Measuring
plots, from a birds-eye view, probably looks a little absurd. You head
for a very specific spot on the ground in the middle of the woods, and
that is the “right spot” to begin measurements. It’s weird to think
that going to one particular point is valid, but just a few feet away
would bias your sample. However, we set up your cruise so that it’s
important to visit that specific patch of woods and take measurements
there, and then move on to the next location.
So, how do you know where to go?
One
approach to designing a cruise is to specify an acceptable level of
confidence and error, collect data, and calculate the variance and
standard error in the field as you sample. If variance is too high, you
keep adding plots until you reached an acceptable level. This is
somewhat inconvenient, though- it requires you to perform calculations
on-the-fly in the woods, and it’s hard to anticipate when you’ll be done
cruising on a given day, or in a given stand. It can also be
inefficient, if you add new random plot locations and have to
criss-cross the property multiple times.
An
alternate approach is to estimate the variation before you enter the
stand, and design your cruise accordingly. That’s the method we employ
when you design a new cruise for Plot Hound. Plots are automatically
generated along a grid across the stand, and you’ve probably noticed
that changing the criteria you specify for variation, confidence, and
error changes the sampling design.
Of these criteria, one of the critical decisions you make is your estimate of the variation within the stand. The estimated variation is where your skill as a forester is really important. Your familiarity with the forest type and area where the stand is located give you the knowledge you need to make that estimate. We’re also going to start including your estimated and the actual variation in your reports, to help you “calibrate” your estimates over time. When estimating variation, also keep in mind the size of the plots you intend to use. On average, there will be less variation between larger (fixed-area) plots than smaller ones.
The estimated variation options we offer range from 0.15 to 0.55 - even a seemingly uniform conifer plantation or aspen stand has some variation, which is why the lowest possible value is 0.15.
For example, if you knew you were visiting a uniform plantation, like this one:
You might safely select a low estimated variation, maybe 0.15.
But if you knew that that plantation had experienced heavy mortality, or had a range of soil conditions, you would reasonably expect that there would be a more variation between plots, and more plots would be needed to characterize the range of structure within the stand- then you would select a higher variation.
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Photo credit: Travis Pond, 2010
In upcoming posts, we’ll go into more detail on the implications of selecting the confidence and error values that also contribute to the cruise you design.
