Heatwaves are not just a summer nuisance; they are a silent killer for Britain's trees, shedding branches and, in some cases, dying. The Royal Botanic Gardens at Kew, a lush oasis on the outskirts of London, has become a stark reminder of the impact of climate change. A brightly painted skeleton of a dead oak tree, known as the Climate Changed Oak, stands as a haunting monument to the changing climate. This tree, along with 400 others at Kew, succumbed to the scorching heatwave of 2022, and predictions indicate that up to 50% of Kew's 11,000 trees could be at risk by 2090. But why are trees falling victim to these extreme weather events? And what does this mean for the future of our forests?
The Cooling System of Trees
To understand the vulnerability of trees during heatwaves, we must delve into their cooling mechanism. Trees, like humans, sweat to stay cool. When water is absorbed by the roots, it travels through the xylem, a network of microscopic tubes, and evaporates through stomata, tiny pores on the leaf surface. This process cools the leaves, preventing them from overheating. However, this system is only effective when water is freely available. In the case of the Fremont cottonwood, often found near rivers, this process can keep leaves 2-5°C cooler than the surrounding air during extreme heatwaves.
As air temperatures rise, the atmosphere draws water from leaves more rapidly, a phenomenon known as the vapor pressure deficit. Concurrently, dry soil makes it harder for roots to replace water losses, placing trees under increasing water stress. Trees respond by closing their stomata to conserve water, but this restricts carbon dioxide uptake, slowing photosynthesis and reducing sugar production. This delicate balance is crucial for tree survival, and when disrupted, it can lead to irreversible damage.
The Fall of Branches
As heatwaves persist, problems can spread beyond just the leaves. With xylem water under constant tension, limited water supplies can lead to the formation of tiny air bubbles, known as embolisms, within the xylem. This process, called cavitation, results in blockages that impede water transport. If enough xylem vessels become blocked, the tree experiences hydraulic failure, and water no longer reaches the canopy effectively. This leads to the death of branches or entire sections of the crown.
Some trees employ a strategy known as hydraulic segmentation, effectively sacrificing parts of their canopy to preserve the main stem. This adaptation helps explain why seemingly healthy trees may lose substantial numbers of limbs during prolonged hot weather. The impact of heatwaves can be felt long after the event, as trees deplete their carbohydrate reserves and reduce growth potential, leaving them less resilient when the next drought arrives.
The Evolving Climate and Tree Vulnerability
Britain's trees have evolved under relatively cool, wet summers, and temperatures above 40°C were unheard of until recently. However, many trees 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, traditional species may increasingly find themselves outside their climatic comfort zone.
Protecting Britain's Trees
So, what can be done to protect Britain's trees from the ravages of heatwaves? Fortunately, several practical steps can help, particularly for younger and more susceptible trees. Occasional deep watering is more effective than frequent light watering, as 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 and moderates soil temperatures, improving soil structure and water-holding capacity.
For mature woodlands, new strategies are needed. 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, and many of Britain's mature trees, having stood for centuries, may be the most effective ways of protecting them for the future. While the battle against heatwaves is far from over, by understanding the vulnerabilities of our trees and taking proactive steps, we can help ensure that Britain's forests continue to thrive for generations to come.
