TITLE: Measuring stick for DBH measurements
DATE: 2022-05-12
AUTHOR: John L. Godlee
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When measuring the diameter of tree stems to estimate above ground
woody biomass, it’s important to avoid stem features which might
skew the diameter estimate and misrepresent the stem diameter.
Burls, swelling at branching nodes, and large wounds should be
avoided if possible. It’s convention to measure stem diameter at
1.3 m (DBH, diameter at breast height), but sometimes this will
need to be changed to avoid these idiosyncratic features.
Recording the height of the stem diameter measurement becomes even
more important when estimating biomass change over time. If the
diameter is measured at different heights in different years then
any biomass change estimates will be unrealistic. Tree stems tend
to taper upwards, so a diameter measurement at 1 m will generally
be larger than a measurement at 1.5 m.
In the field instead of using a tape measure every time you want to
measure the height of the stem diameter measurement, I recommend
using a measuring stick.
I use a 2 m long piece of thick walled PVC pipe with duct tape
markings every 10 cm, starting from 1 m and ending at 2 m. Many
rolls of duct tape are exactly 5 cm wide, so conveniently I can
also measure in increments of 5 cm if necessary. I mark 1.3 m using
a different colour duct tape to the rest as this is the most common
height for diameter measurements. Then I can hold the measuring
stick up to each stem and quickly choose the appropriate height for
the diameter measurement.
The stick also comes in handy for knocking leaves out of trees for
species identification, removing spider’s webs, and probing long
grass for snakes. I tend to carry it with me everywhere in the plot.
Additionally, I use the stick for other measurements in the plots.
Counting tree saplings along 2 m wide belt transects is easy by
holding the stick in the middle of the transect. Any sapling which
touches the stick is counted.
The stick is also useful as a way to make yourself more visible to
other people in the plot when the grass is very long. When we
arrive at a plot generally the first thing we do is lay out tape
measures to mark the plot perimeter. To ensure these tapes are
straight it’s necessary to have one person standing at the corner
so the person holding the tape can walk in a straight line to them.
Waving the bright green measuring stick in the air makes you much
more visible. I normally attach some orange flagging tape to the
top of the stick, or balance my hat on it to make it even more
visible.
I’ve gone back and forth on writing this brief post for a few
years now. I thought that maybe it was far too obvious that a
measuring stick would be useful for measuring the appropriate
height for a DBH measurement. What changed my mind was having some
early stage PhD students with me on fieldwork last month. They
seemed to think it was a genuinely good idea, and they hadn’t
already thought of it themselves, so I figured I would share it
here.
