## BIG BAF
I once heard a seemingly lone, but determined voice say, "We measure too many trees in British Columbia." This simple statement is insightful and significant. I am, of course, referring to cruising timber in BC. And yes, we do use a prism for
point sampling. However, we are - a sampling error of less than 15% @ 95% confidence (for volume/ha), or
- a sampling intensity of
When only full measure plots are used, sampling error is based on the variation (i.e.
CV) of volume/ha between the plots. This CV is typically between 30-50%. Now we do have the option of sampling with full measure plots (all trees fully measured) AND count plots (where simply a tree count is taken). This breaks the calculation of volume/ha into two parts: BA/ha (from tree counts) and VBAR (volume/BA ratio – based on measure trees). The CV’s for BA/ha and VBAR are approximately 45-55% and 15-20%, respectively. Given that we actually do know the typical CV’s for these two variables, we should optimize our sampling. Unfortunately, we do not. A rough approximation indicates an optimal ratio of one measure The following is a description of a project carried out by the second year class of the Forest Resources Technology Program at Malaspina University-College.
- Point-3P – specifically Point-3P tree – (P-3Pt )
- 4P – more properly known as: Point-3P cluster (P-3Pc) or 3P-Point, (hence 4P)
- Big BAF – 2 prisms, one being a big fat one
This paper presents only method 3, Big BAF. Eight full measure plots are required to fulfill Ministry of Forests sampling requirements (i.e. sampling intensity = 1 plot/ha). Note that a minimum of 32 measure trees is required, although more are likely to be measured. The objectives of this project were to: - test the mechanics of executing each sampling method,
- discuss any accuracy issues, and
- extrapolate the efficiencies of these methods to larger block sizes where reduction in work and/or increase in precision will occur.
What follows is a description of the Big BAF method followed by a summary of our findings.
^{nd} growth Douglas-fir. This provides the tree count to be used for BA/ha.
Then, on that same point, a second sweep is made with a big fat prism, say with a (metric)
BAF of 165 (imperial: 165*4.356 = 718 BF/acre). This second
sweep takes in far fewer trees. Only the trees "in" the second sweep are
measured. Objective: take many sample points as BA/ha has a considerable CV (~55%) but measure relatively few trees as the CV for VBAR is small (~15%). A second advantage of this method is that the measure trees are not clumped in a few plots but are instead spread throughout the block. Volume/ha is calculated based on the following: volume/ha = BA/ha * VBAR where: - average tree count (number of "in" trees) determines BA/ha, and
- individual tree measures provide volume/BA ratio (VBAR).
The volume/ha determination is essentially the same as prism cruising with count plots, except two different prisms are used: one for tree count (i.e. BA/ha) and a second for measure tree selection. In the end, more sample points are established than "traditional cruising" but fewer trees are measured. In addition to sampling efficiency and better dispersal of measure trees, a third advantage of this method is a superior estimation of species composition. This is because the BA is more intensively sampled.
Sampling error is based on the variation of It is important to note that, unlike prism cruising with count plots, BA/ha is based solely on the count plots (i.e. the tree counts from the "Big BAF" plots are not used). This is because there will be great variation in BA/ha based on the big BAF – typically no trees in many plots and few plots with one or two trees (with each tree representing a substantial basal area). Mechanics of the Cruise **Office planning:**(for count plots): Given that the estimated BA/ha of our block is 55 m Little BAF^{2}/ha and we are targeting an average of 5 trees/count plot, we would want a BAF of 11 (55 m^{2}/ha ¸ 5 trees/plot = 11 BAF). (Remember the American version is BAF = 48 BF/acre)
**Big BAF**(for measure trees): If we want to measure only 12 trees over 36 sample points, we would on average measure a tree every 3rd plot. This gives us a target of 0.333 trees/plot, therefore, 55 m2/ha ÷ 0.333 trees/plot = a BAF of 165. (American BAF = 718). An alternate way to calculate this would be if you want to measure every 15th tree, use a BAF 15 times the size of the "count BAF."
- Take a sweep with a "regular" size prism, e.g. for 2
^{nd}growth BAF = 11 (or BAF=48 BF/acre), and tally the "in" trees. - Take a second sweep with a much bigger prism, e.g. BAF = 165 (or BAF=718 BF/acre) to select any measure trees. Fully cruise any trees in the second sweep.
Note: it is important to establish the sample point without bias. Specifically to ensure that if the sample point lands inside a tree, THAT is where the sample point is. There is often a bias to establish the sample point where we can physically stand. As the plot radius factor is very small for a BAF of 165 (or 718 BF/acre), it is important to establish plot centers properly. The prime reason is not for BA determination, as it is less affected by a bias of a few centimeters (or inches) in plot location. Instead, if there were an unconscious bias to move the plot centers from the inside of trees, it could result in selecting too few measure trees. Also, note as few trees are measured, it is important to not miss measuring less common
species. This is easily handled by measuring the first tree of a "new" species
when it occurs in a count plot, i.e., it becomes an
^{2}/ha (718
BF/acre).The proper way to compile this cruise is to determine BA/ha solely from the sweeps using the "regular" BAF (in our case BAF = 11, or BAF = 48 BF/acre) and to determine VBAR solely from the sweeps using the big, fat prism (in our case BAF = 165, or BAF = 718 BF/acre). Determination of sampling error was already described under "Statistics."
Thus, life is simple Need I
say more?and efficient!
Malaspina Forestry Grad Class of ‘98 for
the enthusiasm and effort they afforded this project. Also, credit for the ingenuity of
the sample method rests solely with Kim Iles; any errors in its description or execution
are mine. Kim, thanks for your patience and generosity of time. |

*Originally published July 1998*