Case Study Name (Landowner)

Danish Thinning Methods in Red Pine (UMN-CFC)

Stand Information
State or Province: 
Nearest city or town: 
Plant community or habitat classification and growth stage: 
Forest Health Threats: 
Estimated year of stand origin: 
Additional information about stand origin: 
This stand originated under an old stand following a fire in 1911.
Site Index: 
for species: 
red pine
Silviculture System: 
Brief silvicultural objective: 
Trial three different thinning methods in middle-aged red pine. Thinning regimes include A - thinning from below (light), B - selection (from the top), and C - thinning from above (heavy crown thinning).
188 Omega loamy sand, 0 to 2 percent slopes. Sandy, mixed, frigid Typic Haplorthods.
Stand area: 
9 acres
Treatment area: 
1 acres

Experimenting with various Danish thinning methods on red pine and measuring growth over 40 years.

Silviculture Objective(s)

Three plots were thinned according to Danish standards in 1948 and 1960. The treatments included a light thin of plot A, called a thinning from below and above; a moderate thin of plot B, called a thinning from the top; and a heavy thin of plot C, thinning from above and below. These thinning methods will be discussed further in the prescription section. There were no reported considerations for climate change or other forest health issues; rather, measurements were taken at 4 intervals following the initial treatment in 1948 to monitor the response of red pine to these methods.

Pre-treatment stand description and condition
Stand establishment and management history: 

1911: Stand re-initiating harvest and natural regeneration.

Pre-treatment species composition: 

Table 1: Plot species composition in trees per acre (TPA) in 1948 prior to the first thin. Plots A (light) and B (moderate) had very little Scots pine, aspen, and jack pine, while plot C (heavy) had 88% red pine and 12% jack pine.

Plot Thinning Intensity Total TPA Red pine TPA
A Light 1322 1260
B Moderate 1223 1166
C Heavy 951 840
Pre-treatment growth and stocking: 

See the supplemental figures 1, 2, and 3 outlining diameter distributions for the three plots. 

Pre-treatment forest health issues: 

None were noted.

Landowner objectives/situation: 

Part of the mission of the Cloquet Forestry Center per the 2016-2025 Management Plan is to conduct applied research; in particular, to “initiate, attract, and support new and ongoing research related to northern forests, to continue to build credibility through the maintenance of an updated forest inventory, and the application of well-organized and integrated long-term data sets to new problems.” While the treatment took place in 1948, the Cloquet Forestry Center was founded to provide a place for research carried out in northern forests. To that end, applying novel Danish thinning methods to a stand of red pine is well within the scope of the Cloquet Forestry Center.

Silviculture Prescription

The researcher who began this project, Borge Lund-Thomsen, appears to make a distinction between a light thin, described as a thin from below and above, and a heavy thin, described as a thin from above and below (emphasis his). After examining the diameter distributions of the cut trees as a percentage of total trees per acre, it can be assumed with reasonable confidence that this treatment included elements of both a thin from below and a thin from above with different emphasis on certain diameters or crown classes depending on whether the treatment was applied to plot A or plot C. Two entries were planned, with one treatment taking place in 1948 and a second, similar treatment to take place in 1960. The entirety of these plots was treated for a total size of 0.75 acres. Following the second treatment, the stands were to be monitored. The goal of this research project was to see how the Danish thinning methods affected red pine and consequently most non-red pine trees were targeted for removal—though even before the treatment the stands were heavily dominated by red pine from a species composition perspective.

What actually happened during the treatment

1948 Treatment

According to a project abstract from circa 1955, “Felling was done with an axe and all stumps were cut at ground level. Bucking was done after all felling and swamping had been completed. The tops were trimmed to the very tip and the slash was left where it fell.” The light thin removed the smallest amount of basal area, totaling 24 ft2/ac while the selection and heavy thins removed approximately 40 ft2/ac (see table 2). A fourth plot, southeast of plot A (see figure 1), was supposed to receive a heavy thin but was never cut. Trees were only tallied and marked for a cut that would never occur: plot D was not cut later and was not mentioned in subsequent research documents.

Table 2: pre-treatment and residual basal area per acre and QMD following the treatment in 1948.

Plot Pre-Treatment BA (ft2/ac) Pre-treatment QMD (inches) Residual BA (ft2/ac) Post-Treatment QMD (inches) Cut Tree QMD (inches)
A (light) 155 4.6 131 4.8 4.1
B (moderate) 146 4.7 102 4.3 6.5
C (heavy) 147 5.3 106 5.3 5.3

Refer to the supplemental content figures 4, 5, and 6 for diameter distributions of the cut and residual trees. 

1960 Treatment

Following the treatment in 1948, the stand was measured again in 1960. A total tree census was taken at that time, continuing measurements taken in 1948: DBH and crown class were recorded for every tree and tree heights were intermittently taken. Following those measurements, the treatment was renewed in the same year. Plot A (light) again saw the lowest amount of basal area removed in this entry, with 34 ft2/ac, but plots B (moderate) and C (heavy) had 46 and 39 ft2/ac removed, respectively (see table 3). 

