Young Fir Trees Sap Stream Flow

Are the warm summer months making you thirsty? This time of year, young Douglas-fir trees growing on federal forest land in Oregon are too, apparently. Thirsty, fast-growing young trees can reduce the amount of stream water in mountain watersheds during the summer, and the reduction doesn’t seem to let up – even 6 decades after the trees were planted, according to a new Oregon State University study. 

During sunny summer months, stands of spry young Douglas-fir trees grow a good bit faster than mature and old-growth forests. More mature trees are also better able to limit water loss from their needles and withdraw soil water more slowly than faster-growing young trees. Increased water demand from younger forests may reduce the amount of water that would otherwise end up in streams and rivers. 

Coupled with the effects of climate change, reduced stream flows in western mountain watersheds could someday require downstream water use trade-offs between fish and wildlife habitat, irrigation and agricultural uses, and municipal water demands. 

During the first decade or so after a clear-cut, stream flow can increase before steadily declining as planted young trees become established. According to previous research, streamflow reduction in young forests might be as much as 50 percent less than stream flows in neighboring mature forests. This recent paired-watershed analysis conducted by Oregon State researchers supports these same conclusions.

A recent analysis of eight paired experimental watersheds in the western Cascades was completed by the College of Earth, Ocean, and Atmospheric Sciences’ Timothy D. Perry, geography graduate researcher, and Julia Jones, professor of geography. Their results were published in the journal, Ecohydrology. The study received support from the National Science Foundation, the Umpqua National Forest, Willamette National Forest, and the U.S. Forest Service Pacific Northwest Research Station.

“These findings imply that summer water use by intensive plantation forestry might become an issue at a time of increased concern about water scarcity,” said Jones. “Does this mean that forest management reduces streamflow compared to the summer water yield if those stands were still old-growth forest? I think it probably does.” 

With paired-watershed experiments, scientists match two or more adjacent watersheds that are stuck out in similar environmental conditions. That way, when something happens in one of them, scientists can continue to monitor the effects of stuff like logging, chemical applications, or prescribed burning over many years. The “untreated” watershed may then be used as a control. 

“These big paired watershed studies are the only place in the country where these studies have continued since 1980, but they were initially established by the U.S. Forest Service in the 1950s and 1960s. There are only a handful of places in the U.S. where long-term paired watershed forestry experiments are still being maintained,” said Jones. 

These types of experiments are essential tools that scientists use to study landscape-scale, long-term ecological change. 

“It enables a precise determination of the treatment effect,” added Jones.

Questions still remain about how stream flow may be influenced by a range of forest practices, like thinning, that occur on landscapes at different intervals and intensities and create differences in special patterns across forested areas.

“There’s some suggestion that thinning might not have much effect on streamflow, but there’s a need for new experimental evidence,” said Jones.

By Matthew Hunt