Trees are often split into two categories of those that lose their leaves in autumn called deciduous trees, and those that don’t, known as evergreens. During autumn, the leaves on deciduous trees change colour and drop off while on evergreens they don’t, so what’s the difference?
Technically, a deciduous tree is one that has foliage for one growing season but for less than a year – it grows them in spring and loses them in autumn of the same year.
An evergreen tree is one that continues to have some live, green, foliage all year round – but – they do still shed older leaves, they just don’t shed the younger ones.
This means that the extra energy required by evergreens to keep leaves all year means that they tend to grow more slowly.
Deciduous trees use the energy saved because it’s not being used for leaves after they have dropped to be quicker growing and potentially less robust, or slower growing and hardier. From an evolutionary perspective, shedding leaves before high winter winds hit reduces the chances of them being blown over.
Hormonal changes in the leaves causes a cell layer to grow between the branch and the leaf stem (the abscission layer) which presses against the stem and cuts off the leaf. If the leaves stayed on the trees, the water inside the leaf cells would freeze in sub-zero temperatures, expanding as it did so and in effect, causing the leaves to split or burst. Come summer, the tree would have no functioning leaves for photosynthesis, so it wouldn’t be able to synthesize food for energy and would soon die.
Although all trees, including evergreens, shed leaves as an active process, for evergreens it is a deliberate process for the removal of old, inefficient leaves to make way for new, more effective ones.
In fact, the further we go north, deciduous trees diminish in number and evergreens increase. The leaves on these trees have a waxy coating and usually a smaller surface area. This means that they are more resistant to moisture loss and harsh climates.
