Forestry policy is a highly controversial subject in Washington State; however, few people would argue that forests are extremely important for wild and anadromous fish. Trees provide shelter from the sun. They offer streamside protection from predators. And, as this lesson seeks to illustrate, trees also reduce erosion by slowing down water movement and holding the soil with their root systems, insuring clean, water in forest streams.
|Trees and Streams
Trees are also at the center of forest food webs. Young salmon are necessarily a part of these food webs. Leaf litter from trees provides food for both airborne and aquatic insects, upon which the salmon fry depend.
Older forests are complex environments which harbor a diversity of plants and animals and maintain delicate but fertile soils. Clearcutting, coupled with burning of wood waste is devastating to these animal and plant communities, and to forest soils. Forest management laws now provide some protection to salmon through provisions for buffer zones of standing timber along stream corridors. However, timber management still fails to account for the cumulative effects of masssive timber harvests over large portions of a watershed. Over time, habitat for salmon and other forest wildlife is on the decline.
Your students will work in pairs for the activity, “Trees and Streams”. Each pair will make and test two experimental watersheds for erosion characteristics. It is probably most practical to have them do all the work outside, both for access to the natural materials, and to keep the mess out of your classroom!
- Students will experiment with the effects of vegetation on erosion. They will create two experimental landscape models, one with and one without vegetation. The students will observe ways in which these two landscapes differ in susceptibility to erosion.
- Students will relate this experiment to to the effects of clearcutting on watersheds.
- One copy per student, “Trees and Streams”
- 16 plastic paint roller trays
- 8 small shovels, trowels or large spoons
- A source of dirt or sand
- Some “instant” vegetation – leaves, twigs, grass, etc.
- A watering can
- Southern Olympic National Forest Map
- Stream Sediment Loading of Big Beef Creek
- Have the students read the first three paragraphs of the student text, “Trees and Streams”. Stop there to introduce the activity.
- Explain to your students that in each of the two paint trays they will make a miniature watershed landscape, one to represent a forest watershed and one to represent a clearcut watershed. Each watershed should have an identical stream through it. They can use sticks, leaves, pine needles, etc. to represent trees in their forest landscape. The clearcut should have no standing “trees.”
- After their landscapes are prepared and student observers ready, use the watering can to test landscapes for erosion. You should attempt to make all watering episodes roughly identical in volume and force, while students note movement of soil and evidence of silt in the run-off.
- After the experiment, dump dirt and weeds in an appropriate place and clean up tools with a hose if you have access to one. Your students can answer the remaining questions on their Data Sheets.
- Besides just removing trees, logging brings other changes to the forest. Show your students the map, “Southern Olympic National Forest.” Ask them to consider why there are so many more roads inside the national forest than outside.
If your students can’t guess, explain that these are logging roads, cut for access to the timber. They are all primative dirt raods. Timberland in the Skokomish watershed averages 4.5 miles of road per square mile, some of the highest levels in the entire state.
Show your students the graph, “Sediment Loading of Big Beef Creek.” Discuss the impact of road cuts on erosion, as demonstrated by these two graphs.
Ask the students for their ideas about what could be done to ease this land-use dilemma.
(They may suggest restricting the percent of acreage which can be converted to roads, and imposing severe limits in steeply sloping terrain.)
- Answers may vary.
- Answers may vary.
- Students will probably say their results demonstrate that erosion is associated with clearcutting.
- Students will probably recognize that the greater grade, the more serious the problem.
- Silt and excess gravel can smother the eggs and young fish. It can reduce circulation around them, possibly causing oxygen starvation.
- Trees help keep the water temperature low, which helps keep dissolved oxygen in the water high.
- Trees help hold the soil, controlling eroson and siltation.
- Removal of trees reduces the food supply of aquatic insects, which salmon fry eat.
- lumber for building or remodeling houses, paper products: newspaper, paper plates, paper towels…
- using wood and wood products frugally, and recycling paper products.
You might like to make your students aware that lumber is currently being cut for sale to countries like Japan to which the United States has built up a very large trade deficit. Becaude lumber is used to balance this deficit, buying foriegn-made cars, audio equipment, and game computers also puts pressure on our forests.