Forestry Questions

George, it is not as simple as just finding BA. You should also have mean stand diameter. In stands with more than 30% hemlock, 9″ MSD residual BA should be above 120 sqft/ac.. For similar stands with 16″ MSD, residual BA should be 160 sqft/ac or higher.

Every tree that you count (larger than your thumbnail at arms length) equals 10 sqft/ac.

Of course this is all depends on generalizations on a stand-wide basis, and total acceptance of even-aged management. In Hemlock you will notice that it generally reproduces in patches, so you may be reducing BA in one area and not in another. Also Crop Tree management is more of an all-aged management strategy, which is based more on diameter distribution than on BA.

One of the subtleties of crop tree management is recognizing that sometimes there are no crop trees without creating openings where regen can get established.

If you are just trying to keep the overstory stocking high, then the rule of “thumb” will be adequate. I would say you’d be safe keeping the residual BA above 120-140 saft/ac. (12-14 tree bigger than your thumb, or nickle).

Carl

Stand in one spot and slowly turn in a circle counting fat trees as you go around. Don’t worry about distance from any one tree. Your focus is the stand.

Just wondering – did BA replace DBH (diameter breast height) as a more “functional” tool? They are essentially the same thing except that BA converts DBH to an area (I think).

I realize that this must be really confusing for those of us who haven’t spent a lot of time in a forestry classroom… Here is a good article that explains it better (with a lot of stuff about cruise design that you may or may not be interested in):

https://docs.google.com/open?id=1cFbziHEhTr-THRj75RzUUWi3YPUDpIRhsuHzPMNGlg0IJQ2BV1OtKExPZGPA

To answer Geoff’s question:
DBH is still used, but basal area is a way of measuring DBH across the stand. To find the (Quadratic) Average DBH in the stand, you can take the Basal area number and work backwards:
-Take the square root of (Basal Area/ .005454). In this way, Basal Area kind of represents DBH.

A prism (or angle gauge, thumb, or nickel) just takes away the need to measure DBH in order to find Basal area by assuming that each “in” tree (judged by your prism, or each tree smaller than your thumb or nickel) equals a certain area per acre (10 ft2 for a 10-factor prism, the typical thumb or nickel, 5 ft2 for a 5 factor prism, 20 ft2 for a 20 factors prism, etc. (“factor” and BAF are the same, by the way). If you were doing a fixed radius plot (which, by the way, might be a better idea if the trees in the stand are really small), you would need the DBH data in order to calculate Basal Area, but the prism kind of leapfrogs that.

Confusingly, foresters still do measure the DBH of trees “in” their plots, and they use these numbers to extrapolate trees per acre (I think Carl touched on this). If you look around you can find a chart specifically designed to tell you how many trees per acre the tree in your plot, with a certain DBH, represents. The smaller the tree, the more trees/acre it is assumed to represent. Here is the calculation:
43560/ ((3.14(DBHx2.75)^2) = number of trees per acre represented by each tree of that DBH in your plot (at least using a 10-factor prism or equivalent.

I told you, it’s confusing.

As far as hemlocks (a tree with a special place in my heart), the forest service published a hemlock stocking guide. The USFS is not the best about everything, but this should provide some ideas about stocking and management and guide the semi-scientific, semi-intuitive decisions most of us make in the woods.

http://na.fs.fed.us/pubs/fhp/hemlocks/managing_eastern_hemlock_screen.pdf

So the one component that I didn’t have time to get to before is that BA is a stand-wide measurement, and therefore should not be used as a cutting guide in small sections, nor on a tree-by-tree basis.

Understanding the biometrics of stand structure is really only a start at the science of forestry. We (us forester) have all kinds of tables and charts to help us decipher management strategies etc. When we talk about reducing BA we are speaking on a stand, or even landscape, scale. These guidelines are based on an even-aged philosophy of growing trees at their optimum growth rate given competition with other trees. Reduce stocking, increase growth rate.

However, when it comes down to application it is much more art than science. We rarely use a prism, or BA tool, when marking for harvest. Those decisions are based more on specifics of tree health, growth form, and other micro-site factors. If a stand is “over-stocked” the theory is to reduce stocking. There is a minimum stocking level below which a thinning could decrease the growing stock to a point where the stand area is not being used to its fullest capacity. These lower levels may also allow for the establishment of undesirable species. However there are times when we purposely reduce stocking to actually encourage stand regeneration.

All of this is to say that when trying to maintain adequate stocking, the guidelines are just that. If there is a lot of defect and decline in a section of your woodlot, you may want to do a patch-cut, significantly reducing BA. There may be other areas where you may want to maintain a higher stocking than the guidelines call for.

George, I think you should be able to develop a “feel” for what you want to see in your woodlot, and not have to carry a calculator…. I don’t. One important thing to remember is that “overstocked” is a purely human concept. Forests naturally strive to an Overstocked condition, with mortality, blowdown, defect. This condition is what leads to the ecological state known as “forest”. To blindly reduce stocking purely to increase growth rate of trees is too simple. Just look at the way your stand wants to grow, and work accordingly.

Look at the individual trees in the cutting area and decide based on your experience watching them, and from what you know about the ones you have cut, and decide whether or not they need more room, or if there are some that are wasting your resources, or if they can just keep going for a few more years, or whether you need a new stand to start over with.

Carl

@Scott G 31153 wrote:

…. Overstocked is definitely not just a human concept in the Rocky Mountains. In my region, it truly is a measure of a site’s carrying capacity….

I did not mean that it was not a real forest condition without ecological ramifications. It’s just that without human evaluation and intervention, the situation would be self limiting, with some results being perhaps more ecologically important than the maintenance of the covertype or the currently established biological community.

The basis for these stocking guides is timber management, so the underlying definition of “overstocked” has to do with the condition of the forest in terms of producing timber. If there are other objectives such as wildlife habitat, or soil protection, or aesthetics, then the term “overstocked” is out of context.

It is a pet peeve of mine because it sets up a finite expectation for people in how they look at biological communities. Life is a continuum, where extremes are attained, collapse/mortality occurs, and recycling and regrowth follows. Restricting certain extremes from occurring has its place, such as in fire suppression, but on a whole I think we need to allow for more appreciation of broader ecological goals. In my mind letting sections of forest attain an overstocked condition can have very important impacts.

Carl