Heatwave: branches crash down from trees under 'stress' It's notoriously unpredictable so it's very difficult from a tree owner's point of view to meet their duty of care. But certain tree species are known to be particularly susceptible to the phenomenon, including beeches, oaks and horse chestnuts, he said. There is often a warning sign before a branch comes down, described as "a sharp crack not unlike the sound of a shotgun going off." General drought stress is also said to be affecting many species of tree during the hot weather, with birches especially at risk. Mr Smith said: "They are quite susceptible to the fact the ground is extremely dry due to a lack of rainfall. It's essentially a wilting effect. It then makes the tree susceptible to other diseases. It lowers the vitality of the tree. Younger trees are known to be more vulnerable to the adverse effects of the hot weather than older ones, which have often already lived through many hot spells. Austin Brady, head of conservation at the Woodland Trust, said: "Essentially ones with deep roots are probably fine because they can find the water. Younger trees are a lot more vulnerable, not just from the dry weather and the prolonged heat but also from fire. There have been more fires recently and if these fires are going through grassland, which often surrounds the trees, then that might be another risk.