Temperate Deciduous Forests
There is no season such delight can bring,
As summer, autumn, winter, and the spring.
William Browne - "Variety"
Not every region of the world experiences a change in seasons. Some are always blanketed in ice, while others have warm sunshine all year round. For those of us who live in temperate regions, seasonal changes happen every few months. You could be darting across a lake on water skis in July and, 6 months later, you could be racing across that same lake on ice skates!
When you grow up experiencing four seasons every year, you begin to notice and appreciate the seasonal ebb and flow of life around you. Winter is peaceful and quiet; critters that don't migrate stay hunkered down in warm dens, waiting for the Spring (Humans included!) Spring is a welcome awakening; as the snow melts away and leaves begin to bud on the trees, life reappears and prepares for a new year. Summer is a bustle of energy and life; insects are busy pollinating the next generation of wildflowers, and birds sing from every corner of the forest. Autumn is a time for harvesting and a time for goodbyes; mammals fatten themselves up to prepare for a winter food shortage, and birds of a feather flock together to follow their food south.
For many of us, seasonal change is old news. It drums along at a steady, predictable rhythm. But, few people realize that this predictable pattern is what has allowed temperate deciduous forests to evolve and flourish all over the world.
Where can we find them?
When you look at a map of of the world, lines of latitude run east and west, forming invisible belts that circle the globe. These lines of latitude can be grouped into three different categories as you move away from the equator. Regions that lie between 0° and 23° north and south latitude are called the Tropics, regions between 23° and 66° are called the Temperate latitudes, and regions between 66° and 90° are called the Polar latitudes. Temperate deciduous forests are found within the temperate latitudes, just like Tropical Rainforests are found within the Tropics (makes sense, right?)
In the southern hemisphere, temperate forests can be found on the southern tip of South America and in Eastern Australia. In the northern hemisphere, they can be found in northeast Asia (China, Korea and Japan), Western Europe, and the eastern third of the United States.
Abiotic Factors: Temperature and Precipitation
Temperate climates (and all other climates for that matter) are influenced, in large part, by circulating air currents in the atmosphere. Watch this animation of global air circulation and try to locate regions of the world where the clouds accumulate...
As you can see, most of the clouds accumulate along the equator. This is what scientists call a low-pressure zone (Clouds just can't handle high pressure situations). Low-pressure zones are regions of high precipitation. If you watch carefully, you can spot another low-pressure zone in the temperate latitudes.
Due to their global position, temperate forests generally receive about 75-150 cm of precipitation every year (That's a lot, and second only to the Tropics). Remember, though, precipitation can fall in the form of rain or snow because temperate climates experience all four seasons. Temperatures can range from -1°C to -30° C in the winter and 27° to 32° in the summer.
What do they look like?
Temperate deciduous forests are among the oldest and most beautiful forests in the world. Take a walk through the northern hardwood forests of Wisconsin, or taste the sap from a Sugar Maple stand in Vermont, and you will know exactly what I mean. As you walk, tall trees form a leafy canopy above your head, blocking the sun and casting dappled shadows over ground. Leaves from last season crunch noisily under you feet as you scrape through a thick, woody understory. If you dig into the ground, you pull through layer upon layer of wet, decaying leaf litter, and white threads of fungus stand out against the dark soil. Fungus, bacteria and insects underground decompose fallen leaves and organic matter quickly, producing a thick layer of nutrient rich soil, which scientists call humus (Not to be confused with Hummus, Yum!). The humus feeds the trees and supports a biodiverse community of lichens, mosses, grasses and wildflowers on the forest floor.
Not all temperate deciduous forests are created equally, though. Every forest you visit can differ greatly in the species of plants that populate it. In the same way that the Earth can be classified into separate biomes, where specific groups of plants and animals exist within a regional climate, temperate deciduous forests can be classified into different communities depending on the local climate.
Click here to find out more information regarding specific temperate forest communities!
What types of plants can you find here?
You may hear temperate deciduous forests also called "broadleaf forests", because the tree species that populate them have... wait for it... broad leaves! Trees like Maple, Oak, Beech, Chestnut, Elm, Hickory, ect. have big, broad leaves that are attached to the branch by a special stem called a petiole. Unlike pine needles, these leaves are soft and easily digestible to browsing herbivores.
Broadleaf graphic here
In a mature forest, these tall trees form a canopy which blocks most of the sunlight from penetrating through to the plants below. To compensate, the plants that make up the understory and herbaceous layer are shade-tolerant plants, meaning they can survive with a lower amount of sunlight. Due to the seasonal nature of temperate deciduous forests, many of the plants in this region are perrenial, meaning they grow and flower only during the warm, summer months. Thick, woody shrubs like rhodedendron, buckthorn, sumac, honeysuckle, or dogwood dominate the dense understory. This region of the forest is generally the most biodiverse area of the forest and a single forest can have over a hundred different species of plants! Throughout the early spring and summer, shade tolerant herbs and wildflowers like Jack-in-the-Pulpit, May-apple, Bedstraw, Purslanes, and mustards flower and go to seed within a few weeks to months.
