CORVALLIS, Ore. – Riparian zones in many Pacific Northwest forests return to health fairly quickly after forest fires and may eventually provide the same ecosystem services and largely the same species mix, with little need for replanting or management, a new study from Oregon State University concludes.
It has been long understood, researchers say, that riparian areas are among the most important features of the forest: They harbor multiple species, a broad range of plants and trees, provide clean water, and are a key part of both the terrestrial and aquatic food chain. The shade from overhanging trees helps cool streams and nurture fisheries. Because of that, it has often been believed that after a fire they required special attention to prevent erosion and streambed damage, and were often targeted for that.
The new research concludes that the streams often do just fine on their own, and surprisingly quickly.
“In the past, most studies of streams after fire looked only at the short-term effect on fisheries,” said David Hibbs, a professor in the OSU Department of Forest Ecosystems and Society. “And there was a risk-averse approach to streams after fire, because we knew how important they were. So they were often managed fairly heavily after a fire, without a lot of evidence it was really necessary.”
The new study, just published in the journal Forest Ecology and Management, focused on areas burned in the 2002 Biscuit Fire in southwest Oregon and the 2003 B&B Complex Fire in the central Oregon Cascade Range. Areas were studied at two- and four-year intervals after they burned, to measure the amount of vegetation and streamside recovery.
“Other studies have observed that in some cases trees and shrubs were sprouting almost immediately after the fire, within two weeks,” said Jessica Halofsky, a lead author on this study, formerly an OSU doctoral student and now a research ecologist with the University of Washington. “That’s really amazing. And fireweed moved in very quickly to provide shade, cover and bank stability.”
After that, residual live trees dropped seed in following years, leading to pulses of tree seedling development. As often occurs after a fire, there was some heavier stream flow and higher rates of flooding for a period, which caused some erosion and sediment movement.
Some fire effects are temporarily harmful to salmon and other fish species, other studies have found, but in the long run highly positive. Post-fire flooding scours streams and moves sediment around, creating new and clean gravel beds, new inputs of large woody debris, and in general helps to rejuvenate the long-term health of the stream for fish spawning and survival. Hardwoods such as alder that can fix nitrogen, an important forest nutrient, may also play a role in riparian zone recovery.
The natural tree reproduction in and near the riparian zones was impressive – in streams in the B&B Complex Fire, four years after the fire, there were more than 27,000 seedlings per acre. And the study indicated that the mix of species would ultimately resemble what it was before the fire, more hardwoods such as red alder near the streambank and on the flood plain of larger streams, more conifers as you got further away from the stream and higher in elevation near its headwaters.
“The species mix seems to be largely dictated, in the long run, by the hydrology of the local area, not so much the fact that a fire came through,” Hibbs said.
The study also concluded that the tree mortality after a fire was about the same in riparian zones as in the surrounding uplands. However, due to the higher moisture level in riparian soils, there was more organic matter left on the forest floor near streams to help stabilize soil, retain nutrients and result in less exposed mineral soil.
“Generally, this research suggests that a lot of post-fire rehabilitation in riparian zones isn’t necessary following most fires,” Halofsky said. “The cover, stream shade and recovery of plant and animal species occurs rather quickly, without management. These are very resilient ecosystems.”