If you were to cut down every tree in the UK to look at their growth rings (this is not advised), a pattern would start to form. Counting backwards from the bark, you’d see an especially thin ring for the year 1976, meaning that tree didn’t grow very much. None of them did, in fact. This is because the UK was suffering its driest year since 1772, with temperatures reaching 35.9°C, and the forests were not taking it well.

In this way, trees can teach us countless lessons about our planet’s past. The oldest known living tree, a bristlecone pine in California, is almost 5000 years old. When it germinated, woolly mammoths still roamed throughout Russia, and neither Stonehenge nor the Great Pyramid of Giza had been completed. This tree has lived an unfathomable life, but what is it capable of remembering?

One person answering this question is Dr. Estrella Luna Diez, UoB’s very own associate professor in plant pathology. Her research project, MEMBRA, investigates how trees can retain ‘memories’ of stressful events such as drought, disease and insect infestation and eventually pass these on to their offspring.

Unlike human memories, made and kept in various regions of our brains, plants store memories directly using their DNA. Every organism’s DNA carries a unique sequence of chemical bases that act as a set of instructions on how to make every part of the individual. When the organism is under stress, a process called DNA methylation can be triggered, switching off certain parts of the instructions while leaving the sequence itself unchanged. This means that different genetic responses can take place without any change to the organism’s genetic code.

Dr. Luna Diez’s team have found that though all stressors can trigger this methylation, the strength of the effect can vary. Significantly life-threatening events are imprinted more strongly, as are stresses that occur earlier in a tree’s life. This is somewhat consistent with human memory; being hit by a car as a child is probably more memorable than that lower back pain you’ve been getting recently.

Importantly, these epigenetic changes can be inherited by a tree’s offspring. This means that the descendants of a drought-stressed Oak, for example, will be more resilient to similar conditions in the future. This is a particularly hopeful finding for forests who continue to suffer under climate change. In the last 40 years, UK forests have seen a 90% increase in sapling mortality rates, weakening them as potential stores of carbon and as habitats for thousands of animal species. With this new knowledge however, plants can be ‘primed’ by artificially exposing them to stress as seedlings, enhancing their resilience to changing climates.

Despite this, many people will be reluctant to call this process ‘memory’; our brains are what make us humans special, and not everybody is comfortable sharing the limelight with our green cousins. But if we can ascribe memory, and even intelligence (erroneously) to machines, then I see no reason to deprive the term from living beings such as plants.

MEMBRA addresses these issues too, discussing environmental ethics and attitudes through art and humanities perspectives. One limb of the project is The Walking Forest, a ten-year collaborative art project exploring the nature of activism and community within woodlands. It will culminate in the planting of a new forest in 2028 – a living artwork to inspire hope for future generations.

Dr. Luna Diez and her work recently featured in a documentary, The Wonder of Treescapes. The short film features a selection of vignettes from across the UK, interspersed with poetry by Helen Mort. Other contributors to the film include Dr Parker and Scott from Leeds, who discuss the cooling effect trees have on cities, Sophia Brown and her Bristol walking club for women of colour, and Dr Carr who inspires children’s creativity by helping them create tree-planting video games. The documentary takes a beautifully holistic approach to our relationship with forests and is available to watch on Youtube.

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