Pacific Northwest Research Station

Sustainable Ecosystem Productivity

2008 Science Accomplishments

Central Oregon 's sandy loam soils tolerant to postfire logging

Logging activities can compact the soil, reducing its pore size and decreasing oxygen availability and movement of water and nutrients to tree roots. To alleviate compaction, land managers may fracture the subsoil, a practice known as subsoiling.

In this study, scientists examined the effects of compaction and subsoiling after postfire logging on the soil microbes in a mixed-conifer forest in central Oregon. These bacteria and fungi are a key component of forest ecosystems. They perform the complex biological and chemical processes that render essential nutrients available for the healthy growth of forests. They are essential to maintaining soil health and long-term productivity.

Scientists found differences among stands with respect to relative bacterial species abundance, but no difference among treatments. The cumulative number of bacterial species and fungal species did not differ significantly among the compacted soil, subsoiled, or fire-only treatments. Over multiple sampling seasons, however, slightly more bacterial and fungal species were found in the compacted soil treatment.

These findings suggest that microbial communities in the sandy loam soils in this study were generally tolerant of postfire harvest disturbance.

To learn more, contact Jane Smith at jsmith01@fs.fed.us.

Partners: Oregon State University, USDA Forest Service Deschutes National Forest

Intense wildfire alters forest soil

For the first time, scientists were able to directly measure the effects of hot wildfire on forest soils. The 2002 Biscuit Fire burned about half of twenty-seven 15-acre study plots east of Gold Beach, Oregon, established before the fire.

The fire burned at temperatures over 1,300 °F, as evidenced by the melted aluminum tags across the research plots-this is more than twice as hot as typical prescribed fires. Loss of topsoil and combustion of organic material were higher than most previous estimates. More than 10 tons per acre of carbon and 450 to 620 pounds per acre of nitrogen were lost, and nearly 60 percent of this came from the mineral topsoil below the organic layer.

The loss of topsoil and soil carbon can negatively affect a range of processes including nutrient retention and water infiltration. To replace the documented amount of lost nitrogen would require nitrogen-fixing plants to dominate the forest for decades.

This study illustrates the dramatic effects of intense wildfire on soil nutrients and resulting site productivity. Understanding how fire can alter the available nutrients and thus change the productivity of a site is essential when undertaking reforestation efforts and projecting future stand development. Also, a forest that grows slower after an intense wildfire than it did before will have reduced rates of carbon sequestration.

To learn more, contact Bernard Bormann at bbormann@fs.fed.us.

Partners: Oregon State University, Western Washington University