| Correctly
Measuring Girth |
Colby
Rucker |
| Aug
11, 2002 20:32 PDT |
Big
trees on steep slopes often require a variety of measurements to
allow comparisons with other trees.
All height measurements start from the same place - "where
the acorn sprouted." If you stand at mid-slope and sight
down the centerline of the trunk, that line meets the soil at
the central basal contour. You can't determine the contour by
averaging the elevation on the high and low sides of the base;
roots enter the soil abruptly on the high side, but run all over
the hillside on the lower. The central basal contour is a
constant elevation; it extends level from your side of the
trunk, through the spot where the tree first sprouted, and out
the other side. Like all contour lines, it may curve, but it's
at a constant elevation. If you measure upwards with the lean of
the tree 4.5 feet above that contour, that point is breast
height on either side of the tree.
Circumference is measured at right angles to the lean of the
trunk. If you temporarily mark breast height on both sides of a
large tree with a scrap of paper and a thumb tack you can be
sure the tape is even. You can carry some thumb tacks pressed in
a wine cork. Of course, if erosion has left the tree standing on
its taproot, you'll have to estimate the elevation where the
tree first started. If floodplain deposition has raised the
grade, you'll have to accept the existing grade level. We don't
dig 'em out.
On a very large tree, a tape set at breast height may still be
below grade on the high side. In that case, raise the tape to a
higher elevation and attach that elevation to your circumference
record. Actually, a variety of measurements is very useful,
including several taken with the slope, starting at grade. This
will allow others to model the architecture of the base in the
future.
Yes, getting the tape even on the downhill side can be
difficult. I had to use a telescoping aluminum pole to raise the
tape on a 22' 8" tuliptree on a slope above a precipitous
drop at the head of a ravine.
We've found a simple way to measure trunk diameters at any
height with laser rangefinders. This, together with taped
measurements of the base, allows us to calculate the volume of
the entire trunk. For the western giants, volume is the best
means of comparison. Bob Van Pelt is the expert on this, and I
highly recommend his book.
Hope some of this is helpful.
Colby
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