Fixed radius small cruise accuracy

[croyer]

Hi All,

I'd did a 12 acre inventory using (12) 1/10 fixed radius plots. The numbers that came back seemed high so I did a full tally of one species of 14" to 25" trees. The numbers are way off (tree count and boar feet). The app says 147 trees and the count was 68. App is calculating that correctly.

Is this just too small an area to get an accurate estimate using sampling? Do I need more plots?

Thanks.

[stavebuyer]

Nothing wrong with a 10% sample on a fixed radius plot, but with that few plots it would have to be a uniform stand to be representative. Two plots in a poplar hollow, and one with a big Oak leaning toward the fence that they left on the last harvest that will count for 10. Fixed radius tends to be more accurate on young stands where you get a good number of trees in your tally.

[croyer]

Makes sense. Thanks.

[Ron Wenrich]

Since you have the data for a fixed plot, why not run a variable plot at 10% and see how things stack up?  With the variable plot, the big trees may add up, but their representation is a lot smaller.

For example, counting 1 14" tree represents 9.35 14" trees/acre.  A 24" tree represents 3.18.   It should give you a pretty accurate count of the volume and the BA/acre.  It also breaks the stand down by size class and species, if you record and count your data right.  Its a good way of using the data for mgmt purposes.  

The problem I've always found with fixed plot is it takes a long time to do a proper job.  The plot has to be established, and you have to do an accurate radius around it.  What do you do about border trees?  There is a lot of room for bias.  We always want to add the big trees when probably we shouldn't.

The other thing is to have a fairly accurate map which divides your stand into different types.  Without that, you may miss areas that shouldn't be included in the count or there is too much representation in the count.

I wrote several articles on how I did variable point cruising many years ago.  We have it in the knowledge base.  The knowledge base doesn't link very well to the specific article.  This link will take you to the knowledge base:  https://forestryforum.com/board/index.php?action=tips  

From there, press the Forestry category.  There are 3 short articles on point sampling.  Equipment, cruise and number crunching.

[BrandonTN]

No, 12 acres is plenty big enough for plot sampling.

Heterogeneity of the 12 acres matters for determining a good # of plots. How evenly distributed throughout stand are the species you 100% cruised? If theyre all bunched in a section that could make the mismatched results. Did you do plots on a grid covering whole stand? 

If your plots are for silvicultural prescription only, and not timber volume, and the stand is a relatively homogenous mature sawtimber stand, id say 1 plot per acre is plenty. Maybe you used incorrect plot size on the ground? 37.2 ft is radius of 1/10th ac plot

[SwampDonkey]

I've used fixed plot in plantation and stands that where spaced with clearing saws. Had good results. Can verify the density works out as I know roughly the density of the planted or spaced trees anyway. Mature timber where size and spacing is more variable and with many stands I use point sampling. Stratify the woods into homogeneous stands, find the areas also. A GIS with the photo layer or a dot grid tossed on the photo to find area.