Why Don’t Evergreens Loose Their Needles?
Backyard Wisdom - November 2019
by: Gilbert A Smith, ISA Board Certified Master Arborist
Everyone knows that deciduous trees go dormant and shed their leaves so that they don't freeze in the cold winter. So why don't evergreens shed their needles too? Actually, they do loose their older needles continuously throughout the year, as the older needles are shaded out by younger ones. This may happen most heavily in the fall, as is demonstrated every autumn, by our clients who begin to call, worrying about brown needles on their evergreens. The good news is, they never lose all of their needles, unless they’re dead. Evergreens are green throughout the year, but they don’t have the same needles forever!
Photos by Lesley Bruce Smith
I’m generalizing here, but evergreens are in the class of plants called Gymnosperms that evolved much earlier than deciduous plants, in the class called Angiosperms. The evergreens are physiologically simpler and more rugged, able to withstand more difficult (colder, hotter, or drier) environments. So, wherever you see evergreen trees grow natively, you can assume a more difficult environment. If you remember that you see most evergreen forests in the mountains where conditions are often dryer and colder, this makes sense. When we drop down into the lower elevations though, you will find some evergreens, but the dominant plants in our temperate forests are deciduous trees. Deciduous trees, which evolved later, are more efficient at harvesting food from the sun with those lovely broad solar panels, their leaves. Those solar panels also steal all the food (sunshine) from the evergreens. As a result the evergreens stay up on the sunny mountains where they do not get out competed by the deciduous trees.
The needles on the snow beneath this Eastern White Pine are demonstrating the natural cycle of needle growth and attrition. Leaving the needles under a tree helps to keep it healthy by providing a natural mulch.
The Evergreen needles are able to withstand cold temperatures because they are smaller, with less surface area to dry or freeze than the large deciduous leaves. Their breathing holes, called stomata, are on the bottom of the needles and remain closed in the cold of winter to protect them from temperature extremes. Also, the needles and vascular system are saturated with terpenes (think turpentine) which besides giving us that lovely Evergreen smell, protect the trees from freezing, drying, or being attacked by bugs, bacteria, or beavers. Finally the roots, shoots and trunks of Evergreens have a vascular system that is slightly less efficient because it is smaller and not as continuous as deciduous trees, but that structure defends Evergreens from freezing and fungus.
So the Evergreens don't need to lose their needles to withstand the cold winter blasts. They can continue to harvest food from the sun year round, like the slow steady tortoise, not sprinting ahead every summer like the hare.
Mother Nature’s Moment - Why Didn’t the Leaves Fall Off My Trees?