Pollen Analysis

Palynologists typically collect cores of sediment or peat, date layers occurring at various depths, and extract, identify, and enumerate samples of the fossil pollen grains that are contained in the layers. From the dated assemblages of fossil pollen, palynologists develop inferences about the nature of the forests and other plant communities that may have occurred in the local environment of the sampled lake or bog. These interpretations must be made carefully, because as noted above species do not occur in the pollen record in a fashion that directly reflects their abundance in the mature vegetation.

Most palynological investigations attempt to reconstruct the broad characteristics of the local vegetation at various times in the past. In the northern hemisphere, many palynological studies have been made of post-glacial changes in vegetation in places that now have a temperate climate. These vegetation changes have occurred since the continental-scale glaciers melted back, a process that began in some places 12,000-14,000 years ago. Although the particular, inferred dynamics of vegetation change vary among sites and regions, a commonly observed pattern is that the pollen record of samples representing recently deglaciated times contains species that are now typical of northern tundra, while the pollen of somewhat younger samples suggests a boreal forest of spruces, fir, and birch. The pollen assemblage of younger samples is generally dominated by species such as oaks, maples, basswood, chestnut, hickory, and other species of trees that presently have a relatively southern distribution.

However, within the post-glacial palynological record there are clear indications of occasional climatic reversalsfor example, periods of distinct cooling that interrupt otherwise warm intervals. The most recent of these coolings was the so-called "Little Ice Age" that occurred between the fourteenth and nineteenth centuries.. However, palynology has detected much more severe climatic deteriorations, such as the Younger Dryas event that began about 11,000 years ago, and that caused the re-development of glaciers in many areas, and extensively reversed the broader patterns of post-glacial vegetation development.

Other interesting inferences from the palynological record have involved apparent declines of particular species of trees, occurring for reasons that are not known. For example, palynological records from various places in eastern North America have exhibited a large decline in the abundance of pollen of eastern hemlock (Tsuga canadensis), occurring over an approximately 50-year period about 4,800 years ago. It is unlikely that the hemlock decline was caused by climate change, because other tree species with similar ecological requirements did not decrease in abundance, and in fact, appear to have increased in abundance to compensate for the decline of hemlock. The hemlock decline may have been caused by an outbreak of an insect that specifically attacks that tree, by a disease, or by some other, undiscovered factor. Palynology has also found evidence for a similar phenomenon in Europe about 5,000 years ago, when there was a widespread decline of elms (Ulmus spp.). This decline could have been caused by widespread clearing of the forest by Neolithic humans, or by an unknown disease or insect.