Global warming is contributing to the increased frequency of forest fires in the American west. A new study by the US Forest Service shows that hot fires in western forests consume soil carbon and nitrogen, reducing the ability of soils to store carbon and reducing soil fertility. This is a potential feed-forward effect: increasing temperatures due to carbon dioxide in the atmosphere increases fire frequency and intensity, and high intensity fires convert soil organic carbon to carbon dioxide, increasing global warming.
The new study was a fortunate outcome of an unfortunate event: the 2002 Biscuit Fire, which burned about 500,000 acres in southwest Oregon, burned research plots of an ongoing study of forest soils. That meant that there were samples of soils available before and after the fire as well as pre-and post-fire plots measuring tree and shrub growth. Bernard Bormann, the study’s lead investigator, said “Losing our experiment in fire was hard, but the opportunity to better understand fires as a dominant ecosystem process has been very exciting.”
The Biscuit Fire burned soils at more than 1300 F, converting soil organic matter into carbon dioxide, nitrogen oxides and water vapor. Soils lost 10 tons per acre of carbon and 450 to 620 pounds per acre of nitrogen. These losses were higher than estimates from previous studies. Loss of nitrogen to this degree would take at least a century to recover, in the absence of nitrogen-fixing plants.
The loss of soil productivity means that replacement forests may grow very slowly for long periods. These new forests would not be a vigorous sink for carbon sufficient to offset carbon lost from the soil for many years. The exact balance between carbon loss from the fire, both in the standing tree crop and soils, and the rate at which replacement forests will take up carbon from the atmosphere is not known.
Sources:
Press Release from Pacific Northwest Forest Experiment Station – When it comes to forest soil, wildfires pack a 1-2 punch.
Bormann, BT, Homann, PS, Darbyshire, RL, Morrissette, BA. 2008. Intense forest wildfire sharply reduces mineral soil C and N: the first direct evidence. Can. J. For. Res. 38:2771-2783. doi 10.1139/X08-136.
