Across the UK, autumn looks to have come early this year with many leaves turning brown and falling during August’s drought. But this apparent seasonal confusion is a defensive reaction to conserve water.
Leaves are the tree’s lungs. They are covered with tiny pores called stomata that take in carbon dioxide and exhale oxygen. The pores provide a large surface area that makes them highly efficient but can be a liability in hot, dry conditions, as it increases the rate of moisture loss from evaporation.
A tree may respond by closing its pores. Sometimes this is enough, but even with pores closed the leaves continue to lose some water. Capillary action draws up more water from inside, and as the tree dehydrates air bubbles may be drawn into it. These can cause serious damage and death.
To prevent this, if dry conditions persist a tree may start to withdraw resources from its leaves so they turn brown and fall. Sometimes the entire tree is affected, although in some species a tree can sacrifice one branch while the rest stay green.
Drought-induced leaf fall is not in itself a catastrophe, but it is a sign of the additional effort that plants are taking to stay alive. Longer and more severe droughts will cause more stress and increased tree mortality.
