If you visit the Royal Botanic Gardens at Kew, on the edge of London, you will see a brightly painted skeleton of a dead oak tree. The tree, known as the climate changed oak, succumbed in the heatwave of 2022. Instead of removing it, Kew left it in place as a reminder that climate change is already taking its toll on Britain’s trees.

The 2022 heatwave killed 400 of Kew’s trees and predictions show that up to 50% of Kew’s existing 11,000 trees could be vulnerable to climate change by 2090.

A similar story is playing out across the UK. Mature trees shedding limbs and branches during intense periods of hot weather, their canopies thinning, and some dying back altogether. Although these changes can appear sudden, they are often the final visible signs of a much longer process of decline that has been unfolding inside the tree.

The problem is not simply these high temperatures, but that the combination of both extreme heat and drought, increasingly referred to as a “hot drought”, pushes trees beyond the limits of what they can cope with.

Trees struggle to keep cool

To understand why heatwaves are so damaging to trees, one needs to appreciate how trees keep themselves cool. When water is absorbed by the roots, this travels through the xylem (the network of microscopic tubes that carries water from the roots to the leaves), where it evaporates through stomata (tiny pores on the leaf surface). This cools the leaves in much the same way that sweating cools humans.

This cooling system works well when water is freely........

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