Quantitative Silviculture: using BDq to regulate diameter distribution in a stand of Durango and Teocote pine

Introduction

Your task for this assignment is to use BDq to develop targets for numbers of trees to cut and residual stand diameter distributions in a mixed stand of Durango pine (Pinus durangensis) and Teocote pine (Pinus teocote) in northwest Mexico.  

For the purpose of this assignment, we are going to imagine that we are charged with harvesting this stand using the Single Tree Selection silvicultural system, and converting it to a balanced diameter distribution.  

You will harvest the stand over using the BDq technique, with the objective of getting your stand as close to the idealized reverse-J shape as possible in a single harvest.  

The data are based on an actual stand that I sampled during my Ph.D. However, I have randomized the data somewhat around the actual values, so that each of you gets a unique data set for the exercise that follows. Refer to the file: BDq sample Data.xls to retrieve your data, which will be identified by your student number.

Durango and teocote pine are particularly abundant in ecosystems that are affected by frequent low intensity surface fires. These fires sweep through at 5 – 20 year intervals, thinning out the naturally regenerated seedlings and killing some of the younger trees. Fire also promotes further natural regeneration by preparing suitable mineral soil seed beds for natural regeneration. These fire effects promote the development of naturally uneven-aged stand structures.   

I am going to ask you to calculate target stand structures for one value of q (as defined in the lecture), a specified target residual basal area (B) and maximum diameter (D). You get to pick from among three alternative values of q (see below).

The way to calculate BDq is shown in Reading 2 of Section 4 of the Silviculture Reader.

Target Parameters for Residual Stand

Tree numbers in the stand have been calculated on a per hectare basis, and the total numbers vary from about 5,000–6,000 trees ha^-1 (depending on which data set you have). These trees are spread over 16 diameter classes. The first two diameter classes are 2.5 cm wide and represent seedlings and advance regeneration. The other diameter classes are 5 cm wide.  

The stand basal area before harvesting is approximately 45–50 m² ha^-1. This is relatively low, and means that this stand has probably been thinned or selection harvested in the past.

You have the following target parameters to work with:

Target for Residual Basal Area (B) ................................................................... 30.0 m² ha^-1 Target for Maximum Diameter (D) ......................................................................... 60.0 cm.

Choose from one of the following q-values (look at a histogram of your trees first):

q values: q1 ..........................................................................1.2  q2 ..........................................................................1.4  q3 ..........................................................................1.6

Tasks and Questions

1. Complete the data table (which is in the Assignments folder on the Nexus site, as well as on the previous page). This will require you to do some simple formulas in Excel. Recall that basal area of a tree is the surface area of the trunk, given the diameter of the tree, so you will need to recall the formula for the area of a circle, and then extrapolate to the area per hectare for each diameter class (4 marks).
2. For each of three q values (1.2, 1.4, and 1.6), calculate the basal area and numbers of trees in each diameter class in the post-harvest stand. The target basal area is 30 m²ha^-1 and the maximum diameter is 60 cm in all three cases. Show the target basal area and the numbers of trees in each diameter class for each q-value in a table.  (6 marks)
3. Calculate how many trees you should plan to cut in each commercial diameter class and how many trees you can actually cut for each of the three q-values. Recall that you may have surpluses and deficits in some diameter classes compared to the desired composition because of unevenness in the actual distribution. Put these data in new columns in the same table (6 marks).
4. For which q-value will the task of creating a balanced diameter distribution bethe easiest? Give your reasons. Recall that you are trying to move the stand towards the classic negative exponential distribution, but you may not get it all the way there in one round of harvesting (4 marks).
5. Some silviculturists have suggested that more than one q-value should be used in some stands. Specifically, it has been suggested that a different q-value might be needed for smaller diameter classes. Do you think you should use a different q-value for the advance regeneration and seedlings in our pine stand? If so, should it be a larger or smaller q-value. Give the rationale for your choice of answer. (4 marks)

Finally, comment briefly about the structure of the forest stand that will be created using your chosen q-value. Remember, diameter distributions tell you a lot about tree size, the density of the stand, and therefore, indirectly, about light conditions. What sort of stand have you created? Is it more open, more closed than the original? What else can you tell me (One or two short-ish paragraphs: 4 marks).  

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