Heat waves in the UK are causing trees to shed branches and die, impacting ecosystems

On July 9, 2026, a troubling phenomenon is affecting British trees as heat waves lead to branch shedding and tree mortality. The Royal Botanic Gardens at Kew reported that the 2022 heat wave resulted in the death of 400 trees, with predictions indicating that up to 50% of Kew's 11,000 trees may face vulnerability to climate change by 2090.

Impact of Heat Waves on Tree Health

During intense heat waves, mature trees across the UK are exhibiting signs of stress, including thinning canopies and branch dieback. While these changes may seem abrupt, they often signal a longer process of decline within the tree's physiological systems. The combination of extreme heat and drought, termed a "hot drought," is pushing trees beyond their coping limits.

Trees maintain their health by regulating temperature through a process similar to sweating in humans. Water absorbed by the roots travels through the xylem and evaporates from the leaves, cooling them. However, as air temperatures rise, the atmosphere increasingly draws moisture from leaves, leading to a vapor pressure deficit that exacerbates water stress.

Mechanisms of Branch Shedding and Tree Mortality

As heat waves persist, trees may suffer from cavitation, where tiny air bubbles form in the xylem due to reduced water availability. This blockage prevents adequate water transport, leading to hydraulic failure. Consequently, trees may sacrifice parts of their canopy to conserve water, a strategy known as hydraulic segmentation.

The loss of limbs may occur even in seemingly healthy trees during prolonged hot weather. Moreover, the effects of heat waves can deplete carbohydrate reserves, reducing growth potential and resilience against future stressors. Water-stressed trees also become more susceptible to pests and diseases, further compromising their health.

Adaptation Challenges for British Trees

British trees evolved under cooler, wetter conditions, and the recent exposure to temperatures exceeding 40°C poses significant challenges. Research following the 2003 European heat wave indicates that tree species' vulnerability depends on their ability to adapt to local climatic conditions.

As heat waves grow more frequent, many traditional tree species may struggle to thrive outside their climatic comfort zones. Practical steps to support tree health include improving soil moisture retention and selecting species better suited to changing conditions.

• 400 trees died at Kew during the 2022 heat wave.
• Up to 50% of Kew's 11,000 trees may be vulnerable by 2090.
• Extreme heat and drought create conditions known as "hot droughts."
• Hydraulic segmentation helps trees conserve water during stress.
• Carbohydrate reserves deplete, affecting long-term growth.