Ecology

For the love of leaves

Amanda Markle
Strawberry Hill Nature Preserve

(11/2021) There are so many reasons to love autumn; cozy sweaters, hot beverages, pumpkin-spiced everything. It’s a great season to be outdoors. Cooler temperatures and crisp air set the mood for long hikes and backyard bonfires, and autumnal harvests of apples and pumpkins make for picture-perfect family outings (matching flannel optional). For those of us living in a temperate climate, the months of October and November are some of the most visually stunning nature has to offer. As days grow shorter and shadows lengthen, the sunlight itself seems to have extra magic to it; the golden hour on autumn evenings feels extra golden. Adding to the splendor is the most classic hallmark of the season; fall foliage.

The reds, yellows, oranges, and purples that make for such a stunning show come fall are always within tree leaves, but most of the year they are drowned out by the green of chlorophyll.

Leaves are food-making factories for trees, and they are dependent on sunlight. As daylight hours dwindle, chlorophyll begins to break down, and the green pigments fade away, allowing other pigments to shine. Some scientists believe this revelation of colors is not simply a byproduct of chlorophyll breaking down but is actively advantageous for the tree. Researchers studying tree physiology theorize that the way leaves change their colors may have developed over time to help prepare the tree for the winter months.

Anthocyanins, a type of flavonoid, are found in high concentrations in changing leaves, especially ones that are strongly red-colored. Anthocyanins are antioxidants and may help regulate the aging process. The anthocyanins can also help the tree in retaining nitrogen. As chlorophyll breaks down, the anthocyanins act as a sort of sunscreen, protecting the now vulnerable chloroplasts and making it easier for trees to reabsorb nitrogen. Evolutionary biologists think the changing colors may hold strategic benefits for the tree as well. Theories include that the changing colors may confuse and deter animals that typically eat green leaves, or perhaps make it more difficult for pests to camouflage from predators within the leaves. Others believe that the brightly colored leaves may signal the availability of ripe fruit, helping to attract birds and other animals that aid with seed dispersal.

The hue and intensity of autumn leaf colors are dependent upon several factors, including temperature, rainfall, and daylight hours. Cloudy and overcast skies might not be what most people imagine as their ideal fall day, but they are great for bringing out more intense colors in the foliage. These arboreal performances have legions of fans that eagerly await the changing colors each fall. Public opinion surveys consistently find fall to be the most popular favorite season, and when people are asked what aspect of the season they most look forward to, the changing color of the leaves routinely earns the top spot. Every autumn, local new stations eagerly report on which areas are reaching peak color, and leaf peepers flock to the choicest viewing destinations.

All good things must come to an end, however. As the season progresses, leaves dry out, become brittle, and fall. The process is less passive than one might think. Trees play an active role in shedding their foliage each fall. At the point where the stem joins the tree branch, trees develop a special layer of cells called abscission cells. These cells gradually sever the leaves from the tree and push the leaves off. The point of separation when the leaves finally do fall is sealed off, helping to protect the remaining branch. Some trees do keep their leaves all year round, but it’s a risky venture. A rouge warm snap during the winter can trick leaves into food-production mode, drawing water up into their narrow veins. If temperatures quickly drop, and that water freezes, the leaf will die and be useless to the tree come spring. For most trees, it is safer to shed their leaves entirely each fall and invest in growing new ones when longer daylight hours return.

The initial fall of autumn leaves has its charms; many people love the crunch and swish of dry leaves underfoot and jumping in leaf piles has been a favorite pastime of children and dogs for generations. But for many, the multitude of dead leaves on the ground quickly become old, both literally and figuratively. The rainy weather that often comes with the season can make leaves heavy, slippery, and smelly. Raking up and bagging up leaves can seem like a never-ending task and finding ways to dispose of them can be a challenge. While they may not be the most attractive landscaping option, the best thing to do with all those fallen leaves may be to leave them be.

Dead leaves make excellent winter insulation for garden beds and overwintering plants, and as they decompose, they add valuable nutrients to the soil. If an entire lawn covered in decaying leaves seems too much for you (or your neighbors) to aesthetically handle, mowing over dead leaves can help speed up the process of their breakdown. Leaves can also be composted or piled up under bushes and in less noticeable corners of garden beds. It’s almost shocking the number of leaves that can break down over the course of a single winter; a pile two feet high will likely be gone entirely, or simply need one last mowing over by the time the next planting season arrives.

A thick layer of decomposing leaves, or leaf litter, isn’t only beneficial for your garden; it’s also essential for wildlife. Many animals rely on leaf litter for camouflage, nesting materials, food, and shelter. The moisture retained by decaying leaves makes it a perfect environment for amphibians like toads and salamanders. Insects thrive in leaf litter; it’s particularly important for one of summer's most popular natural spectacles (and the official Pennsylvania state insect)- the firefly. Fireflies spend most of their lives as larvae living in leaf litter; hunting and devouring earthworms and other insects they find there. If you want the enchanting glow of the adult form to grace your backyard on summer evenings, one of the best things you can do is leave leaf litter on your property to act as a firefly nursery the rest of the year.

Early November will be the peak season for fall foliage in many parts of Maryland and southern Pennsylvania. Observing and appreciating the changing landscape is a great reason to get outdoors before temperatures start to drop. When the leaves fall and begin to pile up, remember that even though they may be a landscaping nuisance, dead and decaying leaves play an important role in our ecosystem and have many benefits to the environment year-round. No matter the season, there are so many reasons to love leaves!