If you work in forestry, especially with
the type of job you might get right out of undergraduate,
at some point you're going to be doing some forest measurement or mensuration.
This might be as part of a timber inventory, or this
also can be referred to as timber cruising. And there's a lot of reasons
you might do it. I... in some cases it might be just to take
an inventory of a forest stand to get an idea
about what the volume is, what the basal area is, what the condition of the stand
is. It might also be something that's done
before a thinning, or before a timber sale, to get a good
inventory of the trees. Or if you're doing scientific research,
or have ecologic... ecological objectives, it might also be
important to have an idea about the standing timber. So things that
we might be interested in: basal area is something that we're often
interested in. So you remember when we measure trees, we measure them at breast height. Basal area is a measure of the
cross-sectional surface area of the tree, so if you were to imagine
cutting down that tree at breast height and measuring the
surface area of that stump, that would be the basal area of that
tree. So... but we don't want usually just the basal
area of an individual tree, we want the basal area of a stand, and
typically we report that in square meters per
hectare. So square meters of basal area per hectare.
So that's one thing that we're interested in, and along with these other
all sorts of measurements, we often want to have these measured by
species, so knowing the total basal area of a stand
is really important, but also knowing what proportion of that basal area is
douglas fir, versus western hemlock, versus western
redcedar, or western white pine... that's important as well. So basal area
is something that's really important. And another key factor that we want to
measure, or another key statistic that we want to measure, is
volume. There are a number of ways to calculate the volume of a tree
relatively precisely. For our purposes though, we're going to use more of an
estimate approach. If you take that tree trunk and you
imagine that it's a cone, then we can take
the basal area of that tree, so the cross-sectional
area of that tree trunk, multiply that by the height
of the tree, and then divide by one-third, and that will give us a volume
for that tree, or a volume estimate for that tree, in cubic meters.
As I mentioned, basal area is something we can calculate for an individual tree,
or we can scale it up to a stand level and give a measurement
per hectare for the stand. We can do the same thing for volume. We can estimate
the volume of an individual tree, or we can scale
that up to the hectare and get the volume of wood
per hectare for the whole stand, and we often will also split that
up by species, because for management purposes, or timber purposes,
knowing how many cubic meters per hectare of douglas fir versus western
redcedar versus other species you have is very important.