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NATURE NOTES – Number 3 August 2020
Our current topic: How a Tree Grows
Most scouts know that you can count the rings of a stump or log and know how old the tree is. That is correct. But that’s about where their knowledge of tree growth stops. Let’s help them learn a little more.
A tree grows in three directions: Down (the roots), out (the diameter), and up (the height).
As a tree grows, it needs more water and nutrients that the roots must provide, so they go deeper into the soil to provide this. Easy to understand. Soils will be discussed another day.
The increase in diameter is a little more complicated. Not only do the rings tell the tree’s age, but the growing conditions each year. Notice that some rings are wider than others. The quick answer is usually “there was a draught and it didn’t get enough water”. Maybe, but the tree needs more than water to grow. A big factor is sunlight. If a neighboring tree starts to overgrow another tree, the growth might slow because of less sun light. If the overreaching tree is cut down, or falls in a storm, more sunlight can reach its neighbors and they can grow more and increase their diameter quicker. This can be shown to scouts with a few ends of a 2X4 from scrap lumber. The rings are clearly visible and they can see the different rates of growth. A stump or log in the forest might also show signs of insect or fire damage. Different species grow at different rates. In ideal conditions pines may grow an inch in diameter in only 4 or 5 years, while an oak might only add a ¼” in 2 or 3 years. Other factors such as defoliation by insects also affect the growth.
The tree gets taller each year. How does this happen? Does it push up a little more from the ground each spring? Scouts have never been asked to think about this and generally they don’t know. Trees get taller because each spring new twigs sprout from buds all over the tree. Those twigs at the top grow and become branches that reach for sunlight. Next year those branches sprout more twigs. Each year the tree adds to its height with more new growth at the top.
Another feature of tree growth scouts don’t know is about limbs and knots. As a tree grows, bottom branches begin to fall off. They are in the shadow of the tree foliage and are no longer needed to produce food for the tree. When they fall off, what is left behind is a scar on the tree – a knot. This shows up on the trunk of a tree as well as in the lumber sawn from a log.
All of this is somewhat simple, but no one has ever discussed it with scouts. Take them into the forest and look at some trees and discuss how they grow. Use several ends of a 2X4 to explain the growth rings. Look for branches in the forest floor that have fallen from a tree. Look for the knots on the tree trunks.
If you have the resources, cut a 2” or so slice of a pine tree log and a slice of an oak tree log about the same diameter. Let them dry for a couple of months. Then let a scout pick up the two and compare weights. Why does the oak weigh more? Another topic, another time.
Enjoy nature. Understand Nature.