Kew Gardens’ Climate Changed Oak Highlights Trees’ Struggle with Heat

Amidst the lush greenery of London’s Royal Botanic Gardens at Kew, a stark reminder of climate change stands prominently—a skeleton of a once-thriving oak tree, now painted in vibrant colors. This oak, which succumbed during the intense heatwave of 2022, serves as a symbol of the growing toll of climate change on the UK’s trees.

The heatwave that year was devastating, leading to the death of 400 trees at Kew. Forecasts suggest that by 2090, as much as 50% of the garden’s 11,000 trees could be at risk from climate change.

Across the United Kingdom, similar stories emerge as mature trees lose branches, canopies thin, and some trees die back entirely. While these changes may seem abrupt, they often represent the end of a long process of decline exacerbated by extreme weather conditions.

The combination of extreme heat and drought, termed “hot drought”, poses a severe threat to trees, pushing them beyond their limits of survival.

Trees Struggle to Stay Cool

To comprehend the impact of heatwaves on trees, it’s essential to understand their cooling mechanisms. Trees absorb water through their roots, which travels via the xylem to the leaves, where it evaporates through stomata, cooling the leaves similarly to how sweating cools humans.

This system functions effectively when water is abundant. For instance, the Fremont cottonwood (Populus fremontii), typically found near rivers, can maintain leaf temperatures 2°C to 5°C cooler than the air during Arizona’s record-breaking heatwave in 2023, even at air temperatures surpassing 48°C.

As temperatures rise, the atmosphere extracts moisture from leaves faster, a phenomenon known as the vapour pressure deficit. Dry soil complicates water replacement, increasing stress and limiting trees’ ability to sustain normal physiological functions.

By closing their stomata, trees conserve water, but this limits carbon dioxide intake, slowing photosynthesis and reducing sugar production necessary for growth and defense against pests and diseases. Open stomata allow cooling but result in unsustainable water loss.

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Each leaf has a thermal threshold, beyond which it suffers irreversible photosynthetic damage. Surviving heatwaves hinges on maintaining enough evaporative cooling to stay below that threshold, though this margin can quickly vanish. A study on poplar trees found that only 72 hours of reduced soil moisture disrupted leaf cooling, causing leaves to reach harmful temperatures.

Why Branches Fall

Prolonged heatwaves can extend damage beyond leaves. When water is limited, embolisms—tiny air bubbles—form in the xylem, blocking water transport, leading to hydraulic failure and causing branches or crown sections to die back.

Some trees sacrifice parts of their canopy to conserve water for the main stem, a strategy called hydraulic segmentation, explaining why healthy trees lose limbs during extended hot weather.

Heatwaves deplete carbohydrate reserves, reducing growth potential and leaving trees vulnerable to future droughts. Surviving one hot summer may only delay death as accumulated stress takes its toll. Water-stressed trees also become more susceptible to pests and diseases due to diminished defense resources.

Many of Britain’s trees evolved in cool, wet summers, and temperatures over 40°C were rare until recently. Current conditions push species beyond their evolved climatic range. Research from the 2003 European heatwave indicates vulnerability depends on heat extremes and species’ adaptability to local conditions. As heatwaves increase, traditional species may struggle outside their comfort zones.

What Can Help?

There are practical measures to aid trees, especially younger and more vulnerable ones. Deep watering is more effective than frequent light watering as it encourages deeper root growth, accessing longer-lasting moisture.

Mulching with hardwood chips or compost reduces soil evaporation, moderates temperatures, and improves soil structure and water retention as it decomposes. Soils rich in organic matter absorb water more effectively, supporting resilient root systems.

Managing mature woodlands for resilience against future heatwaves requires new strategies. Increasing species diversity, reducing tree concentrations in drought-prone areas, and planting species suited to future climates are key considerations. A diverse species mix enhances resilience to hot weather.

Many of Britain’s mature trees have endured for centuries—these strategies may be vital for their future protection.