This time of year is full of change. The days are getting shorter, temperatures are dropping and animals are searching for a cozy place to bed down. It is easy to observe all of these changes as we see or feel them on a daily basis. What may not be as easy to see are the changes taking place in our trees. Sure, we all enjoy seeing the leaves change color, but why the leaves are changing color and how the tree is chemically preparing for winter is not so easy to observe.

 Trees signal the change of seasons

As the days grow shorter and the temperatures begin to drop, the leaves on a tree receive a chemical signal to stop photosynthesis and reduce production of carbohydrates and sugars. The signal also begins the breakdown of chlorophyll, allowing other pigments like xanthophylls (yellow), carotenoids (orange), and anthocyanins (purple) in the leaves to become more evident as the fall colors we all enjoy. These pigments were ever present, but were not visible due to the dominating presence of the green chlorophyll. At the same time, the place where the leaves connect to the stem begins producing abscission cells, or a thin, bumpy line of cells that seal off the connection and begin to push the leaf away from the stem. Essentially, the tree is forcing the leaves to drop.

The reason this occurs is because the leaves contain water to enable the photosynthetic process. The water in the leaf will freeze during the winter months, and when it freezes it expands. Water expanding inside a functioning leaf would essentially rupture the cells in the leaf, rendering it useless to the tree. Rather than try to grow new leaves after every freeze event, deciduous trees lose their leaves when colder temperatures approach, and produce new leaves for the following summer.

Deciduous trees that don’t lose their leaves

One may notice a few trees in our Michigan landscape that retain dead leaves throughout the winter season. This is referred to as marcescence, and occurs in beech, sycamore, and many types of oak trees. In marcescent trees, the abscission layer does not fully develop until the spring months, if at all. In some cases the leaves are snapped off by wind or other forces. It is thought that marcescent trees may retain the deceased foliage to protect its twigs and buds, containing next year’s growth, from herbivory during the winter months. The dead leaves make the twig and buds less palatable to large herbivores like deer, moose and elk. 

Why conifers don’t lose their needle-like leaves 

Conifers have needles that serve essentially the same function as the annually produced green leaves on deciduous trees. However, conifers needles are typically retained for 3-5 years and lose only the older needles annually. And conifers retain their needles throughout the winter months, usually without damaging the tree. This is due to the physical and chemical makeup of the needles. 

The needles on conifer trees are thick and covered in a waxy layer that helps reduce water loss throughout the year. In winter months, the conifers stop photosynthesizing, and close their stomata, or small openings that allow water to evaporate during photosynthesis. This gives the needles a greyish green appearance (due to the lack of photosynthesizing chlorophyll) and prevents the needles from injury due to frigid temperatures. In addition, the resin content of the needles acts as an antifreeze, further preventing the needles from freezing. 

However, during harsh winter conditions that persist, like those we experienced last year, conifer needles can suffer from “winter burn”, a condition that occurs when the tree dehydrates at the end of a long winter. This usually occurs in March or April, when the sun begins to produce warm temperatures but the ground continues to be frozen. The heat of the sun causes the stomata on the needles to open, releasing oxygen and water. The roots, however, are surrounded by frozen soil and cannot replace the oxygen and water being released. The result is that the needle will turn brown, essentially dying from dehydration. This can also occur during extreme sustained wind events, where moisture evaporates from the needles and cannot be replaced by the roots. It is ironic to consider that a long, windy, cold winter essentially mimics the conditions of a desert environment, where the humidity is low, and the roots have little available water due to frozen conditions in the soil.

More information about how trees prepare for winter can be obtained by contacting Michigan State University Extension. 

Other articles in this series:

• Trees avoid damage from freezing temperatures: Part 2