All right. Now that you’ve set up your monitoring study, it’s time to dive into the actual measurements.
One of the most fundamental tree measurements is diameter at breast height, or DBH.
It’s a measurement of the tree’s trunk that is typically recorded in a tree inventory or your monitoring projects.
I’ve taken the dbh before!
It’s pretty straightforward - a measuring tape is wrapped around the trunk of the tree
at four and a half feet from the ground.
You’re right. This measurement is generally pretty easy to record, but there are a lot
of special circumstances that can arise, and rules to follow in those special cases, so
in this video, we’re going to review those rules.
Recording DBH – and recording it consistently and accurately over time – is important for several reasons.
Urban forest managers as well as researchers might be interested in how mortality rates vary
across different size classes of trees.
They might also want to know how much a tree is growing, with slow growing trees being
interpreted to have poorer overall health.
I’d definitely like to measure growth.
How do I do that?
The simplest case for recording DBH is when it’s possible to measure at 4.5 feet
and the trunk is not leaning.
Measure up from the trunk base, where the trunk meets the ground.
Confirm the best measurement spot.
If 4.5 ft works, then it’s smooth sailing.
Do I really have to measure to 4.5 ft?
Can’t I just know where that is on my body and use that?
No, not really. Not if you want to measure growth accurately.
It’s important to re-measure at the exact same spot in the future to get trunk growth.
Plus measuring up the trunk really doesn’t add much extra time.
I recommend using a stiff household measuring tape to get the right spot on the trunk.
Some field crews might also find it convenient to have a measuring stick or PVC pipe with
markings for 4.5 ft.
So once you find 4.5 ft, hold that spot with your fingers, or mark it temporarily with chalk.
You want to be sure that the measurement is taken at exactly 4.5 ft.
Then wrap the tape around the trunk.
So how are diameter tapes different than a regular measuring tape?
Oh right. The diameter tape, or D-tape as we like to call it, shows measurements divided by the value of pi.
So it converts from circumference to diameter, right there on the fly.
So that means if I don’t have access to a diameter tape, I can just use a regular tape, right?
Absolutely, but then you have to remember to convert from the circumference to a diameter.
Be careful about how you record that!
OK, I’ll remember that.
You mentioned earlier that there are special considerations. What are those?
The D-tape should be wrapped snug, but not too tight.
If a tree trunk is big, one person on the field crew should walk around the tree to
make sure the tape hasn’t gotten twisted or caught on bark.
You know, another really important thing to remember is to read the tape correctly, from right
to left, rather than from left to right.
This measurement is 4.3 inches, not 5.7 inches.
One thing that I see often is that a tree will lean to one side.
How do I measure the 4.5 ft distance?
You would still need to measure the diameter 4.5 ft from the ground.
The key here is to measure the 4.5 ft distance along the underside face of the trunk.
Then measure diameter perpendicular to the trunk.
So one thing that makes measuring any tree difficult is when there are obstructions,
irregularities or things like vines on the trunk.
If possible I usually rip the vines off since they shouldn’t be there anyway.
That’s one good way to do it - you definitely don’t want to include them in the measurement.
But if it’s something you can’t eliminate, like a bulge, a swelling, depression, or branches
right at 4.5 ft marker, measure immediately above that, right where it goes back to normal,
and record that height of the measurement.
For issues like branching flare, where going above the spot makes things worse, you can
measure below the fork to avoid the swelling.
As before, be sure to record the height at which you took the measurement.
That makes sense, but what if the swelling is actually below the 4.5 ft.
I assume I’d measure a bit above all that swelling?
Yeah, you're right.
Our rule of thumb is to go about 1.5 ft above the end of the swell.
OK, so to recap, I want to avoid any sort of irregularity by generally measuring above
it if possible, but if that doesn’t make sense, then below it.
Yep. So I think we’ve covered single stem trees pretty well.
Do you ever encounter multi-stemmed trees?
I’m not sure.
What exactly are multi-stemmed trees?
These are basically trees that don’t have a single trunk.
These trees seem like a pain to measure.
Yeah, they can be, but let’s try to simplify things.
There are three scenarios which differ based on where the tree forks.
The first scenario is for when the tree forks at or below one foot. Record each stem as separate stems.
Think of birches that fork around the ground level.
So when you go to measure, start with the largest stem and record in a clockwise direction.
If there are many stems, measure the six largest.
And of course record the height of where you measured separately for each stem.
In the second scenario, if the tree forks between 1 foot and 4.5 ft, record as a single
trunk, below the fork.
Record as close to 4.5 ft as is reasonable given the swelling near the fork.
The measurement height can be as low as one foot.
I feel like there are certain trees that are more prone to looking like this.
In my experience, cherries tend to fork around eye-level.
Um-hm. It does seem that way.
Generally this scenario covers cherries, crabapples, and hawthorns.
Lastly, for scenario three, if the tree forks at or above 4.5 ft, record as a single trunk,
at 4.5 ft.
I know that was a ton of information so let's try to recap.
When recording DBH, for every tree, the following should be recorded:
the diameter measurement itself. The actual height used for taking that diameter measurement,
so that you can re-measure at this exact same point in the future to get growth readings.
The units used, whether inches or centimeters.
For more information on topics covered in this video, see Urban Tree Monitoring Field Guide and Resource Guide.
Thanks for watching.