PINE THINNINGS - TO MARK OR NOT - HOW MANY TREES TO LEAVE AFTER THINNING – AND TIMING OF SECOND THINNING

By Tom Ebner

In 2006 Dr. Bob Daniels and I published the results of monitoring the growth after thinning in pine stands which had the leave trees marked and in others where the selection of the trees to leave was left to the thinner (operator select thinning). This article was placed on the Web by Lee Laechelt of the Alabama Forest Owners Association in July of 2008 and can be found at www.afoa.org/CILive/CI0807_a.htm .

The recommendations in this article were that marking the leave trees was a good investment (28% return on the marking cost) and that thinning at age 13 and leaving 200 trees per acre after thinning in a marked stand developed the best return.

The growth in these stands has been re-measured annually since 2005 and some additional plots have been added in other stands. The growth prediction equations have been updated with this additional data. The oldest plots have 12 years of growth after thinning

In the process used to develop the data for this article the thinning trigger for the second thinning was set at 130 square feet of basal area per acre. The different combinations of age and trees per acre after the first thinning were grown to 130 basal area and the value of the stand was estimated at that time. The second thinning trigger of 130 basal area was based on what I observed in the field . In fact it was slightly higher than when foresters and landowners in the area of my plots were conducting the second thinning.

Thinning of southern pine plantations has been talked about for a long time but very little thinning was done in East Central Mississippi and Western Alabama prior to 1980. Articles have been published regarding when to do the first thinning but I am not aware of any articles discussing the timing of the second thinning. I have plots on which the basal area exceeds the thinning trigger used in my 2006 paper. The growth on these plots suggests that delaying the second thinning until a basal area 140-160 is reached looks very attractive.

The majority of the stands in which I have plots were thinned for the first time between the ages of 11 and 16. A number of stands thinned at age 17 and older were very highly stocked stands with more than 800 trees per acre before thinning. In some of these stands the diameter growth after thinning increased each year for as long as seven years. For these reasons the age 11-16 thinnings were treated separately from the age 17 and older stands. It will take another 4 or more years of growth data before these older stands can be modeled fairly accurately and therefore the growth of 17 and older thinnings are not discussed in this article.




COMPARISON OF MARKED VERSUS OPERATOR SELECT THINNING

In both the marked and operator select thinnings the average age of thinning in the 11-16 age category was age 14. Prior to the first thinning, the average stand was almost the same (505 versus 521 trees per acre (tpa), 148.7 versus 144.7 basal area, and site index 71.5 versus 70.5 – base 25). The stands after thinning were a little different with the marked stands averaging 186 trees per acre and the operator select stands averaging 222 trees per acre. The marked stands had a 0.76 inch dbh lift from thinning while the operator select thinning had a 0.45 inch lift. The tree quality left after thinning was better in the marked stands with 5.4% of the basal area in fiber quality trees while the operator select thinnings has 9.1% of the basal area in fiber quality trees.

Table 1 shows the results of leaving different levels of stocking (trees/acre) after thinning in the average marked stand and what the stand conditions are projected to be at age 24 (10 years after thinning). Age 24 was selected since at this age the majority of the trees in the stand would meet the minimum merchantability standards for a mill paying a top price per ton for small sawlogs if the owner wanted to harvest the stand or many of the removal trees would meet these standards if the stand was thinned. The minimum log specification for this mill was a 25 foot log with a minimum 7 inch diameter (outside bark) at the small end. To meet this log size a total tree height of 64 feet is needed with a minimum mean dbh of 11 inches.

At age 24 the basal areas for this “average” marked thinning ranged from 162 square feet to 138 square feet at the 170 tree per acre stocking level after the first thinning. If a timberland owner were to use this logic to guide the timing of the second thinning (i.e. trees 64 feet tall and 11 inches mean diameter), it would suggest leaving 210 trees or more per acre at low pulpwood prices ($6/ton) and all the different stocking levels after thinning develop about the same stand value at age 24 if the pulpwood prices were higher ($12/ton).

Table 2 presents the same projection for the average pine stand thinned at age 14 with the operator selecting the removal trees (operator select thinning). In this situation the highest present value at the thinning age of 14 develops with leaving 250 trees per acre after thinning at both the $6 and $12/ton stumpage price for the pine pulpwood.

