Wednesday, 17 July 2013

Palynology: why pollen is not to be sniffed at!

by Michelle Farrell (@DrM_Farrell)

To around 20% of the UK population, pollen is familiar as the cause of hay fever during the spring and summer months. To a very small minority (ironically, many of them well acquainted with the runny-nosed, itchy-eyed symptoms of hay fever themselves), pollen is much more than an allergen. Pollen grains contain the male gametophyte of seed-producing plants, and in order to increase their chances of reproductive success, wind-pollinated plants produce pollen in vast quantities. The small size of pollen grains (generally in the order of 20 to 40 microns, a micron being one thousandth of a millimetre) means that when they are released by a plant they become widely dispersed in the environment. As the plant has no control over where its pollen grains end up, another part of the reproductive strategy is that the grains have a very tough outer casing or exine, made of a substance called sporopollenin, allowing them to survive in less than perfect conditions. This outer wall can be preserved in several environments, particularly waterlogged ones, for tens of thousands of years. This combination of pollen being produced in large quantities, wide dispersal in the environment, excellent preservation under the right conditions, and the fact that pollen grains can often be identified to family, genus and even species level, is what makes them such a valuable tool for research.  

Coring to retrieve pollen-bearing sediments from a bog
on Orkney, Scotland
Pollen analysis is one of the most common methods used for investigating past environments. Pollen is often preserved in waterlogged environments where sediments accumulate, such as lakes and peat bogs. Cores of sediment can be extracted from these locations, and sub-samples from various depths are then subjected to a series of physical and chemical laboratory treatments which remove the majority of the inorganic sediments and large organic debris, leaving behind the fine organic fraction of the sediment. It is this fraction that contains the pollen grains, as well as other tiny organic remains including fern and fungal spores, other fungal remains such as hyphae, and fragments of charcoal from either natural or anthropogenic fires. The study of all these remains together is known as palynology, a term coined by the British scientists Hyde and Williams in 1944 and derived from Greek words meaning ‘the study of small particles sprinkled about’.

The stripes in this core segment indicate that the sediments
were deposited under different environmental conditions
Once you have concentrated the fine organic fraction of the sediment, the next stage is to identify the botanical remains contained within it. Much as a botanist would use a key to help them to identify plants out in the field, palynologists use keys to pollen and spores to aid their identification of specimens under the microscope in the lab. The identification and study of fungal remains is still a relatively new technique, and to date no definitive key to these types of remains has been published. Therefore I’ll focus here on the distinctive characteristics of pollen grains that allow palynologists to distinguish between the different taxa present in a sample. One of the most distinctive features of pollen grains is their apertures. There are two types of aperture: pori (pores), roughly spherical in shape, and colpi (furrows), which are elongated and have pointed ends. Some grains have both colpi and pori in the same apertures, and are known as colporate. The number and arrangement of the apertures is also key in pollen identification. The number of apertures is indicated by the use of the prefixes mono-, di-, tri-, tetra-, penta- and hexa- before the terms porate, colpate and colporate. The prefix poly- is used to denote the presence of more than six apertures. Usually the apertures are arranged equidistantly around the equator of the pollen grain, and this is indicated by the prefix zono-. Panto- is used when the apertures are scattered all over the surface of the grain. Some examples of the way in which apertures are used to identify pollen grains are shown below.

Betula (birch) pollen: trizonoporate,
with three pores arranged equidistantly
 around the equator of the grain
Fraxinus excelsior (common ash) pollen:
trizonocolpate, with three furrows arranged
equidistantly around the equator of the grain

Rumex obtusifolius (broad-leaved dock):
tetrazonocolporate, with four apertures
made up of both colpi and pori
Plantago lanceolata (ribwort plantain):
polypantoporate, with many pores scattered
all over the surface of the grain

The pattern of sculpturing found on the surface of the exine is another crucial factor in the identification of pollen grains. Around fifteen different surface patterns have been described, and two of the more distinctive patterns are shown in the images below.

Cirsium arvense (creeping thistle) pollen, a
good example of echinate surface sculpture

Ulmus (elm) pollen, displaying rugulate
surface sculpturing

Size can also be important in distinguishing between pollen taxa, particularly members of the grass family. The two images below show the difference in size between a wild grass, Phragmites australis (common reed) and a cultivated grass, Triticum aestivum (wheat).

Phragmites australis (common reed) pollen
Triticum aestivum (wheat) pollen

The identification and recording of pollen grains from different depths within a sediment core can be used to provide information on how the vegetation surrounding the core site has changed over time. Sediment cores can often be accurately dated using radiocarbon, allowing the changes in environment to be tied to a chronology. Palynology has great potential for providing baseline data for the development of conservation management strategies, and is also useful from an ecological perspective as it can give insights into how plant communities have responded to climate change in the past, thereby allowing predictions to be made about how vegetation and ecosystems may be affected by future climate change.

One of my main research interests is in the use of palynology as a tool to unpick the ways in which humans interacted with their environments during the Holocene (the period since the end of the last ice age, approximately 11,500 years ago, until the present day). I intend to write more about the archaeological applications of palynology in future posts, but as a taster, it is possible to determine when people began farming in an area, what crops they were growing, where they grazed their animals, whether they cleared woodland to create more land for agriculture, and how they contributed to the development of cultural landscapes such as heathlands. It is also possible to investigate the ways in which people may have managed their environments and responded to climate change in the past, for example managing heathland by deliberate burning in order to maintain the quality of grazing. It may even be possible to detect woodland management practices such as coppicing, and Jane Bunting will write about her work on this in the next GEES-ology post.

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