Taking the pulse of water in western juniper woodlands

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.

[caption caption="Rangeland hydrologist Carlos Ochoa places a temperature sensor in a stream near Dufur, where he is evaluating stream temperatures related to land use in dryland riparian ecosystems. (Photo by Lynn Ketchum.)"] [/caption]

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.