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Forest Ecology

What is an Ecosystem?

An ecosystem is a group of living and non-living components interacting together on a given physical landscape. The size of an ecosystem is arbitrary and could be as small as a few square centimeters if you are looking at a soil microbial ecosystem; as large as thousands of square kilometers if you are looking a biome like the Great Plains ecosystem; or a few hectares if you are looking at a single forest stand ecosystem.

One way to learn more about how a forested ecosystem works is to build a model.

An ecosystem model is an accurate but simplified representation of an ecosystem that can be very useful in thinking about or simulating the actions of a real ecosystem… read more. Because any ecosystem has many different but interrelated components, the best way to understand the system is to break it down into its component parts. To get an introduction to a very simplified forest model, see our Forest Ecosystem Gamewhich gives participants and introduction to how a hardwood forest ecosystem works before and after exotic earthworms invade!

Step One:

The first step in building a graphical model of a hardwood forest ecosystem is to identify its major components.

The components of any ecosystem are those physical things that contain energy and nutrients. In a graphical Forest Ecosystem, these components are often illustrated using boxes like in Figure 1 below.

trees, understory plants, forest floor and soil

Figure 1. Some components of a graphical forest ecosystem illustrated using boxes

A forested ecosystem, by definition contains trees, so that is our first component. In addition to the various species and layers of trees in a forest, there are other distinct ecosystem components.  For example, the understory contains most of the visible plant life found between the sapling layer and forest floor. The forest floor is where one would find most of the plant roots, bulbs, fungi, seeds, years of accumulated leaves and twigs.  The soil is the “dirt” under the forest floor and is composed largely of minerals of various grain size (very small grain size = clay…very large grain size = sand) and organic material that has been mixed in with the mineral component. In addition, there are numerous animals that live in the forest, and of course we cannot forget people. We will add those two components to our forest ecosystem model later… Read more.

Step Two:

Once you have identified the components of your ecosystem model, you need to define the processes that connect the components. This is graphically done by using arrows to indicate the flow of nutrients or energy among the different ecosystem components… Read more. See an example in Figure 2 below.

connections between trees, understory plants, forest floor and soil

Figure 2. The components of our ecosystem model are now connected by processes that result in the movement of energy or nutrients among the components.

One thing to notice in our ecosystem model is that there is no process connecting the trees component with the understory plants component. This is because there are no substantial processes that result in the flow of nutrients directly from a tree to an understory plant or visa-versa (the flow of nutrients always goes through the forest floor first!). In a conceptual diagram, there would be important relationships between the trees and understory plants. For example, trees provide shade to the understory plants. But remember, in an ecosystem model, only processes that result in flow of energy or nutrients are represented. You will see why this is important a little later.

Now let’s add the animals and the people components to our ecosystem. You can see in Figure 3 below that energy & nutrients flow from the trees and understory plants to the animals when they eat the leaves, twigs and buds of trees or graze on understory plants; and when the animal excrete waste products or die, energy & nutrients are returned to the forest floor component. Since people are really just a special kind of animal, you can see that energy & nutrients flow from the trees to people when they eat something from a tree, like maple syrup… Read more.

connecting people and animals to the ecosystem

Figure 3. We have added two components (people & animals) to our ecosystem model, along with some processes connecting them to other components.

Step Three:

Determine the major inputs and outputs of your ecosystem. As you are building your ecosystem model, one thing to think about is whether your ecosystems could be opened or closed. A closed ecosystem is one that has no inputs of energy or nutrients from outside the ecosystem and no outputs of energy or nutrients leaving the system. The earth is an example of a closed ecosystem with respect to nutrients and an open ecosystem with respect to energy (see figure 4 below). All the nutrients that have ever been on earth are here and simply continue to cycle, there are no additions or losses. However, the earth is constantly getting inputs of energy from the sun and simultaneously radiating energy back. The earth doesn’t heat up too much or cool down too much because the earth’s energy balance is in a relatively stable equilibrium, meaning that the amount of energy being input and output are about equal.

energy inputs from sun; outputs from earth

Figure 4. The earth ecosystem and has no inputs or outputs of nutrients which are constantly recycled within the global ecosystem, while the earth has both inputs and outputs of energy that are in a relatively stable equilibrium.

Now, let’s examine some potential inputs and outputs of nutrients & energy to our forested ecosystem (see Figure 5 below).

Just as the Earth ecosystem is closed with respect to nutrients, unmanaged earthworm-free hardwood forest ecosystems are often very nearly closed nutrient ecosystems that there are very few inputs or outputs of nutrients. Rather the nutrients are constantly recycled among the various ecosystem components. In contrast, most agricultural ecosystems require nutrient inputs from outside to function properly (see Figure 5 below).

inputs of rain, sunlight; outputs of evapotranspiration and nutrients

Figure 5. Some typical inputs and outputs of nutrients and energy for forested ecosystems include evapotranspiration, nutrient leaching, sunlight and rain.

Step Four:

Once you have identified the components, processes and major inputs and outputs in your ecosystem model, then you can begin to add the actual values to these parts of your ecosystem by measuring them. For example, you could measure the amount of litter that falls to the forest floor each year (a process), what the biomass of trees is in a given forest (a component), how much light reaches the forest over a growing season (an input), or how much nitrogen leaches from the forest (an output). Needless to say, some of these things are easier to measure than others and for most of these things it would be very hard to directly measure the value for a whole forest. For example, it would be hard to catch every single leaf that fell from the trees in a given year and weigh them all! So, researchers estimate these values taking samples of the given measurement they want to know. In the case of leaf litter, you can put out trays in the forest and after all the leaves have fallen for the year, dry and weight the leaf liter in your trays. They you can use that value to calculate an estimate of the total leaf litter for your forest… Read more. See the Research Methods section to find out more about how scientist sample different aspects of forested ecosystems.

Step Five:

Use your ecosystem model to think about how changes can cascade through an ecosystem or to ask specific questions that can be answered with further research. When the major components, processes and inputs and outputs of an ecosystem are understood, then you can use the model to see how changing one part of the ecosystem affects other parts. For example, if you harvest trees from your forest, that will decrease the amount of leaf litter reaching the forest floor each year which may lead to decreases in available nutrients for understory plants. This is the type of thing forest ecology researchers often study.

For example, researchers may monitor soil nutrient levels for many years after trees have been harvested to see how the real forest behaves compared to what they thought might happen based on their forest model, their understanding of how the forest works. If the results in the real forest are very different than those predicted by their model, then they know that they don’t have full understanding of how their forest works and they may go back to try to improve their model.

In general, when building a model, you start simple and add more detail as needed. To get an introduction to a very simplified forest model, see our Forest Ecosystem Game which gives participants and introduction to how a hardwood forest ecosystem works before and after exotic earthworms invade!

equilibrium conditionsFigure 6. An ecosystem that is in equilibrium doesn't gain or lose nutrients.



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