Pollen analysis is a technique which can be used to get an idea of what plants were around thousands of years ago. The history of our woodlands in the UK since the last Ice Age is a fascinating topic and many people are interested in it particularly if they own woodlands or regularly visit them.
Written records only go back so far and it is very difficult to get much written information from the time before the Domesday Book. The next best thing is pollen analysis.
Pollen has an outer coat (exine) which for some reason is incredibly resilient, especially if it is in a relatively constant environment, i.e permanently wet or permanently dry. This outer coat is made of a substance called sporopollenin and which can survive for thousands of years. It is similar to cellulose in that it is a long chain polymer and this makes getting at the individual building blocks difficult so is not easily broken down, even more so than with cellulose. Just for the record this is its chemical structure.
Mmm…. well you can see the units are long.
The other feature of pollen grains is that different species have different shaped grains with different patterns. Some are very distinctive, for example Pine pollen which is shown in the photo opposite. The grains have two little air sacs to help them float on the wind.
Others, for example grasses, all have very similar shapes so in the main, you can, if you are an expert, identify the pollen grains of each species or at least group of species.
If you can find a good historical source of pollen grains then you can find out what was around many years ago. The good source is to be found in deposits from ponds and lakes. Layers of peat built up from organic matter deposited over hundreds and thousands of years will contain pollen grains that were released by the plants that were growing at that time. You take a core sample using a device called a soil auger and then cut the core up into short 1 cm sections. Each section will be a little bit older as you go down the core.
The next job, carried out in a laboratory, involves treating each section with a chemical which will dissolve away all the organic material except the pollen grains. The chemical used is Hydroflouric acid, fairly nasty stuff which will dissolve most things, including glass, so it is stored in plastic bottles. Years ago it was used to etch designs on glass and people would paint interesting designs on plain wine glasses as a hobby. Fairly disastrous if you spilled it on your lap! Nowadays it is not easily available which is probably just as well.
The excess acid is then washed away and what is left are the pollen grains which of course can be counted and identified using a microscope. This will tell you something about what plants or trees were growing at the time the peat was originally formed. However there are certain problems.
First of all some species produce much more pollen than others. Wind pollinated species, for example, obviously produce a great deal of pollen. This can sometimes be seen in Pine woods in spring where a yellow dust will coat everything, or in wet woodlands or Carr habitats where ponds will develop a yellow scum which is the pollen from Sallow bushes in spring time. So a sample which has 50% Lime pollen and 50% Oak would not mean a woodland with half Lime and half Oak trees. It would mean a woodland almost totally dominated by Lime with maybe just one or two Oaks.
Secondly, the pollen in a sample of peat will be most influenced by the species growing closest to the region where the peat is forming which will be a wet area so the chances of finding pollen from trees like Alder and Ash that like it wet will be much increased and the chances of finding Beech or Oak will be that much less.
When it comes to smaller woodland plants which may be quite common in a woodland such as Bluebells or Ramsons, the amount of pollen they produce in comparison to Silver Birch or Hazel, for example, is quite minuscule, so finding just a few pollen grains is a bit hit and miss, despite them being quite dominant in the woodland flora.
There are other more minor problems but you get the picture – pollen analysis is a guide but it needs careful interpretation.
The other thing to note is that it just provides a sequence of what occurred but no dates. However recently the technique of Carbon dating has been used to work out the ages of each layer from the extracted core reasonably accurately, now making the dating of samples possible.
Normally the results of the analysis are presented in a fairly complicated chart like this.
If you are wondering what Pollen analysis and other evidence has indicated about how our woodland developed and changed since the last ice age then have a look at my two pages:
and Woodland history, part two (the last 2,000 years). Still being prepared.