Table 3: pre-treatment and residual basal area per acre and QMD for the 1960 entry.

Plot Pre-Treatment BA (ft2/ac) Pre-treatment QMD (inches) Residual BA (ft2/ac) Post-Treatment QMD (inches) Cut Tree QMD (inches)
A (light) 199 6.1 166 6.2 5.5
B (moderate) 178 5.6 132 5.1 8.2
C (heavy) 154 6.5 115 6.3 7.3

Refer to the supplemental content figures 7, 8, and 9 for diameter distributions of the cut and residual trees. 

Plans for future treatments

While there were no more treatments, the total tree census was continued in 1966, 1971, and 1985. In 1985 the total heights of all remaining trees were taken (see table 4). Plot A, with a stronger emphasis on trees of lower crown class, had the highest average height. Plots B and C, which were thinned with varying emphasis on trees of dominant and codominant crown classes, had lower average heights that are almost identical.

Table 4: The arithmetic average height of the remaining trees in the treatment area in 1985, in feet.

Plot Average Height (ft)
A (light) 71.1
B (moderate) 63.7
C (heavy) 63.9

In addition, the diameters at breast height of every tree continued to be recorded which allowed basal area per acre to be calculated over time from the beginning of the first treatment in 1948 through 1985 (see S10). The light thin had the highest basal area per acre over time, though that could be accounted for at least partially through a smaller amount of basal area removed at both entries. On an individual tree perspective, however, the QMD of plot A was only 1.1” less than that of plot C despite having almost 80 more square feet per acre of basal area (see S11). The selection thin led to a QMD of 8.2” and 30 more square feet per acre of basal area than the heavy thin.

The ability to continue asking a research question about the response to thinning intensity ended in 1986 when all three plots were part of a larger treatment area that was evenly thinned to an average of 120 ft2/ac in 1986.

Other notes

Hand-drawn covertype maps credit: Cloquet Experimental Forest Compartment Inventory Book. University of Minnesota Libraries, University Archives. Accessed 11 Apr 2019.

Summary / lessons learned / additional thoughts

There's still no one "right" way to thin red pine over the course of stand development but rather there are tradeoffs to the various options in our management tool box. It is suspected that nearly any crown thinning or thin from below treatments will lead to improvements in individual tree growth form, crown development, and timber quality. The most significant tradeoff identified in this case study was between basal area per acre and quadratic mean diameter; recall that the heavy thinning treatment in plot C had the lowest basal area per acre and the highest quadratic mean diameter (see supplemental figures 10 and 11). 

It is important to remember that these treatments were carried out on the plot scale and that the area of all three plots combined totaled less than an acre. In addition, these treatments were not replicated. Lastly, volume was only recorded following the 1948 cut which made it difficult to analyze the value of multiple entries at a short interval over the life of a stand. These thinning methods could be used in the future at the stand scale with replication to have a better understanding of the tradeoffs between these methods and if multiple entries over a shorter interval would be viable. If these treatments were carried out at the stand scale, it would also be valuable to consider site impacts such as soil compaction or changes to hydrology due to shorter entry intervals. This was nonetheless a valuable opportunity to at least examine red pine growth over 37 years and take advantage of the detailed records and stand history found within the Cloquet Forestry Center.

Supplemental content ("appendices")
Figure 1: the coversheet for the Danish thin research records, drawn on trace paper and later digitized. Recall that no action was taken in plot D. Figure 2: Hand-drawn vegetation cover-type map from 1929. The Danish thin plots are in the 0-20 age class area in the upper left, between the two roads running roughly NW to SE. The yellow areas are jack pine, while the green areas are red pine.Figure 3: Hand-drawn covertype map from 1939. The Danish thin plots are in the 28 year old area in the upper left. Figure 4: the hand-drawn legend from the CFC Forest Inventory Book. Figure 5: A red pine from Plot C with its ID tag still (barely!) attached. Most tags had been pushed out by the time the 2018 inventory occurred, though some were found in the duff layer.Figure 6: the dense understory of plot B.
R. Lane Moser
University of Minnesota Cloquet Forestry Center
Lane Moser in Oregon

Lane developed this case study during his 2018 summer research internship at the Cloquet Forestry Center. He is currently wrapping up is undergraduate career at the University of Minnesota and is preparing to start graduate school. He hopes to work in extension forestry in the future.

Summer research intern 2018
Kyle G Gill
University of Minnesota Cloquet Forestry Center
Kyle Gill

Kyle has been the CFC forest manager and research coordinator since the end of 2015. He enjoys exploring stand development and silviculture. He feels it is important to perceive himself and other humans as internal community members of the forest ecosystem community rather than as external.

Forest Manager and Research Coordinator
175 University Road
55720 Cloquet , MN
Phone Number: 
(218) 726-6412

Moser, R.L. and K.G. Gill. 2019. Danish Thinning Methods in Red Pine (UMN Cloquet Forestry Center). Great Lakes Silviculture Library. Cloquet Forestry Center, University of Minnesota. Cloquet, MN. Available at