More than Magic: The Importance of Fall Colors

Introduction

It's that time of year again! Cooler temperatures, the low hum of leftover crickets and katydids in the trees, pumpkin spice everything, and that sweet smell of decaying leaves that permeates local parks and yards. But best of all are the striking fall colors in parks and woodlots across the northeastern United States (figure 1).

Figure 1: Fall color at sunset at Sand Ridge Nature Center in Illinois, 2016.

Every fall in eastern forests, what was once a garden of green transforms to a gallery of color, ranging from bright yellow to blood red to a variegated orange. The glory doesn't last long, and soon the trees are barren and browned as dried leaves flutter to the forest floor (or into your gutters or on your car's windshield!).

I've always had a love-hate relationship with fall, and in an earlier blog post I admitted my distaste for nature's summer finale. But the one thing I will agree on with autumn-enthusiasts is the showy blaze of color that dazzles the roadways and trails throughout the northeastern chunk of our continent.

A question that might be on the back of your mind, however, is why do trees turn colors in the fall? It ends up that not all trees turn colors in the fall and that this is a phenomena that is unique to temperate climate zones across our globe. And while the colorful glory of fall is usually well appreciated by many, the ecological importance of fall leaf-drop is a process of paramount importance to most ecosystems within the northeastern United States.

From food production to self-protection: Why do some tree leaves turn colors in the fall?

Leaves are green...or are they? The majority of leaves are green because of a cellular organelle within the tissue of a green leaf, known as a chloroplast, that releases a pigment known as chlorophyll which reflects green light (see figure 2). Chloroplasts help drive an important process that most plants undertake that allows them to produce food from light energy (there are some plants that do not have chloroplasts, but that is beyond the scope of this blog post). The process driven, in part, by chloroplasts that converts light energy into plant food is known as photosynthesis. Figure 3 shows a highly simplified model of the photosynthesis process.

Figure 2: Chloroplasts within a plant cell
(an individual chloroplast is indicated by the red arrow)

Figure 3: Photosynthesis.
Image from Wikimedia Commons

Plants that photosynthesize are able to produce their own food from light, and the ramifications of photosynthesis for many animals, including humans, is profound. Nearly every animal species that we're familiar with relies on plants for food, including that family member or friend that everyone seems to have that's on that all meat "paleo" diet. That's because most terrestrial and aquatic animals are a part of a food chain that starts with herbivores (primary producers) and concludes with predators (secondary and tertiary producers). The many different food chains that exist across the world are a part of a greater food web that helps maintain ecological sustainability. And another benefit that might be apparent to you if you study figure 3 is that the byproduct of photosynthesis (aka the "exhaust") is oxygen, which we need in order to breath!

In the northeast United States, plants are secretly working away at pumping out oxygen as they convert light energy into plant food. In most tropical and subtropical regions, plants do this throughout the year (figure 4), but in moist temperate regions, such as where I am in the greater Chicago area, most plants cease photosynthesis in the fall as cooler temperatures set in and as the availability of daylight decreases.

Figure 4: In tropical areas, evergreen plants, such as the palm trees
that adorn this beach in Southern Florida, photosynthesize year-round.

Broad-leaved plants in moist tropical environments can expose their foliage to the bright sun year-round, but in colder climates like the Chicago area nearly all broad-leaved plants must protect themselves from cold and dry winter air. Trees in temperate forests must protect their assets in order to survive and ensure that they can continue to produce food to sustain themselves in the future. As winter approaches, they must drop their leaves and go into dormancy until warmer and wetter conditions return in the spring. When the leaves start turning colors in the fall is when you know the transition to dormancy has begun!

But why do leaves that have been green almost the entire year suddenly start turning so many different colors? To let you in on a little secret, the "true colors" of the leaves become visible when chlorophyll begins to dissipate. The relative concentration of chlorophyll, along with the fact that green pigment is stronger than reds and oranges, masks the other pigments in a tree's leaves. As the tree prepares to go into dormancy, photosynthesis temporary comes to a halt. The chloroplasts move to the outer edges of the leaves and begin to decompose, allowing other pigments to become visible. While this is going on, the tree forms an abscission zone at the point where the leaf is attached to the stem, and this disconnects any flow of water and nutrients to and from the leaf (figure 5). Eventually, the leaf dries and detaches from the tree.

