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Is it true that ...Mixed stands need more measured plots?
These are two different questions, although they look alike. The first answer is almost always NO, the second is MAYBE. When you have a rare species, you have to ask "what is rare about it?" What don’t you know because it is "rare." That, after all, is what you will have to make sure you find out with your sampling scheme. Getting the question exactly right is critical. It’s easy to wander off and spend a lot of time, money, and agony getting something you don't need. In the course of a cruise, what is it that you do not know well enough about the 5th most common species? As usual, you go to your records and take a look at your old cruises. Take a look at 3 things for that species.
The number of measured trees to count plots is given by the ratio of these items. If the CV of the basal area is 450% (a typical situation) for infrequent species, and the CV of net VBAR is 45%, then the right ratio is one measured tree in about 10 count plots. If the variability is in basal area, it does little good to measure a bunch of trees for VBAR. Only count plots will help. This is simple math, long understood and well known. Check out the situation in your own records and see what they indicate. When you do a count plot, it affects every species. If you do 40 count plots it will contribute to the answer for every species (even when the count for that species is zero). In most situations of rare species the basal area is poorly known and only count plots will help that situation. It is not wrong to measure lots of extra trees, but it wastes your time. Suppose that the 5th species only shows up on about 4 of the 40 plots. That’s about the ratio you wanted. Does that mean that every plot should be a "measured plot." No, it just means that you should measure the rare species every time it comes along – which is not the same thing as measuring every tree on every plot. If you are measuring all the other common trees to "catch the rare ones" then you are wasting a lot of time, and that time should be invested in something that matters. A sensible cruise compilation program would be able to handle this, but there is such a long history of poorly designed programs that they are usually structured incorrectly and cannot be easily changed. What would be helpful is a handheld program that would keep track of the fact that you need to measure every 20th Douglas-fir, but every 2nd Port Orford Cedar, and a compilation program that could handle that. At the end of the process the computer could compute the VBAR and basal area of each species group independently. Is there a way around this problem with the compliation programs? Yes, but it is a bit clumsy. You can do two or more cruises at the same time, using the same points. On one cruise, count the common trees every time, but measure only one sample point in 5 (for those common trees). In the other cruise (using the same sample points) count the other species, and measure every 2nd sample point (for these more rare trees). Run the data as two separate cruises, and combine the final results. Is all of this necessary? Probably not. The rare trees make up a small part of the stand volume, and often a small part of the stand value. Get some of your old data and find out if taking plots at the right ratio for your main species is also about right for the rare ones. If so, don’t worry about it. If you don’t do that, stop worrying about it anyway. Think about it this way – how could measuring more of the common species help with the rare ones? Example:
In this situation, our records might indicate that for these species we would usually get about 8 trees in every 50 plots using a 1:6 ratio, so the current process works out about right. If we got 15 trees in every 50 plots it would be a few too many, but probably not enough to justify the extra hassle of changing the program or doing two cruises. |
Originally published April 1999