Having spent two years on the North Shore, surrounded by mostly aspens, birches, and evergreens, I was delighted to find that River Bend was full of oak trees. I love all trees, but oaks are among my favorite. One reason I enjoy them is that they stubbornly cling onto their leaves throughout the winter, providing a little splash of color (even if it is mostly brown) in an otherwise drab and naked winter canopy.
But why do oak trees hang onto their leaves? Don’t they know that they’re deciduous trees, and are supposed to lose leaves in the fall? I set out to investigate this mystery.
I discovered that oak trees, along with beech, ironwood, musclewood, and witch hazel trees, have adapted to be somewhere in between evergreen and deciduous trees on the evolutionary scale . Evergreens, like pines and spruces, are the oldest type of tree. They retain green needles all year long so that they can photosynthesize year-round. However, it’s a lot of work to maintain needles, which are subject to frost damage and water loss, and so deciduous trees evolved to combat these issues. They also gained the advantage of having broad leaves to increase their photosynthetic ability in the summer, when the most sunlight is available. In the fall, they remove the green chlorophyll out of the leaves (hence the color change) and drop them when there isn’t enough light to make photosynthesis worth the energy expense.
Most broadleaf deciduous trees drop their leaves in the fall voluntarily. When the weather shifts and the trees deem it a proper time (I’m not privy to the trees’ exact process, but it involves having less daylight), they release enzymes into their twigs, and an abscission layer forms between the leaf petiole and the twig, severing the leaf.
Oak trees and their counterparts tried for the best of both strategies. They don’t photosynthesize year-round like evergreens (except for the live oak and tan oak in the south, which are unique broadleaf evergreens), but they don’t drop their leaves either. This leaf retention is called marcescence.
Scientists have multiple theories as to why oak tree marcescence occurs. For one, marcescence is more common on younger trees or on the lower branches of an old tree. It is possible that the leaves protect the twigs and buds from snow and frost. They may also provide protection from browsing because the buds are hidden from deer. However, a study in Denmark discovered that, when offered a marcescent oak twig compared to a marcescent beech twig, deer preferred the oak, probably due to the leaves’ low lignin content, which is hard to digest. So perhaps oaks don’t rely on their leaves to deter animals as much as other marcescent trees.
Marcescent leaves may protect oak buds from frost.
Another thought is that oaks are preparing for spring. The leaves may help trap more snow in their branches, and when it melts in the spring, all the melt water will end up directly under the tree, giving it a boost of moisture. In addition, when the oaks finally do drop their leaves in the spring, the decomposition of those leaves may provide nutrients at a key time, allowing the tree to outcompete others even in poor soil.
Whatever their reason, oaks and other marcescent trees are a symbol of determination in the face of howling winter winds. They remind us that if we optimistically hold on through the winter, spring will come again.
Northern Woodlands Magazine:
Penn State Extension:
Remember, there’s no such thing as bad weather, only bad clothing and bad attitudes! 🙂