Questions from the Field:
How Else Can I Choose VBAR Trees?
Many people are now using the “Big BAF” method, where you count trees with one BAF and choose measured (VBAR) trees with a BAF that might only get, as an example, 1/5 as many trees. We’ve had many reports of lower costs and faster cruising with this method. We recently did a formal article in the Canadian Journal of Forestry 1. If you want a pdf copy, you can download one here (there is an erratum, click to view).
Suppose you don’t want to pull out an extra prism and readjust your mental picture of trees to check. What other options are available? There are several that come quickly to mind, and perhaps some of them might appeal to you. These techniques will work on fixed plots as well. Consider one if it works well for you and the check cruising system likes it as well.
You can pick every 5th tree. If your paper forms or handheld recorder can be easily set up for that, it would insure a good distribution of the measured trees throughout the stand
You can “roll the dice” in the handheld, and take approximately 1 tree in 5. This can be done by generating a random number between 1 and 5 for each tree, measuring it only when the random number is between 0 and 1.
You can choose all the trees on 1/5 of the plot. It might be easier with 1/4 of a plot, but the principle is the same. If you use a walking staff for a plot center, you can cut slots in the top (or drill sighting holes) to indicate the part of the plot to check. The staff will go in the ground in a different orientation each time, and you will essentially be choosing the section of the plot to measure at random.
You can actually choose the VBAR trees on different plots. There is no requirement that these two functions be done on the same plot. One experienced person can measure trees with a 100 BAF prism along a line, while another crew member just counts along a nearby line using a 20 BAF (just make sure the measurements include at least one tree from each species). Compile the sample as a group of count trees with one BAF and a group of measure trees with another BAF.
None of these systems require the use of a second prism, but might have their own small drawbacks. The compilation mathematics is the same, no matter which method you use to select the trees.
What certainly is a good idea is to measure only some of the trees you count with the prism. No doubt you can think of other ways as well. If you do, drop us a line and suggest any methods you find useful.
What is the consequence of missing a measured tree? Not much actually. When you are counting trees and miss 1 tree out of 4 you miss the stand basal area by 1/4. Bad mistake. When you miss a tree for calculating VBAR, you are just averaging 3 trees instead of 4. Chances are that this will make a very small difference, and that difference will be confined to the answer for that species. This is no excuse for sloppy work, of course. The exception is where you decide to miss a tree because you “do not like it”. Don’t do that. And remember – the count must always be accurate.
1 This was because some outfits feel much better if field methods have been formally published.
Originally published April, 2005