Duration: 30 to 40 minutes
Vocabulary: tree cookie, vascular cambium, natural history
Summary: Students learn about tree growth and how forests change over time.
Objectives: Students will gain a deeper understanding of how forests change over hundreds of years, and understand the age of different trees within the context of other historical events.
Materials: Native PA Tree ID books, tree cookies (cross sections that show tree rings), pictures of american chestnut trees
How many of you have built something out of wood? Like what? What does it mean to say that wood is a sustainable resource, what would that mean? Basically, it means that if you cut a tree down you can always plant a new one. Plastics are not considered sustainable, because though it is a prized material to work with, plastic comes from oil and we can not make anymore oil that what the earth has right now.
Obviously, trees come from forests. However, the forests in Pennsylvania didn’t always look like the do know. When PA was first being colonized, the forests were dominated by pine trees such as the White Pine and Hemlock. These trees were massive. One interesting characteristic of the pine tree is that unlike the maple trees or elm trees, pine trees don’t loose there leaves every year–rather, they are evergreen trees. Because they always had their leaves, evergreens ended up blocking much of the light from hitting the forest floor and prevented other trees and plants that are not shade tolerant from growing in the under story. As a result, besides the trees themselves, not much else was on the forest floor, and you would have had no trouble running or riding a horse at full speed through the forest because of the lack of obstacles. This is also how forests like the Black Moshannon got their name, because it was so dark and shaded inside the forest due to all the pines.
In addition to these pine trees there were also an amazing number of American Chestnut trees. In some areas of Pennsylvania, up to half of all the trees in the forest were Chestnut. Unfortunately, the only chestnut tree usually seen today is the Chinese Chestnut, because a disease wiped out most of the American Chestnut trees in this area. When they were present, though, they would have produce amazing crops of chestnuts. This would provide food for lots of different animals from squirrels, to bears, to birds. The trees are gone now because a very unfortunate fungus arrived in America, likely from Asia, that attacks the vascular cambium in trees, the tissue that allows trees to import nutrients and water up the tree. Once the vascular cambium was killed the tree could no longer do this, and would die (sort of like a person suffocating). As a result, while a few of these trees try to grow, you are not likely to find many of them and if you do, they usually do not get more than a few feet tall before the fungus hits.
The forests in Pennsylvania changed a lot when people began settling. The giant trees growing at the time fueled the wood needs of a growing nation, especially because we are on the east coast where the nation really began. In fact, the forests of PA were clear cut 3 times over!! All the trees you see now are new growth. Much of the old forest was used for fire wood, charcoal production to make steel and obviously, building houses.
Cutting down so many trees had a very serious effect on the ecology of the forests. Perhaps one of the more interesting characteristics is that because the white pines and hemlocks were no longer present, they didn’t shade out the other big leaf trees. It turns out that these big leave trees are more preferred by deer than pines. As a result, the population of deer increased dramatically and has largely stayed this way ever since.
Ask students if they have a favorite tree in their area. Do they have any idea how old this tree might be? How would they find out?
Show students the tree “cookies” to point out the rings. Tree rings represent a year of tree growth. In the spring time, when the conditions are wet and good for growth, new tree cells are big, forming a thick, whitish yellow ring. In the summer, when it may get dryer, new cells continue to grow but they are smaller and more densely packed together (forming a dark ring). The whitish yellow and dark ring together represent one year so to find out how old the tree is, we just count the rings Ask students to count and determine how old the trees these cookies were taken from are.
With the tree rings representing a tree growing each year, tree cores also reveal what the weather conditions were like back then. Students should be able to tell by looking at the thickness of the rings when conditions were good and the thin rings for when the conditions were not so good.
Pass around the tree core from this White Oak tree. Have them look closely at the size of the bands and notice the different conditions. Explain to students that looking at trees gives you a sense of history.
The oldest tree in the Penns Valley Environmental Center was cored by a forester in 2007. This ~215 year old tree was just a baby seedling at around the Whiskey Rebellion (1794) and when Lewis and Clark first explored the Louisiana Purchase (1804). Ask students to name other important historical events this tree saw.
Finally, put the students into groups of 3. Hand each group a Common Trees of Pennsylvania ID guide and ask students to spend time identifying as many trees as they can. This can be made in to a race, to see which team can correctly identify 5 or 10 trees. After enough time, have students gather back together and share which types of trees they identified.
Ask students to explain how tree rings work–what is the difference between the marks made in the spring and in the summer? Have students explain what historical events the trees in this area were probably around for. Ask students to explain how they went about identifying the trees.
Have students make a time line of historical events from the point of view of the tree.
Have students write a short story from the point of view of the tree about what it may have seen in its life.