Sampling+&+Measuring+Trees

In a **timber cruise**, foresters try to figure out how much **merchantable** wood there is in the forest. It's usually done before a forest is harvested.

It's impossible to measure every single tree in a forest -- it would take way too much time, people, and money. Instead, we measure a smaller **sample** of trees, and then project their measurements onto the whole forest (the **population)**. We assume the sample represents the whole population, just like if you sampled cookie dough while baking, you assume what you tasted represented the entire batch of cookie dough.

There are actually many ways to sample. For our timber cruise, we will use plot sampling. A **plot** is a little piece of land. Each group will measure all the trees within their plot. To make sure we draw an accurate picture, we will have to make sure the plots are well spread out and picked randomly.

We will use circular plots (another name is fixed radius plots) that are 1/50 of an acre. To find out how much wood is in an entire acre, we would have to multiply our calculations by 50.

For every tree, foresters usually take two measurements to estimate volume: DBH and merchantable height. **DBH** stands for diameter at breast height, meaning the diameter of the tree at 4.5 feet above the ground. We will measure it with a diameter tape, which we wrap around the tree.




 * Merchantable height** is the height of the tree from the stump up to where the trunk tapers off and becomes much smaller near the top of the tree (basically the part that can be sawed into planks of wood). We measure height with a clinometer.

We have to sight up to the crown of the tree and then sight down to the stump. The total height is both added together. (In the drawing below, total height is AB + BC.)