What is the difference between QMD and QMDS?
Hey Rich,
I've noticed that the stand-level summary data (SummaryByStand) features both QMD and QMDS. Based on our conversation, it sounds like QMDS is what we would traditionally think of as stand-level quadratic mean diameter:
QMD = (BA / (TPA * 0.005454154)) ^ 0.5
where
BA = stand basal area in sqft/ac
TPA = trees per acre
What is QMD, then?
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Hi Eli,
QMDS is from stand level BA and TPA. QMD is the average of plot QMDs.
I was curious to see the difference when plot to plot variability is large. QMD is larger than QMDS in these cases. I think due to the plot avg version weighting each plot QMD equally while stand level in effect weights QMD by tree TPA.
Stand 010302 in the sample database is a good example. 3 plots with ~.1" DBH trees. 3 plots with ~20" DBH trees. 1 empty plot. What is the best estimate of the "tree with average BA"? The plot average: 15" or the stand "avg": 4"? Does it matter?
It's custom to use stand level BA and TPA for QMD so refer to QMDS. I am probably going to rename QMDS to QMD and plot level QMD to QMDP or remove it entirely.
Rich