Friday, April 25, 2014

Setting up a cruise - Estimating Variation

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:

 pine plantation in central Florida 

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.

For example, these two pictures are both taken from the same stand.  There is obviously very high variability within the stand, which would lead to high variation in stocking estimates between plots in these two locations.  More plots would be needed to reach an acceptable level of variance among all plots.

one tree and scattered saplings wide variation in sizes of trees

 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.

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