Deciduous perrenial graphics here
What does "Deciduous" mean?
The term deciduous refers to the plant's ability to lose it's leaves when times get tough. For example, some species of trees and shrubs in the chaparral are called drought-deciduous, which means that they lose their leaves in the dry season to conserve water.
Deciduous trees in temperate forests lose their leaves in the fall to better survive winter conditions like extreme cold and reduced daylight. For a more detailed explanation of how trees lose and regrow their leaves, check out Rob and Jonas' video on plant hormones here!
The ability for a tree to lose its leaves to conserve energy is a useful but costly adaptation. As opposed to evergreen trees, deciduous trees have to regrow thousands of leaves every year. This requires the plant to take precious nutrients from the soil to make them. In some temperate regions, if the soil is too dry or nutrient poor to afford the cost of new leaves, the populations of plants change to suit the environment. For example, in many temperate regions of the United States, the soil is too sandy and nutrient poor to support many deciduous trees and evergreen trees are a big part of the forest. In areas like this, we call it a "temperate broadleaf mixed deciduous forest". Yes, I know, the names get complicated...
Why do the leaves change color?
Temperate deciduous forests are famous for their dramatic color change that occurs every fall. Amazingly, this seasonal effect begins within each cell of every leaf on the tree! To help you understand this better, picture each plant cell as a jar full of red, yellow, and green beads. If there were more green beads than anything else, the jar would appear green. If you took all of the green beads out, the jar would appear red and yellow.
This is essentially what happens within each cell of the plant every year. As the days get shorter and the temperature drops in the fall, plant cells stop producing green pigments (AKA chlorophyll) in the cells of their leaves. When the cells stop producing chlorophyll and the green pigment slowly disappears from each cell in the leaf, other pigments become more vibrant, and the leaves begin to appear yellow, red and orange.
Every year, hundreds of extraordinary migrations occur all over the world: The Swallows of Capistrano, the Monarch Butterflies from Mexico, and of course, The New England Leaf-Peepers. Every year, thousands of New Yorkers and Bostonians make the long journey north to watch the leaves change color; truly this one of nature's most beautiful spectacles (The leaves, not the city people).
Temperate Deciduous Forests Through Time
As we have said, in a typical temperate forest, tall trees create a thick canopy that blocks most of the sunlight from penetrating. This creates a layer of shade-tolerant herbs and shrubs in the understory. But, what happens when the trees fall down? What if a fire burns hot enough to turn everything into ash? What if a tornado sweeps through and delivers everything to the Land of Oz? (It happens you know...) Here, we will talk about how these forests change over time and what happens after a disturbance.
Forests, like any other living thing, grow old and change over time. In fact, every forest you see was once a pile of bare rock! Slowly, lichens and mosses grow over the rocks and decay to form a layer of soil that is capable of supporting grasses and wildflowers. At this point, our temperate forest is not a forest at all, its a grassland! Temperate deciduous forests and temperate grasslands are almost one and the same. The two are often found right next to each other and share many of the same plant and animal species.
Over time, tree seeds from the forest make their way to the grassland and begin to sprout. As the trees begin to shade out the plants under them, the grassland begins to resemble a forest more and more. Grasses in these habitats are not very shade-tolerant. As more trees sprout up, fewer grass seeds are able to germinate and they remain dormant, in the soil, until conditions are right again.
At this point, more shade-tolerant herbs and shrubs are able to grow, and they create a thick understory in this young forest. When the majority of the trees in the forest have reached a mature age, we call it a climax community. Forests that have reached this stage in development are also called old-growth forests for obvious reasons. Many of the old-growth forests in the United States have been cut down to make space for cities and to make use of the valuable timber. But, some old growth forests remain in northern Wisconsin, Michigan and Minnesota. (...and given enough time, more will grow again!)
What makes a temperate forest different from a temperate grassland? Sunlight! Grasslands recieve a lot of it, forests do not. Now, imagine a tree falls in the woods (don't worry about whether or not you can hear it...) Suddenly, the canopy opens where that tree has fallen and light pours into the forest. When this happens, seeds that have been dormant in the soil are finally able to germinate again. Grasses, flowers and other fast growing plants quickly sprout up and take the place of the fallen tree. Over time, new trees will grow up tall enough to shade these plants out again, but that could take years. In the meantime, these areas of the forests remain sunny oases of diverse plant life!
Tree falls, fires, tornadoes and insects are common natural disturbances in these forests. Every time something like this occurs, the forest rebounds within a few years. Sometimes it begins at an earlier successional step, and sometimes the disturbance is just what it needs to clear out the excess and keep the forest healthy.