Modern analog interpretations of vegetation and climate from pollen in sediments.

摘要

This thesis includes development of methods for determining stand scale vegetation histories from fossil pollen (Chapters 1 & 2), as well as the application of similar methods to regional climate history (Chapter 3). The method of interpreting fossil pollen data in each case is the modern analog technique, which consists of comparing fossil pollen with modern pollen assemblages to find modern forests or climates similar to those that could have produced the fossil assemblages.; The goal of pollen analysis from forest hollows is to sample the vegetation on the scale of forest stands of a few hectares, the spatial scale meaningful to ecologists. Although I found that the pollen source area of forest hollows is considerably larger than earlier studies suggested, modern analogs and ratios of pollen types successfully identified stand types on the scale of a few hectares. The method has been used to show that hemlock and hardwood patches have distinct histories, with few instances of stands switching between types (Davis et al. 1998).; The ecological interpretation of stand scale changes requires an understanding of the regional climate in which they occur. Chapter 3 uses the modern analog method to determine mid-Holocene regional climate changes that accompanied the expansion of hemlock in the upper Midwest 6000 years ago and the subsequent hemlock decline (∼5500 years ago). Results suggest that the regional expansion of hemlock about 6000–5500 years ago resulted from a combination of increasing precipitation and increasing winter temperature at the end of a mild mid-Holocene warm period. At the time of the hemlock decline, annual precipitation stopped increasing and January temperatures decreased. A combination of January temperatures similar to modern values and the lower precipitation of the mid-Holocene may have stressed hemlock trees at the time of the hemlock decline or slowed their recovery after insect outbreaks. Rather than a single clear cause independent of climate, a combination of climate factors and insect outbreaks may have contributed to the hemlock decline.