We are well into November and a lot of trees still have their leaves on. However, the leaves did not have their usual fall color and are brown. What is going on?
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| Colorado Public Radio - Royalty Free Stock Photos |
The process of leaves falling involves an understanding of the abscission zone, which is the area between the leaf petiole (leaf stalk) and the branch. The accompanying picture shows the leaf, petiole, branch, and abscission zone. In the abscission zone there are two sets of cell structures, one set of cells is attached to the petiole and the other set of cells are attached to the branch. In a normal fall, the cells attached to the petiole become weaker since the photosynthesis process in the fall is lower. This is the time that the leaves start to turn color as the green covering on the leaves disappears, leaving the natural reds, oranges, purples, and yellows to give the trees their fall beauty. As the days go on, the cells closer to the petiole weaken and can’t hold the leaves on. Thus, the leaves fall off. Meanwhile, the cell structure that is attached to the branch covers up the wound that is left after the leaf falls off. This is to prevent moisture loss from the wound to aid the tree in surviving through the winter.
That is what should happen. What happened this year was prior to the leaves turning color, we had a snow event with cold temperatures. This interrupted the process of leaves turning color by freezing them in place and also freezing the abscission zone. With the abscission zone unable to perform its job of letting the leaves drop naturally, the dead leaves hung onto the tree and turned brown. Some trees like Ash, and some Maples drop their leaves early and did not have a problem. Other trees that lose their leaves later are the ones we see in the landscape with brown leaves hanging on and not looking too great. The ones that fall into this category are the Ornamental Pears, some of the Maples, some varieties of Crabapples, and Hawthorns. A few other trees that fell into the late leaf drop that got caught in this dilemma are Cottonwoods and Aspens.
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| Steve White |
Not to worry. What will happen to the trees that are carrying their brown leaves though the winter is that the winds may blow some off or snow will weigh them down and they will fall. If there are some left on in the spring, the new leaves will push off the dead ones.
There is another group of trees that hold on to their leaves into late fall and early spring when they are pushed off by the newly forming leaves. The trees exhibiting this behavior are Oak, Beech, and Hornbeam. This is natural for these species and is called marcescence that is attributed to the abscission zone failing to develop fully in the fall.