In this experiment, you’ll see how sneaky weeds try to get into the forest and discover how a web of trees acts as a weed filter.
How do weeds move into forests? To find out, we'll look at the population density of weeds inside and outside the forest edge.
- Plant identification book
- Measuring tape
- Before you head outside, familiarize yourself with the local weeds. Common weed species can vary a lot from place to place. If you’re not sure what a weed is, go to a roadside, your garden, or any place where nature is allowed to do its thing. Find the leaves and flowers of the area’s most common plants. Take photos of them and identify them using your plant identification book. If the plants are in your own yard, you can pick a few and bring them to a garden store if you’re not able to identify them yourself.
- Ask questions and do some research about the ways in which these weeds travel and reproduce. Do they move underground through roots, popping up every so often? Do they have seeds that birds eat and distribute throughout the forest? Do their seeds move by wind to land on a new patch of ground to grow in?
- Get familiar with native plants as well. What’s local to your area? What naturally grows in the forested areas around your home?
- Now, find a forest! Whether it’s in a local park or in your own backyard, you’ll need an area where the forest meets an open place. This area is called the forest edge. The more forested area you can find, the better.
- Create a hypothesis, your best guess about what is going to happen. Will the forest edge or the forest interior have more weeds? What about the areas in between? What percentage of plants in each area will be native species that are local to your area, and what percentage will be weed species?
- Set up study plots in three zones: a zone at the forest edge, a zone 10 feet or more inside the forest, and a third zone 10 feet deeper into the forest. Measure a 2-foot by 2-foot study plot at the forest edge, and mark it with pegs and string. Now, walk in a straight line 10 feet into the forest, and mark another 2 foot by 2-foot plot marked with pegs and string. Finally, walk 10 feet more (or farther if you wish) and mark another plot with pegs and string. Make sure you have permission to go off the trail!
- Using your notebook and pencil, take note of all of the plant species that you see in each square. Use your camera to photograph those that you don’t know, and bring your photos to a local nature center or garden store so that they can help you identify the plants. Create a bar graph of the different species that you see in each plot. Compare the graphs with each other. How are the plant populations different from one area to the next? How many weeds are there on the forest edge versus the forest interior?
At the forest edge, there are more weeds and fewer native plants.
Although you might not think that plants move, they need to move to reproduce. Some throw their seeds into the air, like the dandelion and the maple tree. Other plants such as burdock (burrs) hitch a ride on animals and people. Some plants have light spores that travel through air or water to a new destination. Some move under the ground, pushing their roots along until they find a good place to pop up and grow. Still others rely on animals to travel from one place to another. The animals eat the seeds, and the seed makes its way through the animal without being digested, emerging as animal scat.
Many weeds are good edge plants. They love the sunshine and are tough, so they don’t mind if people step on them. Native plants can be pickier than these durable weeds, and they may not survive on the forest edge. In the interior of the forest, native plants are at home, and they sometimes grow better than the weeds do.
There’s another reason why there are more weeds on the forest edge. While weeds are very good at growing and reproducing, the forest acts as a barrier to weed seeds. The forest edge plants and trees catch seeds in a giant net of leaves and branches, keeping them from entering the forest interior and forcing them to fall closer to the edge of the forest.
Knowing what you now know about edge effects, do you think that forests act as a filter for other things, like pollution? Why? Maybe you could find a way to investigate!