Tuesday, June 9, 2015

How to Use Plot Hound for the Most Accurate Estimates of Merchantable Height

When you cruise with our Plot Hound app, you have the option to record a “Merchantable Height” for each stem. The purpose of this post is to explain how we use that value in our volume reports, so you know how the merchantable heights you report affect the final data.


If you’d like to receive volume reports, contact us and we’ll get started setting you up right away.

In developing Plot Hound, one of the biggest challenges SilviaTerra faced was determining how best to handle ‘stopper’ heights in hardwood trees. Unlike conifers, which typically have a predictable growth form (mostly straight and up), hardwoods have a lot more branchiness. Botanically, this is the difference between excurrent and decurrent growth.

From a forester’s perspective, the highest possible merchantable height would be the height where the stem tapers to the minimum usable diameter for your product. (For example, the merchantable height for pine pulpwood would be the height at which the diameter tapers to 4”). However, in reality, true merchantable height is only as high as the stem goes before it becomes too branchy to produce a log of sufficient quality for your product. On decurrent hardwoods, this is often well below where you’d like it to be.

If you record a merchantable height for your tree, we treat that as a hard ‘stop’ in our virtual bucker (that system is described here).  We don’t calculate any volume for that particular stem above that height. So, if the stem is only going to be used as a sawlog, merchantable height should be recorded as the top of the sawlog. However, if the stem is going to be used for both sawlog and pulp log, the merchantable height should be recorded as the top of the pulp log (the higher of the two numbers).

If you don’t record a merchantable height, what happens?  For softwood species, we simply calculate the taper-based merchantable height and stop computing volume at the minimum diameter for the product.  

For hardwood species, because so many hardwoods have a true merchantable height well below the calculated merchantable height from the taper equations (due to branchiness, top breakage, and all of the other reasons you might record a lower merchantable height in the wood) we calculate a shorter merchantable height for each hardwood stem and ‘buck’ to that point instead.

The merchantable height we use for hardwood volumes is calculated from stem diameter and total height. We predict a variable ratio of merchantability calculated from a large dataset of hardwood stems on which both merchantable and total height were measured. This ratio is applied to the measured (or predicted) total height for that stem to get the merchantable height used in volume calculations.  We’ve found that the estimates of merchantable volume for hardwood stems and hardwood stands using this approach are much closer to published studies and foresters’ expectations than when we rely on taper equations alone to estimate the upper point of merchantability for a hardwood stem.

To sum up: Plot Hound reports the most accurate estimates of merchantable volume for hardwood stems in your cruise when you record a merchantable height for each stem. If you don’t, our process for estimating it has you covered! If there are other heights you need, such as stopper heights for a single product (such as sawlog height in a stem that will be used for saw and pulp), use the ‘Notes’ field to record that information.

Don’t forget to contact us before your next cruise to set up volume reports!

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