Author: R. Scott Anderson
Fossil pollen has become a major tool used by paleobotanists and paleoecologists in assessing what an environment may have been like many hundreds, or more commonly thousands, of years ago. All of the common trees, shrubs and herbs in the region produce pollen, which carries the male genetic material of the species.
Once pollen is produced it travels from one plant to another either by means of the wind or by animals, primarily insects. One very precise strategy of pollen distribution occurs when pollen is collected by insects or other animals and is moved from one plant to another. However, most trees within the region produce and distribute pollen by the wind-pollinated strategy. This strategy means that a plant must produce a vast amount of pollen, allowing the wind to blanket the landscape with pollen, and hope that some of it reaches the female portion of another plant. This assures that a considerable amount of pollen falls on the landscape, on lakes and on streams. Because many tree species produce large amounts of pollen, pollen analysis is ideal for understanding the past composition of wind-pollinated vegetation communities such as woodlands and forests. The branch of science that studies fossil pollen is called palynology.
The chemical composition of pollen makes it among the most resistant organic materials known to decomposition. Given the proper environmental conditions pollen can be preserved for thousands of years in a variety of locations. On the Colorado Plateau, for example, fossil pollen has been extracted from the sediments at the bottom of lakes, bogs and wetlands, from alluvial sediments along streams, and from cave deposits. Fossil pollen has also been identified from packrat middens within the area. Studies using these pollen assemblages can help us understand the history of plant species and the variations of plant communities and climates through time. Fossil pollen has also been identified and analyzed from archaeological sites, and has helped us understand the types of food and building materials used by ancient Americans.
Depending upon the context paleoecologists have developed specific methods for working with fossil pollen. For instance, at the higher elevations of the Plateau, where pollen from local plants has been deposited in the sediments of lakes, scientists are able to extract columnar cores from the bottom of these lakes and then analyze them to speculate or reconstruct what the land cover of an area may have looked thousands of years ago. These cores are often horizontally stratified as new sediments are deposited into a lake or pond each year. Using radiocarbon dating techniques, paleobotanists and paleoecologists are able to date certain layers in the core. Having established an age for a layer, the scientist can then analyze the abundance and type of pollen that is preserved within the sediment. Similar methods allow us to analyze pollen from caves and alluvial sediments at lower elevations where lakes and wetlands are rare or non-existent.
The usefulness of fossil pollen lies in the fact that many plants have distinctively shaped pollen grains on a microscopic level, and scientists are easily able to distinguish, say, sagebrush pollen from spruce pollen. However, it is very difficult to delineate between some closely related species using fossil pollen. For instance, scientists can distinguish between pollen of ponderosa pine and pinyon pine, but it is much more difficult to distinguish between ponderosa pine and lodgepole pine. Because of the fact that most pollen can routinely be identified primarily to plant genus or family, fossil pollen is important for providing a generalized view of past vegetation. Of course, paleoecologists are constantly developing new methods to allow for greater detail in interpretation of past vegetation and climate.
Paleobotany and Paleoclimate of the Southern Colorado Plateau. The biota of the Colorado Plateau during the middle (50,000-27,500 B.P.) and late (27,500-14,000 B.P.) Wisconsin time periods was dramatically different from that seen today. Differences were primarily a result of major climate changes associated with the last major glacial period. This site examines the environment of the southern plateau during this time. Adapted by R. Scott Anderson from his journal article.
Packrat Midden Research in the Grand Canyon. On the Colorado Plateau the ice age (Pleistocene) vegetation of the Grand Canyon has been determined through the analysis of plant fossils preserved in caves and fossil packrat middens. Large changes occurred as the most recent ice age ended and the Holocene era began. Adapted by Kenneth L. Cole from his journal article.
Late Holocene Environmental Change in the Upper Gunnison Basin, Colorado. The Upper Gunnison Basin is a high elevation (3100 to 3600 m) region on the edge of the Colorado Plateau in southwestern Colorado. Its unusual ecological characteristics include an absence of plant and animal taxa that should occur here. Fossil and archaeological evidence indicates that many of the missing species existed in the Basin during the late Pleistocene to middle Holocene. Authored by Steve Emslie.
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