Fixed radius sampling

Good Feller · July 12, 2008, 11:07:44 PM

I was wondering if someone could give me a refresher course on fixed radius sampling? Does anyone even use it? Seems like to me point sampling is 10 times easier. What are the pros and cons of each method? Thanks

WDH · July 13, 2008, 12:27:09 AM

Fixed radius is easier to understand. It is my preferred approach.

Either will work if you do it right.

In fixed radius, the plot size is always the same. You can develop a good sense of the plot radius with experience, and that leads to consistency.

The point sampling plot size is dependent on tree diameter, so with large trees, they can be a long way from plot center. In thick brush, you may not even see them.

Stand conditions come into play as well. Both methods are relatively easy to apply if the stand is consistently stocked.

SwampDonkey · July 13, 2008, 06:33:57 AM

Here's some formulas for US units. Tally trees by species, diameter and height (measure some and estimate most). Might even separate dominance classes (dom, Int, suppr).

Density = Tally by diameter class x 12 x plot radius / (diam class x # of plots)

BA = 0.005454 x diameter class² x Density

Vol = Density x Vol calc per diameter class ^* in my area its a volume table based on site index and breast height diameter (standard volume table)

I use a loggers tape (or string box) and flagging tape to mark off the N-S, E-W points on a circle perimeter with compass at 10 meters (33 feet) from a centre stake and interpret in an arc between them on the perimeter. I measure one quadrant at a time in each 90° pie slice.

tonich · July 13, 2008, 06:44:11 AM

Haven’t used this method since I’ve graduated the university.
I’ve been using mainly Bitterlich method of direct basal area determination since. Very easy and fast to perform:

Quote
Plotless Sampling

One of the earliest stereological methods that employed PPS was introduced by Bitterlich in 1939 to improve the efficiency of fieldwork in the forest sciences. Bitterlich developed a sampling method that revolutionized the forest sciences. Up to this time the sampling quadrat method proposed by Pond and Clements in 1898 was still in use. Laying out sampling quadrats at each sampling site was a difficult process at times due to the physical obstructions of the natural world. Besides the physical issues it was also a costly procedure. It took a considerable amount of time to lay out a rectangle and to measure the trees included in the quadrat. Bitterlich realized that PPS could be used in the field. Bitterlich proposed the use of a sampling angle. All of the trees selected by a fixed angle from a sampling point would be counted. The quadrat, or plot as it was often called, was not required.

The quantity being estimated by the researchers was tree volume. The original sampling method was to choose a number of sampling points. The researcher traveled to each sampling point. A quadrat, rectangular sampling area, was laid out at each sampling point. Measurements of the trees in the quadrats was used to estimate tree volume. A typical measurement is basal area.

Bitterlich's method was to choose a number of samplng points. The researcher traveled to each sampling point just as in the quadrat method. At each sampling point the researcher used an angle gauge to see if a tree had a larger apparent angle than the gauge. If so, the tree was counted. No quadrat and no measurements! Just count and go. The result of this procedure was an estimate of tree volume.

Grosenbaugh realized the importance of Bitterlich's work and wrote a number of articles describing the method. Soon a host of devices from angle gauge, to relascope, to sampling prism were developed. The Bitterlich method, employing PPS, and these devices profoundly increased the efficiency of fieldwork.

/ ;D

SwampDonkey · July 13, 2008, 06:57:19 AM

I suspect the lad will be using point sampling, but he asked for input on plot sampling. ;D If you ever encounter trespass cutting, you will likely use a plot to outline the sample and measure stumps and convert to breast height diameter from sampling neighboring standing trees to correlate. Or volume in tree length tables, based on stump diameter. I found the relationship between stump and breast height is pretty much on an R² = 1 or a typical 45 degree graph with sample points pretty much on the line. A prism would be of no use for stump tallies. ;)

We had an elderly gentleman living alone on a farm. He pretty much had no one. Some Quebec mill, desperate for wood hired a couple logging crews to harvest 100 acres of hardwood, some softwood mixed in. I say desperate because of distance from the mill. The mill only paid the guy the typical $10,000 that many mills did around here in the 60-80's for the stumpage. The old guy never seen that much money at one time. Typical situation around here. Then a group of neighbors, acting in interest of the land owner had asked him what he got out of it because the marketing board does do management work for woodlot owners. That's when we were asked to go in and measure stumps and get an area for what was harvested. He never got paid for even half the wood, nowhere near it. The loggers where completely innocent in this, they never got all their pay either. But, the marketing board sent mill a bill, just for the check off (levy), based on our assessment of volume taken. They only paid half that bill I think and it was, and still is, only 2.2 % of volume. The old guy never did get all his pay. We have a lot of these fly by the seat of the pants out fits taking advantage of the ignorant and unaware folks.