Week of October 23, 2022 – October 29, 2022

Sunny Guyette Staff Member and Environmental Educator

by Sunny Guyette, Environmental Educator

Crunch, crunch, crunch! As the season changes, we may notice the trees that turn from green to vibrant shades of yellow, orange, and red are losing their leaves, while other trees stay green year round. But why does this happen? Why do some trees lose all of their leaves in the fall and some keep their leaves all year round?

Just as we know the signs of season change and have our preparation routine, trees do too! All summer long, the trees are using most of their energy to disperse chlorophyll to the leaves in order to store as much nutrients, and water as possible. But as we head deeper into fall and eventually into winter, we may notice that the days are getting shorter, and we do not have as much daylight as we did in the summer. The temperatures are dropping, and rain slowly turns into snow. The trees detect this change in season and decide that it is time to stop dispersing chlorophyll to the leaves in order to save energy as we head into a drought of water and sunlight. Hence why we see the color of the leaves change from green to yellow, red or orange. With this, the trees will also decrease the production of auxin, a growth hormone. This reduction in auxin will weaken the abscission layer resulting in the leaf breaking from the branch with even the lightest gust of wind.

Trees that lose all of their leaves in the fall are called deciduous trees, and these trees often have thin, broad leaves. Some examples of deciduous trees that we see in Upstate New York are maples, birch, ash, or aspen. The trees that have thick, needle-like leaves and keep their green leaves or needles during the winter are called conifer or evergreen trees. Examples of coniferous trees are pine, spruce, hemlock, or balsam fir.

Now you may be wondering about why conifers do not lose their needles. Well, although there are evergreens that will lose all of their needles in the fall like the Tamarack or Eastern Larch, most evergreens will shed their needles, just not all at once. Evergreens will continue to grow back their needles all year round because the needles are built to retain water and nutrients unlike the thin broadleaf deciduous trees. Evergreen needles have a thick, waxy layer that helps reduce water and nutrient loss, so it is beneficial for the trees to expend their energy all year long.

Falling leaves are something we all expect; it is something that we just know happens. But understanding why it happens allows us to see the process from the tree’s perspective and can even cause us to wonder if trees are able to consciously make these decisions?

To learn more about the abscission layer, click here!