Walter Dodds has watched the Konza Prairie Biological Station landscape slowly change since he started studying it in 1990. Woody vegetation replaced drifts of grasses and wildflowers along the streams, choking their flow and disrupting the ecosystem in this 3,500-hectare preserve in the Flint Hills of northeastern Kansas.
About a decade ago, Dodds, an aquatic ecologist at Kansas State University, and others cleared much of the woody vegetation to help restore the prairie, which is jointly owned by The Nature Conservancy and Kansas State. But the effort has failed to help grasses return, according to a new study in Ecological Applications on how the landscape has fared.
The balance seems to have shifted, perhaps permanently.
Natural cycles in the prairie ecosystem once held woody plants at bay. Konza’s 500 species of grasses and wildflowers once thrived in a system of checks and balances known as disturbances. These include seasonal weather extremes, grazing animals that regenerated the ecosystem, and fire that cleared spent vegetation and redistributed nutrients.
The balance seems to have shifted, perhaps permanently, Dodds said.
Despite grazing and managed burns, woody vegetation took hold, fundamentally altering the riparian zone. Sun-loving algae, the base of the prairie’s aquatic food web, was shaded by the canopy. Across 3 decades, streamflow declined by 40%–50%.
“Along with that, there has been an increase in days with no flow. Therefore, there are strong negative impacts on stream biota as well as less water available for human uses downstream,” Dodds said.
A Drastic Change?
In 2010, Dodds enlisted dozens of professionals and volunteers to remove woody plants along 4.5 kilometers (2.8 miles) of stream channels. The group cleared shrubs and trees with brush cutters and chainsaws. They piled up all the debris and conducted scheduled burns. To regenerate the native plants, they cast bagfuls of seed in the cleared areas.
A decade later, the trees were still gone, but the landscape remained changed. The shrubs not only persisted, but also expanded their hold. And the grasses resisted reseeding.
The grassland riparian zone had lost its resilience.
Water quality changed too. Dodds and his colleagues saw huge increases in nitrates after clearcutting, which can be problematic for human health and cause excess plant growth in waterways. The nitrates flowed downstream, only to be reabsorbed by the rebounding shrubs.
Zak Ratajczak, an ecologist also at Kansas State University and a coauthor on this study, has been researching the landscape-scale effect of woody encroachment using current and historic remote aerial images. In some areas of the prairie, shrub coverage has grown to 45%–50%, despite grazing and periodic burns, he said. The rate of growth for these trees and shrubs has become startling, Ratajczak said.
Dodds conceded that increasing nitrogen availability could have spurred shrub regrowth. Also, modern grazing practices do not fully match conditions of centuries ago, and today’s managed fires likely do not replicate the heat and intensity of ones in the past.
“It’s getting to the point where it seems like there are very few options to prevent this process from happening.”
But something else has changed relating to an essential function in plant biology—photosynthesis—in light of climate change and atmospheric carbon dioxide enrichment, said Kelly Lyons, a botanist and ecologist at Trinity University in San Antonio who was not involved in the study.
“The process of photosynthesis has an inherent trade-off between water and carbon,” Lyons said. “Warm-season prairie-adapted grasses and wildflowers optimize carbon capture and minimize water loss, maintaining an advantage under lower precipitation.”
Researchers theorize that with rising atmospheric carbon, encroaching woody species have greater carbon capture and are less disadvantaged, allowing them to get a foothold and spread quickly. Historical disturbances such as frequent fire are no longer keeping shrubs and trees in check.
“Woody expansion is the biggest threat to tallgrass prairies,” Dodds said.
“It’s getting to the point where it seems like there are very few options to prevent this process from happening,” Ratajczak said.
—Kimberly Hatfield (@fieldnotes2014), Science Writer