Another autumn has arrived which may mean another season of constantly raking up leaves in your yard. Unless of course your yard has evergreen trees that don’t shed their leaves all at once in the Fall. We’ve all taken note that some trees predictably loose their leaves sometime between late summer and early winter, whereas other trees seem to hold on to their leaves all year round, but why do these different behaviors exist for trees? The terms we use here are deciduous trees for those that loose their leaves all at once in the fall and evergreen trees that keep their leaves through the winter. Let’s explore why these two strategies exist.
Winter is a challenging season for most organisms on this planet. Cold temperatures and shorter days present many difficulties for life, and trees are no exception. As days start to get shorter in the fall, trees begin to slow their metabolism to accommodate for the reduction in sunlight. Plants grow due to their leaves feeding on sunlight through a process called photosynthesis. Photosynthesis requires sunlight, water, and carbon dioxide. Sunlight is in low supply, and water may be frozen and unavailable come winter. Trees that retain their leaves can quickly become dehydrated when there’s a lack of water because leaves constantly release water into the atmosphere regardless if any new water is being absorbed by the roots. And as temperatures decrease, the risk of frost damage to sensitive leaves increases. Furthermore, the additional surface area of leaves can act as a sail during winter storms, making it much more likely that branches will break when strong winds arrive. These unfavorable growing conditions along with the risk of injury cause many trees to simply drop all of their leaves and go into dormancy. As they shed their leaves, these deciduous trees are able to suck out some of the carbohydrates from the leaves and store this extra energy in their roots to better prepare for the long winter ahead.
*As a side note, some of the carbohydrates that are transferred out of the leaves are from Chlorophyll, the pigment that makes leaves green. This allows other pigments such as anthocyanins and carotenoids to reveal their yellow, orange, and red properties, gracing trees with their celebrated fall color.
A significant disadvantage to being deciduous is that once spring comes and trees are ready to photosynthesize again, they have to dedicate an enormous amount of energy and time to growing new leaves before they’re able to grow again. Evergreen trees avoid this issue by retaining their leaves all winter, but they require some additional adaptations in order to do so. The main characteristic that is often seen on evergreens is much hardier leaves. Think of a pine needle or a coast live oak leaf. These leaves are hardened by wax, decreasing the amount of water they lose, and better protecting them from harsh winter conditions. The leaves of evergreens are also often much smaller than their deciduous counterparts, decreasing the surface area for water loss and lessening the sail effect during storms. Evergreens still dramatically slow their metabolism during winter, but the retention of leaves does allow them to continue to photosynthesize in winter, albeit at a much slower rate than in the spring and summer. These trees also have the advantage of having a full canopy of leaves when favorable growing conditions return.
Both deciduous and evergreen trees have their reasons for displaying such strategies, and we’re lucky to have such a variety of both types of trees in our Sonoma County landscape. If you’ve been considering getting any of your trees pruned, now is a good time to schedule that. Winter is a great time to prune most trees to set them up for a healthy growing season in the spring and summer. If you have any questions, reach out to Cameron Long and LongView Tree Care today for a free consultation!