Forest Understory and Wood Production Response to Ponderosa Pine Thinning Treatments in Southeast Montana

Forestry Technical Note Number MT-32

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Forestry Technical Note Number MT-32 (PDF; 56 KB)

September 2007
By Robert D. Logar, State Staff Forester

Introduction

A ten year study was performed in Southeast Montana to document the effects of pre-commercially thinning Ponderosa pine stands. Part of the study was to evaluate the potential for chemical thinning. The results from this study will assist planners and producers in estimating the effects of pre-commercial thinning on forest understory and wood production. The results from the study will also provide information on chemical pre-commercial thinning -- kind, rate, cost, limitation.

General

Background – There is approximately 1.65 million acres of forestland in 14 southeastern Montana counties. A conservative estimate is that ten percent or approximately 165,000 acres are overstocked stands of trees that are in need of thinning.

Purpose – The project demonstrates forest understory and wood production response to several Ponderosa pine thinning techniques. Two thinning techniques will be compared – mechanical and chemical thinning. A cost analysis for each of the methods along with an evaluation of the success of the chemical thinning will be made.

Procedure – Forestland sites were selected in Carter, Custer and Powder River counties. Sites were characterized as having moderately deep- to deep-loamy or heavier-textured soils. Slopes were generally less than 15 percent. Efforts were made to maintain consistent aspects at all sites. Average annual precipitation for the sites ranged from 15 to 19 inches.

The following six treatments were used at each site.

1. Mechanical thinning at 10-foot spacing
2. Mechanical thinning at 16-foot spacing
3. Chemical thinning at 10-foot spacing
4. Chemical thinning at 16-foot spacing
5. Un-thinned site to quantify wood and forage production
6. A site without trees to quantify the forage production.

Each replication was about one-quarter to one acre in size. Treatments were installed in the spring of 1996. Monitoring and characterization of the treatment responses occurred one, two, three, five, and ten years after the treatments were installed.

Results

Mechanical Thinning
There was 100% mortality to the trees that were removed mechanically. The cost to mechanically thin increased with the number of stems removed.  Graph 1 displays the costs per acre to mechanically thin based on the number of trees per acre removed. The cost to remove 2,000 trees per acre is about $140 per acre. The cost to remove 6,000 trees per acre is about $300 per acre. A typical thinning operation removes about 2,000 to 4,000 trees per acre.

Chemical Thinning
Two Dow chemicals were used as a comparison for effectiveness and cost analysis. Garlon and Pathfinder were the two chemicals used. Garlon is a concentrate that is mixed with a carrier. Pathfinder is a pre-mixed herbicide. The labels indicate that they can be used on both ponderosa pine and Douglas fir seedlings and saplings. Both were applied using backpack sprayers. A six-inch zone of herbicide was sprayed on three sides of the stem of a sapling. Foliage applications were applied to seedlings. There was about 90 percent mortality on the seedling/saplings using Garlon with diesel as a carrier. There was about 50 percent mortality on the seedling/saplings using Pathfinder. The chemicals were most effective on stems less than three inches in diameter breast height. Diameters greater than three inches resulted in greatly reduced mortality. The costs associated with chemical tree removal are displayed in Graph 2. The cost to remove 2,000 trees per acre is about $60 per acre. The cost to remove 6,000 trees per acre is about $170 per acre. About three-quarters of the costs in chemical thinning is labor-involved, not the chemical.

The most effective rate was seven ounces of Garlon per gallon of diesel. Use about one gallon of mix to treat one acre averaging 2,000 - 3,000 stems per acre.

Forest Understory Response
Clipping data was taken in the fall of 1997, 1999, 2001, and 2006. Overall, the sites showed a significant increase in forage production in the thinned stands as compared to the un-thinned stands. Plots that were mechanically thinned to ten-foot spacing produced an average increase of understory vegetation of 121 percent compared to those in the un-thinned sites. Those mechanically thinned to 16-foot spacing produced an average increase of understory vegetation of 153 percent compared to those in the un-thinned sites. Plots that were thinned chemically resulted in a 119 percent increase on the 10-foot spacing and 102 percent on the 16-foot spacing. The reduced amount in the 16-foot spacing chemical treatment was due to reduced kill of trees greater than three inches in diameter.

Wood Production Response
There was a significant increase in wood production as a result of the thinning. The mechanical thinning showed the most consistent response and largest increase in volume as a result of the thinning. The trees were generally taller and at least twice the diameter on the thinned sites when compared to the control (un-thinned sites). There was generally twice the volume on the thinned sites then the control tree sites. This is especially the case in the trees that were thinned and spaced 16 feet apart. The attached Table 1 shows the results of the sites and measures the percent change in height and diameter after the thinning.

Summary

1. There was 100 percent mortality to the trees that were removed mechanically.
   a) The cost to remove 2,000 trees per acre is about $140 per acre.
    
2. There was about 90 percent mortality on the seedling/saplings using Garlon with diesel as a carrier.
   a) Chemical thinning was most effective on stems less than three inches in diameter breast height. Greater than this the results dropped dramatically.
   b) The most effective rate was seven ounces of Garlon per gallon of diesel. Use about one gallon of mix to treat one acre averaging 2,000 - 3,000 stems per acre.
   c) The cost to remove 2,000 trees per acre is about $60 per acre.
    
3. Overall the sites showed a significant increase in forage production in the thinned stands when compared to the un-thinned stands.
    
4. There was about twice the wood production on the thinned sites when compared to the control (un-thinned sites).
   a) The mechanical thinning showed the best response to the thinning and largest increase in wood production.
    
5. Ten and 16-foot spacing were both effective for increasing forage and wood production. However, the 10- foot spacing canopy was starting to close after the ten year evaluation period. Recommend using the wider 16-foot spacing to continue good forage and wood production.

Chemical and mechanical thinning increased the amount of forage and wood production on the site. Chemical thinning was effective and was about one-half the cost of mechanical thinning. Garlon with diesel was the most effective chemical. There was about twice the wood production on the sites that were thinned. Forage production was significantly greater on the mechanically thinned sites when compared to the control (un-thinned sites).

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Last Modified: 08/20/2008