Dr. Paul Adams

Woodland management almost always involves using a heavy vehicle, especially when some timber is harvested. Crawler tractors, rubber-tire skidders and farm tractors are the common types seen in the woods during logging operations. But traffic in the woods can have some consequences, and soil compaction is one result that can be a management concern.

Productive forest soils ("good earth") are usually loose and porous, which also mean they're relatively weak when it comes to supporting heavy machinery. That's why when forest roads are built, topsoil is removed to expose deep soil that's harder and stronger, and even that soil often is compacted intentionally to strengthen it even more.

Soil compaction may be good for roads, but out in the woods it can impact site productivity. Studies by Oregon State University (OSU) researchers and others have shown that both seedling and residual tree growth can be reduced substantially where soils are compacted. And these impacts are seen on a wide range of soil types (e.g., sand, clay and ash soils) and commercial tree species (e.g., pine, Douglas-fir, hemlock, true fir).

But I've seen lots of seedlings on old skid trails

Old skid trails can be a great place to find seedlings and saplings. But look closer and consider the growing environment. Bare soil is a good seedbed for many species, and local brush competition has been reduced. You may notice the seedlings that are growing best are near the trail berms, where the soil is loosest. And there's a good chance you'll find fewer or smaller seedlings in the trail ruts where the soil is most compacted.

Soil compaction growth effects can be subtle and not even visible to the eye. This is especially true for older forest stands where diameter growth may be reduced by compaction by only 5-15 percent. But even this level of impact means lower timber volumes at harvest time. An OSU study estimated that the "net present value" of forest stands might be reduced by as much as $500 per acre (i.e., a lot, especially considering these are 1987 dollars) by volume losses from soil compaction. Such economic impacts show that it can pay to protect the good earth.

But I only use small equipment

Small tractors or other vehicles can leave a lighter touch on the soil. But they're still heavy enough to compact it, especially if they make several passes over the same area. An OSU study found that after 6 to 10 passes, soil compaction by a rubber-tire skidder (high ground pressure) and a torsion-suspension vehicle (low ground pressure) was similar. Interestingly, a crawler tractor produced somewhat greater compaction than the other two vehicles, even though its ground pressure was intermediate. Tracked vehicles, regardless of their ground pressure, can effectively transfer vehicle vibrations that increase soil compaction. That's the idea behind vibrating rollers and other devices that are used to intentionally compact roadbeds and foundations.

But I only log when the soil is dry

The dry season is a great time for logging (unless fire danger is high), and it does reduce soil disturbances like rutting and puddling (i.e., wet soil that flows under pressure). But contrary to popular belief, dry soils can and do compact. In fact, some soil types (such as sandy soils) can be weaker and even more susceptible to compaction when dry than when slightly moist. One way to appreciate this is to think about how beach sand supports you where it's dry, moist or very wet.

More effective ways to control compaction

To protect the good earth on your property, you should start by assuming that your woodland is very likely susceptible to compaction. And this is probably true throughout the year and on the different soil types found on your property, except for areas that already are compacted.

It's nearly impossible to avoid all compaction during logging, unless you're willing to pay for helicopter or full-suspension cable logging. A more realistic goal for most landowners is to limit the area of compacted soil through a carefully planned network of designated skid trails. When combined with techniques like directional felling and line pulling from vehicles that stay on the trails, it's fairly easy to restrict compacted areas to no more than about 7-12 percent of the harvest unit. The costs of designated skid trails also can be reasonable, often just planning and layout time. Some loggers use designated trails as a routine practice because they've found they improve logging efficiency.

Of course, there will be some situations where new or existing compaction is extensive enough to significantly impact seedling or stand growth. One option for reducing such impacts after final harvest is to till compacted soils to help restore their productivity. Although specially designed equipment like "winged subsoilers" can be most effective in loosening heavily compacted soils, even simple scarification can encourage plant rooting and soil organisms that promote soil recovery.

Tillage may not be needed everywhere compacted soil is found-consider reusing existing skid trails if they provide a reasonable traffic pattern. And if you're using a well-planned designated skid trail network from the start, you can think of it as part of the permanent access system for your property. With that approach, growth impacts shouldn't even be an issue and all you'll need to think about is controlling runoff on slopes with water bars or other trail drainage features.

Paul Adams is a professor and Extension specialist in the Forest Engineering, Department at Oregon State University in Corvallis. He can be reached at paul.adams@orst.edu.

Some suggested readings

Soil compaction on woodland properties, EC 1109, OSU Extension Service, Corvallis.

Designated skid trails minimize soil compaction, EC 1110, OSU Extension Service, Corvallis.

An evaluation of four implements used to till compacted forest soils in the Pacific Northwest, Research Bulletin 45, OSU Forest Research Lab, Corvallis.

This article appeared in the Northwest Woodlands Magazine, Summer 2001- Published quarterly by the World Forestry Center as a benefit of membership in the Oregon Small Woodlands Association, Washington Farm Forestry Association, Idaho Forest Owners Association and Montana Forest Owners Association.