Brad Withrow-Robinson, OSU Forestry & Natural Resources Extension agent for Benton, Linn and Polk Counties
In this series about young stand thinning , I’ve worked on the assumption that people know the density of trees in their woods. I realize that in many cases, people don’t really know that, so cannot easily apply that information to deciding if they have enough room for healthy growth or if trees need to be thinned.
If you know what distance the trees were said to have been planted, you may have a fair idea of the density (a 10’ x 10’ spacing is about 440 trees per acre, a 12’ x 12’ is about 300 tpa). This is a good start, but not necessarily very accurate. Actual planting spacing can vary quite a bit according to the conditions in the field and experience of the planters. And of course some seedlings die during establishment, or some other trees may seed in from outside. So it is probably a good idea to go out and get a better idea of what you’ve got. The basic way to do this is to measure some plots.
We commonly use circular plots for this since they are easy to install and measure accurately. We choose a radius for the size of the plot we want, (typically 1/100, 1/50, 1/20 or 1/10 of an acre). We use larger plots for larger trees, smaller plots (and typically more of them) for smaller trees. See the table at right. It is never too early to get an idea of this. Checking the work of a planting crew often involves checking planting density with a lot of small plots.
So how is this done? Let’s walk through the process together.
First, if you have not measured a plot before, make it easy for yourself. Choose some easy ground with trees that have been pruned up and are not overrun with blackberries. Something like this, to the left.
I realize this may not describe the young stand you are actually interested in measuring, but since this is a training practice, that is fine. If you don’t have anything that fits this description, maybe ask a neighbor, or someone in your OSWA chapter to practice in theirs.
For this exercise you’ll want a few stakes, some flagging, paper and pencil, and a tape measure (a loggers tape is best). Oh, and maybe bring a friend along to help.
Go into the woods and toss several stakes out around the stand (each with a piece of flagging). Those will be the centers of your practice plots. Working together, figure out which trees fall within the radius of your plot (for example, 16’ 7” feet for a 1/50 acre plot). Some will be easy to tell, others will have to be measured from the plot center. If on the line, count it as “in” only if the center of the tree is within the radius at breast height (which is why it is good to have a helper). Let’s say you count 6 trees within your 1/50 acre plot. What’s that mean? That represents a plot density of 300 tpa. To find that, you multiply your plot count by the denominator of your plot size to get density (or 6 trees x 50 =300tpa). Repeat on the other practice plots, or until you get the hang of it.
For bonus points, go back and measure the diameter of each of the “in” trees in the plot and record their diameter at breast height (dbh). If you figure the average and compare that to the illustration and description from the earlier post you can learn how much competition those trees are contending with now, and how much room they have to grow in the future.
So that is the idea. Not that difficult, really. Getting an accurate measure of a whole stand requires some rigor we will not go into here, but you’ll have to read more about that elsewhere. But even a few plots can give you some important insight, so I’d encourage you to put in some plots, and start getting an eye for what you have. It is easy to do when trees are small, before crown closure. Yes, it can be hard work if your stand is brushy and full of blackberries. You may want to do a little pruning and clearing in your plots to make it easier. But winter is a great time to do that sort of work. Good luck.