Pollen and environmental reconstruction

Indroduction
Pollen grains are generally transported by wind, insects and other animals and may find their way into deposits of lakes, oceans, swamps, mangroves and peat bogs. Their cell wall consists of very resistant material, called sporopollenin, which aids in preservation under anoxic conditions over long periods of time. This characteristic allows for the use of pollen and spores as indicators of past vegetation types.
The figure on the left shows a light microscopic view of the pollen grain of the tropical genus Bombax (Bombacaceae). Sculpture and structure of the wall of pollen grains is highly diverse. Pollen grains are in general very small between 20 and 40 µm (0,02 - 0,04 mm) and can be observed under a light microscope.

The pollen analysis allows the reconstruction of vegetational changes and shifts of the complete vegetation zones. Furthermore, it is also possible to illustrate past plant diversity, stability and dynamic of ecosystems. Due to the strong relationship between vegetation and climate (temperature, precipitation), the identification of past vegetational changes allow to reconstruct climate variability in the past. Pollen and spores can also be indicators for climate changes. It is for example possible to study vegetational and climate changes in the tropics during the Ice Age period (Pleistocene) and the post glacial period (Holocene).

When humans change the environment, they leave 'foot-prints', indicated mostly by changes in the vegetation and by fires. These changes can be identified by pollen and carbonised particle analysis, for example those deposited in lake sediments. This allows the determination of first human impacts, the reconstruction the history of human settlement and changes in land use. One can thus determine in what way and to what degree humans changed the coastal regions during historical and prehistorical times (e.g. by fire).

Methods
The 'Livingstone corer' is a useful tool in obtaining cores of lake sediments. The ajacent figure shows a coring operation on a tropical lake in South America. Sediment cores can be compared to a 'book' about the natural history of an area in the sense that each page would corresponds to a sediment layer. The words on the pages of this 'book' would represent the pollen grains and spores of a sediment sample. Just like the page of a book has many words, one cm3 of lake sediment may contain more than one million pollen grains and spores. The identification of the pollen grains and spores yields the name of the plants present in the area at the time. In order to correctly identify the pollen and spores it is important to have a good knowledge on the ecology of the plants. Only then is it possible to interpret past climate changes.

Pollen diagrams help illustrate the pollen analytical data and help to reconstruct vegetational changes. They show the number of identified pollen and spore types in the different sediment layers, as a direct count or as a percentage of total tree pollen (e.g. 300 to 500 pollen grains).

Many pollen analytical studies have been conducted on ecosystems in Europe and North America, but the natural history of past tropical ecosystems such as rainforests and savannas is largely unknown. For example, Germany has more than 5000 study sites, while Amazonia (which is 15 times larger than Germany) has less than 30 study sites. Mangroves and salt marshes are of special interest for the purpose of interpreting changes in sea level through time. Mangrove sediments can be several meters thick and also contain pollen grains, which allow for the reconstruction of the development and dynamics of mangrove ecosystems. The distribution of mangroves and salt marshes is related to the height of sea-level. Therefore, changes of these ecosystems also allows for the reconstruction of sea-level changes.

In addition, cores taken by bore-vessels from the sea ground may also contain pollen and spores. Those were transported by rivers and wind to the ocean and deposited on the sea floor. Pollen and spores in marine sediment cores provide information on environmental changes in coastal regions and in the hinterland.

Pollen analysis can make an important contribution to the reconstruction of local and global environmental changes. The understanding of natural environmental changes, such as changes of ecosystems,and related climate and sea-level provides important information on how to understand modern and future environmental changes. Additionally, the knowledge of natural environmental variability helps in the interpretation of how severe past anthropogenic environmental changes have been.


Text, photos and graph copyright © 2005 by Hermann Behling