Evergreen trees, such as many conifers, often have narrow, needle-like leaves and a thick waxy coating and these adaptations help them to conserve water during winter.
Deciduous trees, such as oak, ash and beech, shed their leaves in the autumn. On frosty winter days, the water in the soil is frozen, so it cannot be taken up by the roots; the air temperature may be quite warm if the sun is shining, so if leaves were still on the trees they would lose a lot of water and wilt. This would result in the death of the tree. So dropping the leaves before winter sets in is the most sensible thing a deciduous tree can do! They can ‘tick over’ during the winter months using stored energy in their roots.
In the autumn a corky layer forms at the base of deciduous leaves, cutting off water supplies. This causes the green colour (chlorophyll) to fade, revealing shades of yellow, orange or red beneath.