotones in alpine and subalpine ecosystems are not in equilibrium with the current climate, but are instead a relict of an earlier warm period. Both of these facts point to an important role for legacy effects in alpine climate/ecosystem interaction. If glaciation is viewed as a climate "disturbance," then Elias's findings also suggest that the climate/vegetation interaction does not return to its previous state (i.e., a hysteresis effect) when a climatic disturbance event is completed. The lag effect between climate variation, which often occurs abruptly, and ecosystem response, which lags in response, results in a system where feedback mechanisms associated with previous climate cycles might often overlap. Thus, simple correspondence between climate "event" and ecosystem response is not a suitable framework for analysis of this ecosystem at millennial timescales.

Martinson and coauthors (1998), in presenting a science plan for decade to century-scale climate variability and change, note that the paradigm used for the study of climate variability at seasonal to decadal timescales may not be applicable to decadal and longer timescales. Paleoclimate and historical records are often too short to apply the process of generating hypotheses and quickly evaluating them. Martinson et al. (1988) argue that making progress at these longer timescales will require improved and faster climate models, and expanded paleoclimate data bases. Understanding processes at these longer timescales is essential because it is at these timescales that, as Elias (p. 387) notes, "ecosystems form, break apart, and reform in new configurations." Also, Martinson et al. (1998) note that it is over these time periods that the life prospects of future generations are defined by climatic variability. They argue that informed stewardship of Earth's resources requires a sustained effort to understand processes on these longer timescales.

References

Berger, A. L. 1978. Long-term variations in caloric insolation resulting from the earth's orbital elements. Quaternary Research 9: 139-167.

Gillespie, R., F. A. Street-Perrott, and R. Switsur. 1983. Post-glacial arid episodes in Ethiopia have implications for climate prediction. Nature 306: 680-683.

Hall, D. O., and J. M. O. Scurlock. 1991. Climate change and productivity of natural grasslands. Annals of Botany 67: 49-55.

Kutzbach, J. E., G. Bonan, J. Foley, and S. P. Harrison. 1996. Vegetation and soil feedbacks on the response of the African monsoon to orbital forcing in the early to middle Holocene. Nature 384: 623-626.

Kutzbach, J. E., and F. A. Street-Perrott. 1985. Milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr BP. Nature 317: 130-134.

Martinson, D. G., K. Bryan, M. Ghil, M. M. Hall, T. R. Karl, E. S. Sarchik, S. Sorooshian, and L. D. Talley. 1998. Decade-to-century-scale climate variability and change: A science strategy. National Research Council, National Academy Press. Washington, D.C.

Part V

Climate Variability and Ecosystem Response at Selected LTER Sites at Multiple Timescales

This page intentionally left blank

0 0

Post a comment