Examining the response of oak trees to a changing climate

Upland oak woodlands are beautiful. As their name suggests these woodlands occur above 250 metres and are dominated by Oak trees. They harbour a rich collection of mosses, liverworts and lichens as well as a unique combination of other plants and animals which make this type of woodland internationally recognised. One of these rare species is a Lichen (Graphina pauciloculata) which is considered endemic to the UK (quite a rare thing), with over 50% of all sightings occurring on Dartmoor!

Wistmans wood is probably the most famous of these upland oak woodlands on Dartmoor and like the two other isolated woods (Black a Tor Copse and Piles Copse) is located on the west facing boulder covered slopes of an empty Dartmoor stream. Whilst the origin of the wood is still debated there is little argument as to its charm; Wistmans Wood inspires intrigue and romanticism in equal measure. As you walk past the emerald green moss covered boulders, curtains of lichen festooning the hunched trees and ferns spring from almost every tree limb, it is difficult not to feel a sense of respect as well as a sense of excitement about what Dartmoor and the UK uplands more widely can be.

As part of my PhD research I decided to look at the response of the Oak trees at Wistmans Wood (Quercus robur) to climate variables. The research should be important in the years ahead in helping us understand the response of the wood and other upland oak woodlands to changes in climate.

With the permission of Natural England (who manage Wistmans Wood), I collected cores from 26 oak trees within the wood in July 2017 using an increment borer (essentially a large apple corer). The cores collected were very thin (about 10mm in diameter) and the resultant holes are plugged and sealed so trees are left healthy with no long term impact. Once the cores are collected they are dried and sanded to make the growth rings clearer. The slow growing nature of the trees made identification difficult meaning only six from 26 cores could be used to construct the ‘master chronology’, essentially an averaged ring width timeline of the cores used. This master chronology is known as a standardised ring width index and shows whether the trees were growing more or less than average for a particular year. Once the master-chronology was made it was compared to climate data for the area made available by the Met Office.

One of the dominant factors impacting on European climate particularly on the western coastal fringes (like Dartmoor) is the Atlantic Ocean. The North Atlantic Ocean experiences multi-decadal cycles known as the Atlantic Multi-decadal Oscillation (AMO index) which swings between warm and cool phases over a 60 year period. The warm phase is associated with wet and warm summer weather and the cool phase drier sunnier weather. We are currently at the peak of the warm phase.

Intriguingly the results of my study show the growth of the oak trees at Wistmans Wood are significant positively correlated with North Atlantic sea surface temperature during spring and summer. Isn’t it great to think that the growth of the oaks is influenced by oceanic cycles taking place thousands of miles away in the depths of the Atlantic Ocean. This type of relationship brings home the fundamental connection between ocean and land, and shows that if tree growth can be thought of as the lungs of the planet (breathing in carbon dioxide and releasing Oxygen) then our planets breathing is controlled by the oceans and the large cycles which control their temperature (by influencing climate). Next time you take a walk in Wistmans Wood think of the sea!

For those interested in finding out more I hope to publish the final results in the months ahead.