Trees Threaten Growing Grasslands

Trees are currently threatening growing grasslands, turning tallgrass prairies into shrublands and forests, and a pair of researchers is concerned that if nothing is done to stop or slow down this process, an important part of the ecosystem may soon be lost.

Two Kansas State University biologists, doctoral student Allison Veach and professor Walter Dodds, are researching grassland streams and the expansion of nearby woody vegetation, such as trees and shrubs, to see what can be done to save growing grasslands.

"This is an important issue regionally, because as trees expand into these grassland areas, people who are using grassland for cattle production have less grass for animals, too," Dodds said in a statement.

Grasslands in North America and across the globe are rapidly disappearing, thanks to greedy trees that are expanding and converting grasslands into forest ecosystems. This change in environment, Dodds says, can affect stream hydrology and biogeochemistry.

In their latest research, the biologists studied 25 years of aerial photography on Konza Prairie Biological Station in Kansas and observed the expansion of trees and shrubs in riparian areas, which include areas within 30 meters (98 feet) of streambeds. They focused on three factors: burn intervals; grazers, such as bison; and the historical presence of woody vegetation.

Described in the journal PLOS ONE, analysis showed that burning, in particular, had a significant effect on threatening trees. Burning every one to two years actually slowed the growth of trees and shrubs.

"Although we can reduce woody expansion by burning more frequently, we can't prevent it from occurring over time," Veach noted. "Woody plant encroachment may not be prevented by fire alone."

So why are trees encroaching on grassland territory? The research shows that bison do not spend much time near streams, so they likely don't influence the growth of nearby trees and shrubs. But the researchers suspect climate change may have something to do with it. Grasses and trees compete for carbon dioxide (CO2), and grasses are much better at conserving water and efficiently using CO2. But with more CO2 in the atmosphere due to human-related activities such as the burning of fossil fuels, it becomes easier for trees to gather the greenhouse gas and gives them a growing advantage over grasses.

Soon, Dodd warns, "grassland streams basically are going to switch to forests and will not be grassland streams anymore."