Wonders of Autumn

Fall is finally here, and it is a beautiful time of year, full of colour and crisp air. As the days shorten and summer heat disappears, you will likely find yourself donning sweaters again. But how do plants and animals manage this seasonal change? What's going on beneath the surface of the soil, and in Ontario's many lakes? Read on to learn about some wonders of autumn and enhance your seasonal experience!

This year, September 22 officially marked the beginning of fall. Fall is a season associated with change, characterized by ephemerally brilliant colours, cooling temperatures, and shortening days. Deciduous leaves dazzle us with warm, vivid hues before falling to the ground, where they line sidewalks, stiffening and crunching under our footsteps. Apples are plump and ripe for picking, alongside winter squash such as the classic orange pumpkins placed on porches to communicate a willingness for hordes of trick or treaters to come marching up on Halloween. If you enjoy the outdoors, you're probably already familiar with certain fascinating changes in nature associated with autumn, such as the emergence of unique mushrooms and clever critters.

How do plants and animals adjust to differences in daylight and temperature as fall sets in? 

For plants, the process of losing leaves is triggered by consistently cold weather and shorter periods of light. Leaves appear green due to the pigment chlorophyll, which is abundant in warm weather. However, leaves also contain other pigments. Chlorophyll is produced at a slower rate at the end of summer which results in the loss of green pigmentation. Carotenoids, which are responsible for yellow and orange colouration, and anthocyanins that produce brilliant reds, as you may see in red maples (Acer rubrum) either remain the same, or can even increase in quantity at this time of year. Carotenoids are present year-round, but are obscured in summer by the high concentration of chlorophyll. However, they outlast chlorophyll in fall, giving us an annual gift of amber tones. Anthocyanins occur in the presence of sugars and ample sunlight. When the days are consistently sunny and warm, and nights are cold, sugars that are produced in excess during the day will remain in the leaf cells. Consistently overcast days with warmer nights will result in less anthocyanins; this is why in some years, leaves appear redder than in others. 

Leaves are ultimately dropped by deciduous trees to protect themselves from freezing temperatures. They are thin, and unable to photosynthesize (produce energy) in winter, as the water required for the process would expand when frozen and damage their cells. Many evergreens such as the eastern white pine (Pinus strobus), Ontario's provincial tree, have formed a different adaptation, boasting thick, waxy resins that defend their needle-like leaves. 

Animals, on the other hand, have developed different fall routines. They exhibit cyclic behaviours in which their actions change seasonally. Certain birds, fish, and insects prepare to migrate to warmer areas for the winter months, and other critters such as squirrels hoard nuts for hibernation. Many animals, including some snakes and bats, hibernate as a way of mitigating the issue of limited food in winter. Certain frogs, such as the bull frog, hibernate in the water once temperatures become freezing. Others, like the spring peeper, have adapted to form ice crystals in their body at -5C or lower, allowing ~40% of their body's water content to freeze as a unique form of hibernation. 

All the while, below ground, a whole other set of changes are happening. Increased moisture in the air and generally cooler temperatures influence the soil, creating conditions that are ideal for fungi to fruit. Under ground, these fungi exist as hyphae, fuzzy-white branching filaments that form a complex, wide-spread network throughout the soil known as mycelium. Fungi often form unique symbiotic "mycorrhizal" (myco = fungi; rhizal = root) relationships with plant roots. They can also be found inhabiting organic matter, such as decaying logs, or live plants. The fruiting bodies of fungi (mushrooms) are its more easily recognizable form. Mushrooms and molds are considered to be macroscopic fungi, while yeasts and spores are generally microscopic. You may be familiar with the classic Amanita muscaria, or fly agaric mushrooms that inspired the mushroom emoji. Here in Ontario, you can find the variant known as formosa (Amanita muscaria var. formosa), which are yellowish-orange in appearance, but still very closely related to the bright red version found out west. Like many mushrooms, these are lovely to admire from afar, but toxic to consume!

One tell-tale sign that summer has passed is the presence of increasingly windy, chilly days. While this may seem inconvenient to us, the process is critical for freshwater systems like lakes. Known as fall turnover, this allows for nutrients and oxygen to move from the bottom of a water body to the top. It also occurs in spring, in an event known as spring turnover. In these transitional months, a thermocline (water layer with vastly different temperature than the surrounding sections) forms in northern lakes. Water is at its most dense not when it is frozen, but when it is at ~4C, which it cools to in fall. This shift in density causes the newly heavy water to sink and displace the lower sections. This results in oxygenated water shifting to the bottom of the lake while nutrients rise to the surface. If this did not happen, organisms that rely on oxygen to be at lower, warmer parts of the lake in the winter, such as fish and decomposers, would struggle to survive.

Autumn is an exceptional season. While taking in pleasant sights this year, you might enjoy considering some of the fascinating processes taking place behind the scenes to make these wonders happen!