Pollen and spores are critical parts of the life cycles of vascular and, because they have very resistant walls, pollen and spores typically are the most abundant, easily identifiable, and best preserved plant remains in sediments and sedimentary rocks. Therefore, their presence and abundance in sediments can be used as a proxy indicator for composition of plant communities growing at and around a site over both recent and geologic time.
Spores, as referred to here, include the reproductive bodies of lower vascular plants such as club mosses, horsetails, and ferns. The earliest occurrences of spores apparently produced by land plants in the fossil record are in Lower Silurian rocks, slightly preceding the appearance of the the first vascular plant megafossils (Cooksonia). Pollen grains are the sperm-carrying reproductive bodies of seed plants, including gymnosperms (such as conifers and cycads) and angiosperms (the flowering plants). Pollen first occurs in the Upper Devonian rocks, corresponding to the occurrence of the earliest fossils seeds (Archeosperma) in North America. Both spores and pollen have very resistant walls composed of a substance known as sporopollenin, and the resistance and inert nature of this wall allows preservation of pollen and spores in sediments under a variety of conditions. Pollen and spore walls of each plant species bear certain types of sculpture and have characteristic apertures; these features are among those used to identify the types of plants represented in a suite of pollen isolated from a given site. To study pollen grains and spores, a compound microscope must be used because pollen and spores are very small, typically between 10 micrometers (µm) and 200 µm (There are 1000 µm in 1 millimeter, and a millimeter is about equal to the diameter of the head of a pin). Pollen and spores typically are isolated from sediments and rocks using both chemical and physical means, and they ultimately are mounted on microscope slides for examination with the light microscope. For most geological and environmental applications of pollen and spore analysis, scientists count and identify grains from each sample using the microscope and generate pollen diagrams of the relative (percent) and absolute abundance of pollen in samples from a site.
Environmental Studies - Studies of human impacts on the environment commonly have relied on the historic record, which may cover only the last few decades, to determine the response of plant communities to environmental changes. By analyzing pollen from well-dated sediment cores collected at critical sites, it is possible to obtain high-resolution records of vegetational change with decadal-scale resolution and to document community changes over the last few centuries and millennia. For example, in the Everglades of southern Florida, scientists at the USGS have documented the response of vegetation to changes in water management practices, nutrient influx from agricultural activities, and disturbances such as road and canal construction and fire over the last century. Additionally, they have established the natural levels of variability over the last few millennia. Comparison of vegetational trends over both time scales is needed to determine whether human activities really have had significant impacts on the ecosystem. These types of data are critical for land-use managers in optimizing management practices and in planning restoration activities.
Biostratigraphy - The evolution of land plants since Silurian time resulted in great variety of plant types and spore and pollen morphologies. The relatively broad dispersal and great abundance of pollen and spores in sediments and rocks throughout this time has facilitated the development of pollen zonations for use in biostratigraphic work. Using biostratigraphic techniques, pollen and spore assemblages can be used to correlate stratigraphic sections several hundred kilometers apart; for example, spore floras have been used to correlate coal beds of Pennsylvanian age from Pennsylvania across the midcontinent into Kansas.