Tuesday, December 17, 2013
Most people are aware that deciduous trees lose their leaves seasonally while evergreens hold onto their leaves and do not lose them all at once. While it is true that the shorter day length and less opportunity for photosynthesis play important roles, we should also be aware of the vital part water loss plays in this occurrence. Tree leaves constantly lose water through transpiration, but have many adaptations to mitigate this and are of course getting replacement water through their roots.
In winter however, water can freeze and not be available. Deciduous trees lose their leaves to help prevent this loss. Evergreen trees are able to hold onto their leaves and still photosynthesize to some extent due to some other adaptations. Most evergreen leaves for example have a thick waxy cuticle covering to reduce water loss. Trees with needle-like foliage (such as the White Pine in the first photo) have smaller surface areas and fewer pores to lose water. Other evergreens can roll their leaves inwards and/or droop their their leaves to also reduce water loss (such as in the second photo of a Rosebay Rhododendron).
Other adaptations adaptations help evergreens to shed heavy snow, such as downward pointing drip-tip leaves (as in the American Holly photo) and being extremely flexible. Another problem however is that herbivores would especially target these green leaves when most trees have lost their leaves or have just dry, hanging foliage available. Many evergreens therefore are distasteful or have some other means to make themselves less desirable (such as the spiky leaves of the American Holly).
Evergreens have been symbolic of renewed life and hope for greener seasons for centuries, being the reason for Christmas trees and wreaths in our winter households originally. Understanding why they can do this just adds to their mystique.