Figure 5: Leaf abscission.
Click here for image credits.

More than just pretty: Why leaf fall matters
Many species of broad-leaved trees in the northeast United States drop their leaves in the winter and go into dormancy to prevent themselves from desiccation (drying out). The process of entering dormancy leaves us for a brief amount of time a forest filled with color. The leaves fall to the ground, and the tree replaces its lost leaves with new ones in the spring.

The question we need to ask ourselves about fall color and leaf drop, however, is so what? Summer ends, we are dazzled with a display of showy colors, and then the snow starts to fall and it's winter. We huddle by the fireplace inside, or we frantically do our holiday shopping, all the while missing out on the most important part of fall: rotting leaves.

Figure 6: Soil horizons, or layers.

You heard me right: Rotting leaves. The process of leaf drop and the subsequent decomposition cannot be overstated in terms of its ecological importance. The decomposition of leaves replenishes the soil with nutrients that plants need to grow, thus eastern temperate forests actually self-fertilize! The assortment of rotting leaves and other decaying material is known as detritus, and detritus eventually breaks down further into hummus, or topsoil (figure 6). Topsoil is one of the most important layers of the soil profile, and most of our crops rely on the fertility offered by natural topsoil. Thick mats of fallen leaves also help prevent erosion and moisture loss from woodland areas, and it creates habitat for dozens of decomposers, including insects, crustaceans, fungi, and even animals like salamanders that prey on decomposers. Figure 7 shows a common animal that is among the decomposers, the "rolly-polly" isopod that many are familiar with.

Figure 7: Isopod, a common decomposer. Image source: Click here

So you see, fall leaf drop is not just important for its aesthetics, but also for its necessity in maintaining ecological stability. The health of our forests and many other habitats rely on this annual phenomena, adding top soil and replenishing the environment with critical nutrients. We benefit by having the opportunity to witness the many colors of fall, and our soil and waters are more productive as a result.

Threats to the Magic: Changing our Landscape and our Climate
One of the more obvious threats to autumn color is the removal of trees and forests and replacing those areas with urban and agricultural landscapes. If there's no trees, there's not much fall color, and in return no new topsoil being formed nor replenished. In the region I live and work in, most of the landscape is urban with only a few pockets of natural woodland and prairie found in small "islands" (figure 8). But in areas where green grass still grows, we don't see much of a hummus layer, especially when we're talking about green turf grass in manicured suburban yards and parks (figure 9). While raking leaves can be a lot of fun and is an important landscape maintenance task, one could argue that we've gone overboard. We are removing too much of that nutrient-packed, moisture preserving detritus layer provided to us for free from the trees and flowers growing around us.

Figure 8: A highly urbanized area just outside the city of Chicago.

Figure 9: Highly manicured landscaped areas that are cleared of fallen leaves are deficient in topsoil and are not easily replenished with nutrients nor protected from desiccation.

Climate change, especially in my region, is causing summers to become longer, subsequently extending the growing season closer to the first hard freeze. What affect this will have on local ecosystems in my area remains to be seen, but I can tell you that the unusually warm and wet season we have had is making for a disappointing fall (everything is just kind of wilting and turning brown, or is infected by fungal diseases like powdery mildew). Shorter winters could alter the pace of decomposition, and with heavier rains this could lead to topsoil erosion and nutrient overload into nearby waterways. The consequences of climate change will be significant in my region, and fall might not arrive with the same glory that it has in the past.

Conclusion
Autumn leaf drop is a magical site that is a privilege to witness for those that live in or visit moist temperate regions of our planet. Besides being pretty, the process of leaf abscission and the resulting formation of a hummus layer in our soil is a critical event that sustains our ecosystem. Threats to this very process has the potential to affect us negatively in many ways.

So be sure to show some appreciation for nature's conclusion to summer! To help you out, I will end with some scenes of fall colors from the many places I have been within the last several years. Enjoy!