News Release

MADISON, WI, JUNE 5, 2009 -- The vast amount of wood used as a resource throughout the world has for decades led to rampant forest clearing, a situation which is having ecologically disastrous consequences. For years, the protection of these native forests has been addressed by plantation forests, which are grown specifically to supplement the world’s supply of wood. However, practices on these plantations that increase tree growth, such as fertilization or weed control, may have unintended effects on belowground carbon and nitrogen storage if they influence plant litter production or organic matter decomposition. Belowground carbon and nitrogen storage is important as it is directly correlated to the amount of soil organic matter present, and is also important for sequestration of atmospheric carbon dioxide.

A team of scientists from Oregon State University (now at Lawrence Livermore National Laboratory and Virginia Tech) and the U.S. Forest Service investigated the effects of fertilization and weed control on belowground carbon and nitrogen storage in ponderosa pine plantations in Northern California.

The research was funded by the USDA Forest Service, the Sierra-Cascade Intensive Forest Management Research Cooperative, and Sierra Pacific Industries. Results from the study were published in the May/June issue of the Soil Science Society of America Journal.

Carbon and nitrogen storage in forest floors and in mineral soils to 1 meter depth were measured at three experimental plantations with different soil types, parent materials, and productivity. All three sites had been planted with ponderosa pine and experimental plots were left untreated or treated with fertilizer and/or weed control with herbicides. Belowground carbon and nitrogen was measured after two decades.

Fertilizer increased tree growth at all three plantations, but the effect was proportionally greater at less productive sites where nutrients are more limited. Weed control increased tree growth at the less productive sites, but not at the most productive site where nutrients and water are more readily available.

Effects of treatment on belowground carbon and nitrogen storage were less pronounced than treatment effects on pine growth. Fertilizer increased forest floor mass, carbon and nitrogen storage at all three sites, and the magnitude of the effect diminished as site productivity increased. Fertilization also increased mineral soil carbon storage at the less productive sites, but the effect was more pronounced in forest floors than mineral soils.

Weed control only increased forest floor carbon pools at the least productive site and had no effect on nitrogen pools. However, at one site where weed control removed nitrogen-fixing shrubs, weed control may have reduced litter quality including nitrogen concentrations. Whether this has any influence on sustained productivity remains conjecture.

This study suggests that the major mechanism for increased carbon sequestration due to forest management in the ponderosa pine region of northern California is through increased wood production. Soil and forest floor carbon sequestration may provide a lesser, but still important pathway for enhanced carbon storage. Differences in response to treatment among sites indicates the importance of considering site-specific factors when making forest management decisions related to sequestration of soil carbon. These research plantations are being maintained and may be revisited in the future to investigate longer-term effects.