Julia Jones webinar

Please mark your calendars for an educational webinar with OSU professor Julia Jones, who will speak about her research, based on data dating back to the 1950s, about the long-term effects of forestry practices on stream flows. Her findings, especially in regards to summer stream flows and deficits, have important implications for understanding how forestry has changed coastal streams over the past half-century and what may happen in the future.

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Forestry in the Pacific Northwest has a variety of effects on streamflow, which vary depending on the age of the forest, the season, and the type of streamflow response of interest: water yield, peak flows, and low flows. Long-term studies (since the 1950s) in Oregon show that when mature and old-growth forests are harvested, and the land is replanted with Douglas-fir plantations, there is an initial increase in water yield and peak flows, including 1-yr floods, which can persist for up to 40 years. When old-growth and mature forest was clearcut without anyriparian buffers, summer flows also increased initially, but by the time the plantations were about 15 years old, summer flows declined to levels lower than those from mature or old growth forest. This “summer water deficit” means that summer flows from 25 to 50-yr-old plantations can be as much as 50% lower than those from mature or oldgrowth forests. In other words, plantations of this age range provide higher peak flows in winter, but lower flows in summer, compared to mature or old forest. Most long-term studies of forestry effects on streamflow take place on federal forest land, where clearcutting did not leave trees standing in a riparian buffer zone. A long-term study in the Alsea basins in the Oregon Coast Range included recent harvests of 40-yr-old plantations using Oregon Forest Practice rules for buffer zones. In this case, although clearcutting of the plantation (leaving trees in the riparian buffer) led to a slight increase in flows relative to the former plantation, summer streamflows remained lower than those in the reference watershed, which is mature forest (a mix of Douglas-fir and red alder). Within a few years, despite the fact that the replanted forest was only a few years old and a narrow riparian buffer occupying <3% of the watershed, the clearcut watershed produced summer streamflow deficits that were as large as those from the prior 40-year-old plantation. Summer streamflow deficits from watersheds with plantations also tend to be larger (more severe) during dry years. Collectively these findings have interesting implications for interpreting how summer streamflow has changed in the Coast Range over the past half-century or so, and how it may change in the future.

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