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Criterion 4 Indicator 22
Rationale
Area and Percent of Forest Land With Significant Soil Compaction or Changes in Soil Physical Properties, Resulting From Human Activities
 
Soil compaction occurs when soil particles are physically compressed, eliminating the air spaces, or pores, between the soil particles. Soil compaction is a concern because it can reduce forest growth and increase soil erosion. Forest growth is reduced because compaction makes it more difficult for roots to penetrate the soil as well as eliminating the soil’s ability to hold air and water in its pores. Compaction also inhibits the ability of water to infiltrate or drain from forest soils. When compaction becomes severe, water can flow across the surface of the ground and carry soil into streams.

Can This Indicator Be Quantified
 
Not at this time.
 
Soil compaction can be caused by moving heavy equipment or logs across the forest floor, or by repetitive actions like heavy recreation traffic or grazing. Cafferata (1992) reported that 10 to 40 percent of a harvest unit could be compacted during tractor logging, but that the area compacted could be reduced up to 5 percent with careful planning. The degree of compaction can vary tremendously depending on factors like the number of passes over the ground and soil moisture conditions. Timber harvesting methods that lift or partially suspend logs also reduce compaction. For example, cable logging generally compacts less than 5 percent of the area. The amount of compaction is also influenced by the silvicultural system. Selective harvesting systems, or even-aged systems with several intermediate harvests, require multiple entries and may require more extensive skid trails. Both of these factors can increase the amount of compaction.
 
The severity of compaction is related to many factors including: the soil type, soil moisture, the amount of organic matter covering the forest floor, the weight per unit area the soil must support, and the number of times the soil is compressed. It is difficult to predict compaction’s effects on soil productivity because of all the variables, but McNabb and Froelich (1983) estimate that stand growth losses can range from 5 to 13 percent and compaction’s effects can last 30 years.
 

Trends
   
Soil compaction is not considered to be a widespread problem in Oregon’s forests, though little quantitative work is available to support that conclusion. The Oregon Forest Practices Act requires operators to "avoid ground based yarding on unstable, wet, or easily compacted soils and on slopes which exceed 35 percent." In western Oregon, 62 percent of the private forest land is on slopes that can be accessed with ground-based yarding, and most private harvesting in eastern Oregon is done with ground-based systems. Educational programs have been targeted at yarding operators for many years, teaching methods to reduce compaction.

Data Source and Availability
   
Data is limited to small geographic areas associated with individual studies of soil compaction.

Reliability of Data
 
NA.

Scale
 
NA.

Recommended Action for Data Collection
 
It would be difficult to design a successful sampling system that can estimate the extent of compaction problems, because different soils and tree species respond differently to compaction. Harvest units could be sampled to estimate the area in skid trails, and changes in the soil’s bulk density could be measured on a sub-sample of harvest units. Bulk density changes would need to be associated with factors like soil type. In order to calculate how much tree growth is lost due to soil compaction, better information is needed on how individual trees respond to soil compaction.

Definitions
 
Bulk density — The weight of soil solids per unit volume of soil.
 
Soil compaction — "… is the densification of soil by the application of a dynamic load to a soil, thereby causing a decrease in the air voids within the soil due to changes in the relative positions of soil grains or aggregates." (Li, 1956)
 

Selected References
 
Cafferata, P. 1992. Soil compaction research. In: Skaugset, A., editor; Forest soils and riparian zone management: the contributions of Dr. Henry A. Froehlich to forestry, pp. 8-22. Oregon State University, Corvallis, OR.
 
Froehlich, H. A. 1979. The effect of soil compaction by logging on forest productivity. Final report for USDI Bureau of Land Management, Portland, OR.
 
Froehlich, H. A., and Azevedo, P. Cafferata, D. Lysne. 1980. Predicting soil compaction on forested land. Oregon State University, Corvallis, OR. 120 pp.
 
Li, C. Y. 1956. Basic concepts on the compaction of soil. ASAE, Journal of Soil Mech. Found. Div. 82:1-20.
 
McNabb, D. H., and H. A. Froehlich. 1983. Conceptual model for predicting forest productivity losses from soil compaction. Proceedings of the Society of American Foresters National Convention, pp. 261-265.