Arboretum Paths: Why are some leaves not falling from trees?
Published 7:39 pm Wednesday, January 4, 2012
Have you ever noticed that some trees have leaves that never seem to fall? The next time you take a walk or drive through Pearl River County, look around. Obviously, you’ll see that most of our area’s deciduous trees have shed their leaves. However, you may notice other tree species that are still holding fast to their crop of brown leaves.
Trees with persistent leaves along our local roadsides and woodlands will most likely be species of oak, and are now extremely easy to distinguish from the surrounding bare trees. They may also offer your family a fun opportunity for learning. Because you can now discern these trees, you can learn to recognize their branching structure and form, and get a better idea of the areas where they prefer to grow.
On a road trip to Tenn., earlier this month, I enjoyed observing the exquisite patterns of the South’s forested hillsides sweeping past my car window. Especially attractive were the groups of American beech sprinkled in the forest understory, which is one of my favorite trees. Like the aforementioned oaks, beech also retains its leaves during the winter. And, high on a hillside overlooking Dayton, Tenn., I had another treat when parking under the spreading canopy and structural beauty of a beech tree in my brother’s front yard. This tree, perhaps forty or fifty years in age, was thoroughly laden with beech nuts and distinctively shaped buds.
Beech tree leaf buds are very different in appearance than those of the oaks. Oaks have clusters of many buds at the ends of their twigs, while beech have single terminal buds and look like tiny rolled cigars, pointed on the ends. When new leaves appear in the spring on our beech trees at the Arboretum, they are fascinating to watch. Rapidly, trees once covered in last year’s dead leaves are completely transformed, soon cloaked with the emerging, delicate new leaves. Children are pleasantly surprised when they discover that the backsides of beech leaves, whether green or brown, feel exactly like velvet.
Because beech trees (Fagus) share the same trait of persistent leaves as seen with some of the oaks, it will probably not surprise you to learn that they are members of the same family, Fagaceae, known as the beech family. Oaks (Quercus) are the most well-known family members. Chestnuts (Castanea), which includes chinkapins, are also part of this family.
Tree species that have persistent leaves are not totally evergreen, or deciduous. This condition of retaining dead leaves through the winter months is known as marcescence.
Evergreens were the earliest occurring types of trees. Over time, deciduous tree species which lose their leaves in the fall developed as plants gradually adapted to seasonal temperature changes. While evergreen trees have leaves that are retained year-round, allowing them to continue to provide nutrients to the leaves through the process of photosynthesis, deciduous trees will rest or “hibernate” during the colder winter months. Deciduous trees also will shed their leaves as a survival response to stresses such as drought or disease, in order to conserve water or avoid further infection.
Ecologists have debated whether trees with markescent leaves have developed a permanent, advantageous adaptation, or if they are simply evergreen species that are slow to develop into being fully deciduous.
Scientists have theorized that marcescence, or delayed leaf drop, may have evolved as a beneficial trait for trees growing in dry, infertile conditions. By waiting to release the nutrients contained in their leaf matter until the spring, these trees conserve their organic matter until the optimal time it is needed for growth. In nutrient-poor environments, even a small difference in timing such as this may allow the trees an advantage, allowing them the maximum benefit through the efficient release of nutrients once the leaves decompose.
Other theories for leaves persisting through the winter are that the adaptation provides protection for the leaf buds, for instance, by making them less susceptible to browsing by wildlife. In colder climates, retained leaves will capture snow, and this may benefit the tree by providing more water, as the snow melts, to the tree’s root zone.
Remember the days when you took great delight in asking your parents and teachers – why? Or, perhaps you have the task today of entertaining children or grandchildren who never seem to lose interest in posing this simple question. Many botanists and ecologists have certainly spent time pondering the existence of marcescent leaves and theorizing about their purpose. Consider this: Both beech and oak trees yield nuts that provide tasty eating for numerous species of wildlife. Would not a tree with persistent leaves provide the ideal spot for birds and mammals to munch? That nice crop of brown leaves gives them some great cover. Certainly it shields them to some degree from chilly winter winds, and it most definitely gives the critters some camouflage in forests now primarily populated with bare trees. Doesn’t it just seem like nature has a way of working it all out?
Mark your calendars! Many secrets will be revealed on February 4 on methods for identifying trees in winter by their buds, bark, and other characteristics on the Crosby Arboretum’s winter botany field walk with Dr. Glenn Hughes at 10 a.m.
But if you can’t wait until then, come take a walk Wednesday through Sunday from 9 a.m. to 5 p.m. The Arboretum is located in Picayune, off I-59 Exit 4, at 370 Ridge Road (south of Walmart and adjacent to I-59). For more information on the Arboretum, please see our website at www.crosbyarboretum.msstate.edu, or contact the office at 601-799-2311.
For further exploration: What is a mast, and what determines a “good mast year”?
Research the tree species that produce mast in our area and learn which wildlife species consume the nuts.