QMD vs QMDS in SummaryByStand
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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?
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
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