Imagine this: you’re on holiday. After visiting an idyllic town or an adventurous hike through the mountains, you contentedly seek out a sunny terrace. You lean back and feel the sun’s rays warming your face. After a while, you notice your face getting very warm; it even starts to burn. Once back at the holiday home, you look in the mirror and are shocked. Your whole face is bright red!
Sunburn in Trees
Sunburn is not only a common problem for people but also for trees. Sunburn occurs when trees are exposed to intense sunlight, especially during extremely hot days or after a period of cloudiness. The latter may sound strange, but when the clouds suddenly disappear, the amount of UV radiation reaching the trees quickly increases. This abrupt increase can overwhelm the trees’ protective mechanisms, leading to sunburn. Additionally, there is often a lot of sun reflection from pavement, buildings, and heat radiation from surfaces like asphalt. This also contributes to the development of sunburn in trees.
| Trees are our natural parasols.
How Does Sunburn Occur in Trees?
At the hottest part of the day, the sun beats down on the trunk, often on the south side. Beneath the bark, the tree’s sap flow takes place. The sap flow acts like a radiator in the tree, normally providing cooling. In extreme heat, the tree’s sap flow cannot cope, leading to stress and potential damage. The result is that the cells overheat and the cell walls burst. As a result, the cells die. This is also known as ‘necrosis’; the death of healthy, living cells. Necrosis comes from Ancient Greek and is derived from ‘Nekros’, translated ‘corpse’. The consequence of necrosis is that the bark cracks open. Fungi, mushrooms, insects, and moisture (rot) can nest in these cracks, weakening the tree and ultimately causing it to die.
| Trees protect us from sunburn, but they also need our protection against it.
Trees damaged by harmful UV radiation.
Young Trees, Heavily Pruned Trees, and Thin-Barked Trees
Young trees, old trees that have been heavily pruned, and trees with thin bark (a sensitive ‘skin’) are particularly vulnerable to sunburn. Examples of tree species with thin bark include Fagus (beech), Carpinus (hornbeam), and Tilia (lime). Sunburn can lead to bark damage, moisture loss, and ultimately the death of branches or even the entire tree. Besides direct damage, sunburn weakens trees’ natural defences. This makes them more susceptible to diseases and pests.
Continuous Increase in UV Radiation
The study “Increasing Surface UV Radiation in the Tropics and Northern Mid-Latitudes due to Ozone Depletion after 2010” (2023) shows that UV radiation levels remain high worldwide in 2023. In some regions, there is a continuous increase in UV radiation caused by the delayed recovery of the ozone layer. This increase has been observed particularly in the tropics and temperate areas of the northern hemisphere since 2010. Despite international efforts like the Montreal Protocol to restore the ozone layer.
Trees Protect Flora and Fauna from UV Radiation
Researchers have found that trees can protect flora and fauna from harmful UV radiation by providing shade with their canopies. Additionally, many tree species produce protective substances such as flavonoids and phenols in their leaves, which act as natural UV filters. These substances absorb and scatter UV radiation, reducing damage to plant cells. These mechanisms not only help the trees themselves but also the underlying vegetation and animals that depend on these trees. This is discussed in the chapter “Ultraviolet radiation and its effects on plants” in the book Plant Stress Physiology (2023).
| Did you know that a good sunscreen not only protects people from harmful UV radiation but also trees?
Trees Need Protection
Effective prevention of sunburn in trees can involve measures such as selecting climate-resistant tree species and using trunk protectors. Trunk protectors made of materials like coconut, bamboo, and jute are breathable and protect the tree against sunburn. Additionally, trees can be smeared with a type of ‘tree sunscreen.’ This white trunk protection paste, called ArboFlex, reflects sunlight instead of absorbing it. This helps keep the trunk temperature low and prevents overheating.
Regularly providing enough water, especially during hot summer months, helps trees stay hydrated and resilient against UV stress. Good water management is crucial to prevent dehydration and damage from UV radiation. To provide the right amount of water, water reservoirs, underground irrigation systems, and watering bags are frequently used.
Take Care of Trees
Trees, especially mature ones, are invaluable for cooling our planet in the fight against climate change. Therefore, we must take good care of them. Since 1995, GreenMax has been the specialist in trees in public spaces. Our mission is to give trees a place among concrete, pavement, and buildings. We are happy to work with you to keep the trees in your area healthy and thriving during a completely non-binding consultation. Will you join our mission?