How Do We Know?
Fossil Pollen

Big bluestem in flower. The exposed, yellow anthers are the pollen-bearing structures of the bluestem flower

Palynology is the study of pollen. Scientists at the Illinois State Museum study fossil pollen that has been preserved in peats and lake sediments. Pollen preserves best if the sedimentary environment lacks oxygen or is acidic, conditions unfavorable for the organisms that decompose pollen. Fossil pollen is an important kind of data for reconstructing past vegetation. Because vegetation is sensitive to climate, fossil pollen is a very important kind of proxy data for reconstructing past climates.

Most plants are either insect pollinated or wind pollinated. In insect pollinated plants, insects, especially bees, transport pollen from one flower to the other. Because the insect vector of pollen transport is highly efficient, insect pollinated plants produce relatively less pollen and little of this pollen is released into the air. On the other hand, wind-pollinated plants produce large amounts of pollen because the probability is small that an individual pollen grain will actually land on another flower of the same species. Many of our forest trees are wind pollinated, as well as important prairie plants such as grasses, ragweed, and sage. Some insect pollinated plants do release significant amounts of pollen into the atmosphere, and pollen from these plants is also important for reconstructing past vegetation.

Because of their very small size, pollen grains, which are living plants, are easily dried out and destroyed. Thus, they have evolved a very tough and resistant outer "shell," which preserves well and which enables the scientist to concentrate the pollen for study. Some pollen eventually falls into lakes or peatlands where it accumulates with other sediments, layer after layer, year after year. Lake sediments typically lack oxygen necessary for decomposing organisms, and if the sediment remains wet, the pollen will preserve for thousands or even millions of years.

Pollen is an especially valuable kind of fossil for reconstructing past environments for a number of reasons:

• The sediment of virtually any permanent, natural lake will contain abundant fossil pollen. It is not hard to find. A cubic centimeter of lake sediment will typically contain tens or hundreds of thousands of pollen grains.

• Pollen is released into the air and is transported some distance, the assemblage of pollen in the sediment is representative of the vegetation from the general region around the site, not a single, small, perhaps special habitat.

• All higher plants produce pollen (or spores), the pollen assemblage will represent wide variety of plants from the region.

• Pollen accumulates continuously, year after year, scientists can reconstruct vegetation change over thousands of years and determine when and how rapidly changes occurred. Some lakes even have annual layers, like tree rings, that make it possible to reconstruct short-term climate variability over hundreds or thousands of years.