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Do trees really help to keep our brains healthy?

Scientists are interested in the effect of trees on the human brain
Scientists are interested in the effect of trees on the human brain Credit: Sergey Nivens / Alamy 

How your brain develops is linked to the environ­ment 
you experience. Studies show clearly that rats’ brains fail to develop normally if they are kept in cages with the essentials for life – food, water and bedding – but nothing else. For proper brain development, rats need an “enriched environment”, which means larger cages with toys, running wheels, and ideally some company.

So far, so reasonable. But what, if any, are the lessons for human brain development? What kind of “enriched environment” do we need for healthy brains? On the one hand, the classic hamster wheel seems like quite a good metaphor for modern city life, so maybe cities provide an enriched environment? On the other hand, maybe not; plenty of studies show that rural life is generally linked to better mental health.

To try to get to the bottom of all this, German researchers looked at the brains of 341 older (mostly retired) residents of Berlin. Their results were published in the journal Scientific Reports. Using MRI scanning, they looked at three regions of the brain, including the amygdala, which regulates the response to fear, anxiety and stress. They then looked for any connection between indicators of “structural integrity” (a measure of normal, healthy development) of the amygdala and features of the environment near where the study subjects lived: specifically, how much woodland, water (lakes, rivers, canals), wasteland and urban green space there was within a 1km radius of home. Green space could include trees, but was mainly parks, gardens and fields, while woodland was anywhere with tree canopy greater than 30 per cent and tree height greater than 5m.

The results were very clear and also a bit surprising. A healthy amygdala was linked to more nearby woodland, but not to any other land use types. In case living near woodland was related to being better off, they included the subjects’ income in the analysis and found it made no difference.

The reason these results are slightly surprising is that plenty of other studies have shown that improved well-being is linked to experience of green space; but here measurable effects on brain structure were linked specifically to woodland, so why should that be? It is possible that something else linked to woodland is what actually makes you feel better, such as less noise or lower air pollution? On the other hand, there is evidence that woodland itself has direct beneficial and calming effects on the brain. Japanese research has shown that shinrin-yoku (“forest bathing” or “taking in the atmosphere of the forest”: basically walking in or just contemplating woodland) leads to lower salivary cortisol (reduced stress) and reduced activity in the prefrontal cortex (greater relaxation).

Like much interesting research, all this raises at least as many questions as it answers. For example, if green space with a few trees doesn’t work as well as woodland, how many trees do you need? And how does woodland compare with other kinds of “wild” landscape that are harder to come by in cities, such as mountains or seaside?

But from a gardening perspective, if just looking at some trees is good for your brain, think how much better actually planting one or two might be.

  • Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively; his most recent book is Darwin’s Most Wonderful Plants. Visit books.telegraph.co.uk