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.
Jeff Eden/RBG Kew, Author provided (no reuse)
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 available. A clear example can be seen in Fremont cottonwood (Populus fremontii), which is often found close to rivers. This tree used this process to keep its leaves between 2°C and 5°C cooler than the surrounding air during Arizona’s record-breaking heatwave in 2023, even when air temperatures exceeded 48°C.
As air temperatures rise, the atmosphere draws water from leaves more rapidly, in a process plant scientists call the vapour pressure deficit. Concurrently, dry soil makes it harder for roots to replace those losses, placing trees under increasing water stress and limiting their ability to maintain normal physiological functions.
By closing their stomata, trees conserve water. But this in turn restricts carbon dioxide uptake, slowing photosynthesis and reducing sugar production required for growth, repair and for defence against pests and diseases. When stomata are open, this allows for continued cooling, but comes at the cost of unsustainable water loss.
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Every leaf has a thermal limit beyond which its ability to photosynthetise suffers irreversible damage. Surviving a heatwave therefore depends on maintaining enough evaporative cooling to stay below that threshold, although this safety margin is known to disappear surprisingly quickly. A recent study on poplar trees found that only 72 hours of reduced soil moisture and access to water was enough to disrupt leaf cooling. The leaves that had previously remained cooler than the surrounding air temperature became hotter than the air itself, reaching temperatures damaging to photosynthetic tissue.
Why branches fall
As heatwaves persist, problems can often spread beyond just the leaves. With xylem water under constant tension, when water supplies become limited, tiny air bubbles known as embolisms begin to form within the xylem in a process called cavitation. This results in a blockage impeding the transport of water. Ultimately, if enough xylem vessels become blocked in this manner, the tree experiences hydraulic failure, with water no longer able to reach the canopy tree effectively. This causes branches or entire sections of the crown to die back.
Some trees effectively sacrifice parts of their own canopy, reducing the need for water to keep the main stem alive. This strategy is known as hydraulic segmentation and helps explain why seemingly healthy trees may lose substantial numbers of limbs during prolonged hot weather.
Trees can continue to pay a price long after a heatwave is over, as this extreme weather depletes overall carbohydrate reserves and reduce growth potential. These effects can persist for years, leaving trees less resilient when the next drought arrives. A tree may therefore survive one exceptionally hot summer, only to succumb after another as the stresses accumulate over time. Water-stressed trees also become more vulnerable to insect pests and fungal and bacterial diseases because they have fewer resources available to defend themselves from attack.
Britain’s trees evolved under relatively cool, wet summers, and temperatures above 40°C were unheard of until recently. Many are now being exposed to climatic conditions well outside those under which they evolved. Research following the 2003 European heatwave suggests that vulnerability depends not only on how hot it becomes, but also on how well species can adapt to local conditions. As heatwaves become more frequent, many traditional species may increasingly find themselves outside their climatic comfort zone.
What can help?
Fortunately, there are several practical steps that can help, particularly for younger and more susceptible trees. Occasional deep watering is much more effective than frequent light watering because it encourages roots to grow deeper into the soil, where moisture remains available for longer.
Mulching, creating a thick layer of hardwood chip or compost, reduces soil surface evaporation rates, moderates soil temperatures and, as it breaks down, improves soil structure and water holding capacity. Soils with high concentrations of organic matter can absorb water/rain more effectively, which supports more resilient root networks.
Managing mature woodlands to increase their resilience to future heatwaves will require new strategies. Increasing woodland species diversity, reducing the concentration of trees in drought-prone areas and planting species better suited to future climates should be considered. A greater diversity of species helps resilience to hot weather.
Many of Britain’s mature trees have stood for centuries – these may be the most effective ways of protecting them for the future.
