Autumn is probably most known for 3 things (well, 4 if you count pumpkins…): Fall Foliage, Colder (and shorter) Days, and Bird Migration, but there is way more going on! Why do those leaves change color, who uses those fallen leaves and why are they important to the survival of so many? Let’s examine some of these lesser looked-at changes as summer gives way to autumn.
“Tree Clocks” and Falling Leaves
Trees can “count”, meaning that they can keep a record of time in order to know when is the right time to enter the next season’s mode of operation. They keep track of night vs. day time hours, which makes sense, seeing as the plant relies on sunlight to photosynthesize and has unique adaptations and functions during the night. They also can keep track of temperature. This, among other cues, help to tell a tree how to behave.
In fall, there is less daylight hours, meaning that the tree can’t photosynthesize as efficiently. It takes a lot of energy to produce chlorophyll, the pigment needed to photosynthesize, and if the tree is not producing energy at the same high levels as in the summer in order to make that green pigment, the tree simply takes back the chlorophyll out of its leaves and keeps their molecules for later! When the overwhelming green pigment is taken away, the other pigments that were always there, are revealed.
Eventually, there is so little light that the tree doesn’t find its leaves useful anymore. It takes back all of the pigment, leaving only brown, crinkly leaves. The petioles (stem-like structures that adhere the leaf to the branch) detach as a strong wind blows. The leaf falls to the ground, leaving a tiny scar on the branch. This is called abscission, and trees do this at different times, if at all! Some deciduous trees, like beeches (pictured here) and some oaks, retain some old leaves, and only shed them in the spring when new buds burst into baby leaves and push the old leaves out of the way. This retaining of old leaves is called Marcescence. Examining the shape and size of the scar can actually be a great way to identify tree species in the winter!
These leaves tend to fall exactly where the tree wants. Creating those leaves took a lot of nutrients, which decomposers enjoy munching on throughout the season. This leaves decomposes and sits at the base of the tree, where fungal decomposers can return the nutrients to the tree.
Insects Overwintering in Fallen Leaves
Not only do fallen leaves act as food for decomposers and the parent tree, but they also provide vital habitat to overwintering insects. The endangered Rusty-patched Bumblebee, isopods, Hairstreak moths, ladybugs, spiders, Luna Moths, Wooly-bears (Isabella Tiger Moth caterpillars), and many other arthropods and insects spend their entire winter safely tucked in and insulated beneath these leaves. So please, leave the leaves in the autumn to help protect these insects!
Not all insects require leaves, but another sort of protected home. Some insects produce glycerol, a chemical utilized in antifreeze, as part of a foamy house that protects them over the winter. Others live as eggs and hatch inside of stems and twigs in houses called “Galls”. The adult lays an egg inside the plant tissue, which results in the plant hormones running wild! Instead of growing normally, it poofs out like a balloon or a fluffy bunch, to create a “living room”. Throughout the winter, the insect is protected inside the plant, then chews its way to freedom when it’s warmer!
Decomposition of Leaves Replenishes Exhausted Soil
Earlier, I mentioned the importance of leaves staying where they fell. The leaf litter does feed more than just the trees, though. By the time summer, the growing season, has ended, the soil is often exhausted! It takes a lot of work to feed all of the plants growing quickly!
In a healthy forest ecosystem, the forest itself is a land manager and each part helps maintain one another. The leaf litter becomes a layer of valuable organic matter, which provides good nutrients. Microbes, insects, fungi, and other larger squirmy-wormies break down the organic matter and crawl about in the soil, adding air and space for water to flow. The air flow prevents sensitive plant roots from rotting after a rain storm, and allows for good water infiltration. Unfortunately, healthy woodlands aren’t super easy to come by anymore. Unsustainable development, invasive species spread, and human-caused climate change are all negative impacts on habitats. In curated habitats, like urban and suburban environments where trees are managed by people, ecosystem health needs a helping-hand.
When we remove the leaves from the environment, the health of all plants and animals within that system suffer too. Those decomposing leaves feed the parent tree, but also the rest of the plants growing in the area. Native flowers, pollinators, secondary consumers, and ultimately local apex predators are all at risk of suffering if soil health suffers.
For example, Pennsylvania’s native Joe-Pye Weed relies on moist, loamy soil. Loam is a blend of silt, sand, and clay. Organic matter feeds the loamy soil. The Joe-Pye Weed grows tall and with many blossoms during the summer growing season. These blooms feed over 40 species of caterpillars, provide nectar for countless adult butterflies and moths, and provide food for seed-eating birds in autumn. The hard stems provide overwintering homes to Mason and Leaf-cutter bees, and some Black Swallowtail pupae find shelter in their chrysalis attached to the stems. Then, in fall and winter, as leaves fall around the old Joe-Pye Weed stems, soil microbes, fungi, and other decomposers return those leaves to the plants in the spring in the form of nutrients! The cycle continues and the soil stays healthy.
So, Respect Overwintering Lifeforms and Soil Replenishers!
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