Western juniper, an iconic tree in the arid West, was once mostly confined to rocky ridges and isolated patches of shallow soil. Since the 1880s, however, it has increased its range tenfold in central and eastern Oregon, elbowing out native sagebrush and grasses and sucking up more than its share of water.
In many places where encroaching junipers have formed dense stands, bare soil is all that lies beneath the trees. Ranchers have witnessed springs drying up as junipers became abundant in a watershed. And, almost miraculously, when the junipers are cut, the water returns.
The issue of juniper encroachment is complicated; plants are just one of many interacting factors that influence how water moves across the surface and underground. In an effort to understand the impact of junipers on the water cycle, OSU scientists helped establish the Camp Creek Paired Watershed Study in the Crooked River watershed southeast of Prineville. Begun in 1993, the researchers monitored groundwater levels and stream flow in two neighboring watersheds with similar physical characteristics and vegetation. After collecting 12 years of pre-treatment data, in 2005 all except the oldest junipers were cut from one basin, leaving the other basin intact. Within months, the researchers measured an increase in groundwater and spring flow in the cutover basin.
But the research didn’t end there. Now, almost 12 years after cutting, the researchers have wired the two watersheds, and the valley between them, with instrumentation including soil moisture probes, transpiration sensors, weather stations, flumes, and groundwater wells to take the pulse of hydrologic changes.
“The effects of juniper removal results in a redistribution of water budget components, largely due to the lack of tree canopy interception,” said Carlos Ochoa, an OSU rangeland hydrologist whose research determined that up to 70 percent of rainfall is intercepted by the juniper canopy and never reaches the ground.
Along with Tim Deboodt, an OSU Extension rangeland specialist, Ochoa has confirmed that the cutover watershed has increased water flowing in streams and springs. They also found that soil moisture and shallow groundwater remain longer in the cutover basin, nurturing greater perennial grass cover.
“Connections between upland water sources, groundwater, and downstream valleys make a big difference to ecosystem services, including water quality and quantity, far downstream,” Ochoa said.
Currently, the researchers are developing best management practices for juniper control and evaluating its socioeconomic benefits.