Eastern redcedars creeping into the tallgrass prairie

Redcedar invasion has been a known problem for nearly 80 years and it does not seem to be going away.

An ongoing Oklahoma National Science Foundation Established Program to Stimulate Competitive Research project conducted by Oklahoma State University discovered eastern redcedar encroachment in the tallgrass prairie really sucks.…up a lot of water.

“Eastern redcedar encroachment into grasslands is a major issue in Oklahoma as it can impact streamflow and water supply,” said Chris Zou, ecohydrologist in OSU’s Department of Natural Resource Ecology and Management.

Zou collaborated with NREM colleagues, Donald Turton, Rodney Will, Lei Qiao and Elaine Stebler to study how eastern redcedar encroachment affects the annual water runoff in the tallgrass prairie.

“Our results showed that less than 2 percent of annual precipitation became runoff in watersheds encroached with eastern redcedar, which was significantly lower than about 5 percent from grassland watersheds for the four years of our study,” Qiao said. “We also found that eastern redcedar encroachment resulted in the reduction of both surface and subsurface flows and the magnitude of reduction depended on annual rainfall.”

This has important implications because eastern redcedar uses more water than tallgrass prairie vegetation, leaving less water available for streamflow, which is important for ecological and human uses.

The researchers used seven experimental watersheds, three of which were on grassland sites and four on adjacent sites heavily encroached by eastern redcedar. Watersheds are widely used in studying water budgets associated with a given land use type or land cover type to understand climate and land-use impacts on water quantity and quality.

The study, which began in 2013 and will conclude this year, determined how wet the soil was before and after rainfall events in both grasslands and areas encroached by the eastern redcedar trees. The wetter the soil, the closer it was to saturation.

“It is more likely to produce runoff or produces more runoff when the soil is more close to saturation just before the rainfall event, vice versa,” Zou said. “Our finding is that the soil of grassland watershed tends to be wetter (closer to saturation) than redcedar woodland in most cases and we measured less runoff from redcedar watersheds even under the same precipitation input.”

Even after large amounts of rain, the redcedars were making an impact.

“Our soil moisture data indicated that the soil profile under eastern redcedar rarely became saturated and did not stay above field capacity very long even after a heavy rainfall,” Stebler said.

The fact that redcedar encroachment is a problem is not new but if the problem continues to grow it could have devastating effects. While Oklahoma rangelands are somewhere between two and 10 percent covered with the species, the researchers estimate a complete conversion of rangelands to eastern redcedar woodlands would result in reductions of up to 40 percent in annual streamflow for the upper portion of the region and approximately 20 percent for the lower Cimmaron River basin.

“The impact is more a potential threat rather than a fact at this stage, but to sustain our water resources in the Southern Great Plains, watershed managers should use best management practices to control or prevent woody plant encroachment into the grassland,” Zou said.