Virtual DBH’s are used to process diameters of forks and reductions when processing data from PlotSafe to YTGen.
A virtual DBH is a DBH applied to a stem where the stem does not exist at breast height. It represents the over bark diameter that, in combination with taper and volume methods and the full stem height, best define the shape and size of the stem in the parts where it does exist.
When a tree includes a diameter reduction or fork the diameter is estimated by the crew, according to Figure 1 a fork is present at 9m and the crew has entered these as 325 and 350mm at 10.4m.
When the data is processed, using YTGen a virtual DBH replaces the estimate by crew. Figure 2 displays the principals – forks and reductions are calculated using the volume and taper function designated.
The structure of an YTGen population file (.ytf) is displayed in Figure 3
Figure 4 is the .ytf equivalent of the PlotSafe MARVL stem features displayed in Figure 1.The file contains two stems L0 and L1 (fork), main stem has a measured DBH of 53.2cm at 1.4m, while virtual DBH used for the fork. Figure 2 displays this principal in the use of virtual DBH, in this example the DBH of D (reduction) is 38.9841 and L1 (fork) is 36.2006cm at 1.4m (primary and secondary leader).
It is important to note that the diameter present in a .ytf file is the virtual diameter at breast height (that being the virtual DBH), and not the diameter recorded in PlotSafe (by the field crew) as shown in Figure 4 vs that recorded in Figure 1.
A good way to understand Virtual DBH is to test it using the test stem feature in a methods file within YTGEN. Right click in the methods windows dfialog and set the DBH of the primary stem. Then selecting a volume and taper method tou can see the resulting diameter up the stem in 1m increments (as shown below in Figure 5 and 6).
By the use of the volume and taper function, Figure 5 visualises the projection by using measured DBH at 1.4m (53.2cm) to the point of reduction. Figure 6 is the virtual DBH equal to 38.9cm at 1.4m for the reduction (primary stem). Figure 7 is the virtual diameter of the fork, DBH = 36.2cm.
Using Virtual DBH to Validate Upper Stem Diameters from Field Inventory Staff
One of the common questions from people new to YTGEN and PlotSafe is why is it ok for field staff to estimate upper stem diameters? should we not be measuring them more precisely? This comes down to how we actually check on the field assessment of the diameters called by the field crews. The upper stem diameter is adjusted if;
the virtual DBH of the upper stem diameter (be it from a fork or a reduction) is greater than the actual DBH of the tree, at this point it is adjusted to match the DBH of the tree.
the cross section area below the fork must be greater than or equal to the sum of the cross sectional area of the resulting stems above the fork. The assumption is that the crew have not measured 325 and 350mm, they have guessed them and to PlotSafe and YTGen it is the relative size of each stem that counts and get used in calculation the virtual DBH’s so that the proportions of each forked stem remain the same after any adjustment. Therefore the field crews should be trained to get the stem proportions correct i.e. that fork 1 is 50% smaller than fork 2. During conversion if any adjustment is made in calculating the Virtual DBH then these proprotions will still hold true.
Alternatives to using a Virtual DBH
An alternative to using a virtual DBH to define a secondary stem is to treat the base of the secondary stem as if it were at ground level. In this case, biometric methods receive as input
• the overbark diameter at breast height plus stem base height as a proxy for DBH
• total stem height less stem base height as a proxy for height
i.e. the stem is treated as a small tree rather than a small part of a big tree. YTGen does not use this approach because of the effect it has on the modelling of tree growth. In many individual tree growth models, small trees don’t get any bigger and then they die. Small parts of large trees continue to grow.
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