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Dead leaves cling on to some trees

  • Beech trees and leaves Ruth Smith—For the Monitor

  • Beech trees hang on to some of their leaves through the winter and only drop them in the spring, when new growth forms. Ruth Smith / For the Monitor

  • Red oak trees are among those that keep some of its leaves during the winter instead of dropping them in the fall. Ruth Smith / For the Monitor

  • Beech trees and leaves Ruth Smith—For the Monitor



For the Monitor
Friday, November 03, 2017

Recent heavy winds and rain have brought down many of the autumn leaves, but there are some that will remain on the trees long past autumn, through the winter until next spring.

I’m not just referring to the leaves or needles on the evergreen trees. There are broad-leafed trees that keep their leaves, even though they are dead and brown. The American Beech, some oaks trees, hornbeams and witch-hazel are known for this trait. Retention of dead plant material is called marcescense.

Before we explore marcescense, it’s important to understand what happens to leaves in the autumn.

The green leaves of summer get their color from the light-absorbing pigment called chlorophyll. I consider this substance to be magical because it converts the sun’s light into sugars and food that we can eat. We owe our existence to chlorophyll.

But it is rather ephemeral and requires considerable amounts of light for its production. As daylight is reduced with the changing seasons, leaves stop producing chlorophyll and other colors are revealed and produced in the leaves. Carotene (orange) and xanthophyll (yellow), which have been present in the leaves all along but are overpowered by the green, are revealed. Trapped sugars in the leaves will trigger production of anthocyanin which causes leaves to turn red.

The timing of fall foliage depends on temperature and moisture, but the progression of change is chemically triggered within the leaves.

While all of these color changes are going on, hormones in the leaves are halting growth. A layer of corky cells known as the abscission layer is produced where the leaf connects to the twig. This seals off the tree and prohibits the transfer of nutrients from the leaf to the tree and water from the tree to the leaf. This layer has less lignin, the glue that holds cells together. Without this adhesive, the leaf is only hanging on by its thread-like veins and can easily be torn off by wind or heavy rain.

In the case of the beech and oak trees, the abscission layer begins to form later in the fall. The leaf blade and petiole (leaf stem) die, but the very base remains active until it is paused by colder weather. Thus, the beech and oak leaves are forced off in the spring when the abscission process is resumed.

Why this occurs in these particular species is a subject of much speculation. There is no single answer but a variety of factors probably have contributed to this peculiarity. One theory has to do with the fact that members of the beech family (including oaks) evolved in more southern regions. Many species that are still found further south are actually evergreen, retaining some of their leaves all year. As the American beech, red and white Oaks and other species moved north in their distribution, some of the residual traits have been retained. These species are an example of the continual evolution from evergreen to fully deciduous.

Other theories have to do with the beneficial nature of the retained leaves. They have been shown to protect the buds from browsing deer and moose. In addition to concealing the buds, the dried, rough leaves are unpalatable and therefore deter the herbivores. This may also help explain why it is generally lower branches and younger trees where the leaves are retained, places more accessible to browsers.

Regardless of why beeches and oaks retain their leaves through the winter, now is the time to enjoy their final act on the foliage stage. The sunny yellow and butterscotch beech leaves and russet oaks have taken over from their more flamboyant predecessors, the lemon yellow birches, crimson reds and flaming oranges of the maples. The beech family may not be as brilliant but even as their colors fade, they will add tan and chocolate colors to the stark winter landscape as well as texture, movement and sound as the wind rattles their dry blades.

Marcescent leaves also provide an opportunity for observation. Since the scientists haven’t arrived at all of the answers, it seems that some secrets are still waiting to be revealed. As you explore the woods, can you come up with an idea of why the leaves are remain on the trees?