There are 24 different stands in the marked thinning sample and 22 stands in the operator select sample. The average marked thinning stand has been thinned for 5.6 years while the average operator select thinning stand has been thinned for 5.8 years. The average diameter growth in the marked thinnings is .0.446 inches while in the operator select thinnings it is 0.34 inches, an increase of 31% for the marked thinnings.
The operator select thinning has a slightly larger basal area after thinning (70.2 versus 68.5) but after ten years growth it is roughly 20 square feet of basal area (or approximately 3 years growth) behind the marked stand. The present value of the stand at age 24, discounted to the time of the first thinning at 6% and adding the value of the age 14 thinning income is $330 per acre lower than the marked thinning at the 250 tree per acre thinning level and over $450 per acre lower for the 170 tree per acre thinning level.

This increase in present value per acre for the marked thinning results from several factors. The leave trees after thinning are larger and the larger trees were growing faster before thinning. The plot data also shows the larger trees growing faster after thinning. The tree quality is better in the marked thinning. In the marked thinnings, 5.4% of the basal area was in trees graded as pulpwood, regardless of diameter, while in the operator select thinning 9.1% of the basal area was in pulpwood quality trees. Finally, the annual mortality (as a percent of beginning basal area) in the operator select thinnings is larger than in the marked stands (1.282 versus 0.652). While it is not quantified, the distribution of the trees after thinning appears better in the marked thinnings.

ACTUAL BASAL AREAS VERSUS THE PROJECTED “AVERAGE” STAND

Charts 1 and 2 show the projected basal area for the average marked and average operator select thinnings along with the basal area of all the stands in the data base at their 2009 re-measurement. I now have a number of plots in the marked thinnings with basal areas of 120 or greater with good diameter growth and no increase in mortality. This certainly suggests that the thinned stands could be grown to higher basal areas before a second thinning. The plot data I have suggests that a basal area of around 160 would be a good maximum basal area before the second thinning. This would give the timberland owner more time to span periods of low log prices.


DOES A SECOND THINNING MAKE SENSE?

Table 3 shows the stand parameters of age, trees per acre after the first thinning, basal area, mean diameter, and trees per acre if all the different stocking levels after the first thinning were grown to a basal area of 160 with the resulting stand values discounted to age 14 (the time of first thinning) at 6% and adding the stumpage income from the first thinning at $6 and $12/ton for the pulpwood.

Table 3 would suggest that if an owner did not want to thin the stand a second time or if there was a need for the cash flow from a harvest cut rather than a second thinning, then a lower stocking after the first thinning (170 tpa) would develop a 6% or better return through age 28.

The lower stocking levels after the first thinning should lead to a lower marking cost and possibly a lower logging cost with the higher removals per acre. The estimated diameter lift from thinning from 505 trees per acre to 170 trees per acre of 0.79 inches is an average of some very good marking and some very poor marking in stand of generally excellent tree quality. A good marker in stands of good tree quality should better that gain.

A second thinning in stands with 200 or more trees per acre after the first thinning appears to develop a higher present value at age 28 than the 170 tpa option after the first thinning. In my plots which have had a second marked thinning, I am averaging a dbh lift of greater than 0.75 inches and the stands are averaging 0.48 inches in diameter growth when thinned to 75 trees per acre.

In projecting a stand with 200 trees per acre left after the first thinning and thinned for the second time at age 23 with a basal area of 139 to 75 trees per acre with a 0.75 diameter lift and an average annual diameter growth of 0.5 inches/yr between the ages of 24-28, I develop a present value which is $240/acre better than for the 170 trees per acre option at age 28 shown on Table 3.


HOW MANY TREES PER ACRE TO LEAVE AFTER THE FIRST THINNING

I think the answer to this question is to leave the larger sawtimber quality trees with good distribution. I have 50 plots in which I have mapped the location, diameter, and quality of each tree on the plot. When I do a paper thinning of these plots with the above criterion I rarely find more than 300 trees per acre meeting this standard. With a one row in five removal you will lose 20% of these trees which would leave 240 trees per acre after thinning. My data suggests that any stocking level after a good marking and thinning which leaves 170-250 trees per acre will develop approximately the same present value without a second thinning and a higher present value if more than 200 trees per acre are left after the first thinning and a second marked thinning is conducted around 140 basal area.