Transcript
JULIA COOKE
How long have oak trees been in the British Isles, and how did they get there? In order to understand ancient plant ecology on large scales, a way to track the presence of plants over time is needed. We can do this by studying pollen, which can be well-preserved and unique to the plant species.
There is a relationship between soil depth and the age of the soil, and we can find out if pollen is buried at various depths. Preserved pollen is used to establish the presence of the species. Radiocarbon dating is used to date the soil core depth.
Finally, we can study the DNA of existing plants to discover how closely oak trees in different parts of the country are related to each other. This is because DNA is passed on from generation to generation. Ancient pollen and extant DNA are powerful tools for looking at ancient ecology.
Between 23 000 and 15 000 years ago, ice sheets and glaciers covered much of the British Isles and sea level was much lower than today. What are islands today were joined to each other and to mainland Europe. Oaks, including Quercus robur, the English oak, did not occur in the UK before about 10 000 years ago. It was too cold. However, land bridges between continental Europe and what is now the British Isles provided a source of plants and animals which could colonise the area as the climate warmed.
Palynologists, researchers who study pollen, have tracked the colonisation of the British Isles by oaks using pollen preserved in peat and soil. Pollen is released from male catkins, or flowers. Oak pollen has a distinctive shape, so it can be identified as coming from this type of tree.
Soil cores, such as the ones shown here, are extracted from locations where peat or sediment have been laid down over long periods. Here is an example of a core. The depths are marked. The age of the soil at several depths is determined using a technique called radiocarbon dating.
Slices of the core are examined for oak pollen. Where the pollen was found in this example is shown by a plus sign, and a minus sign shows where none was found. From this core, it appears that oaks were present at this site from 1000 years ago until today.
The dots on this map show where cores have been collected and analysed. The age at which oak pollen first appeared in each core has been worked out, and a research paper from 1989 collects the results. We can see that oaks spread from south to north. Each line represents the northern limit at a particular time. The dotted lines are where the pollen evidence was limited, and so there is less certainty.
The rate of dispersal was probably limited by how quickly oaks could grow and produce acorns, and how far these acorns were spread by animals. Oak distribution may also have been limited by environmental factors, such as temperature.
In 2005, other researchers published an analysis of DNA from extant oaks across the British Isles. The three colours indicate three genetically distinct populations of oaks. Using the information from the pollen analysis, the researchers suggested how these oaks could have spread, as shown by the arrows, to explain the patterns we see today. Together, the two studies suggest the likely colonisation routes and timing of oaks in the British Isles.