At Kansas State University’s (KSU) Konza Prairie Biological Station a study carried out by Allison Veach and Walter Dodds shows that encroachment of trees and shrubs near grassland streams may one day turn these grassland watersheds into forested streams.

All around the world grasslands are disappearing, being converted to forest ecosystems by woody plants. According to Dodds, these conversions in local environments can adversely affect stream hydrology and biogeochemistry in the area. This study focused on the expansion of trees and shrubs near grassland streams on Konza’s 8,600 acres of tallgrass prairie in the Flint Hills of Kansas.

Dodds, Veach, and Adam Skibbe poured over 25 years of aerial photographs from the station, using three independent variables to gauge the expansion of woody plants: intervals between fires in the area; the impact of grazers, like the bison that roam around the station; and the presence of woody vegetation prior to the window of time being studied.

The results showed that time interval between burns was the most important factor. Though the expansion of these woody plants was reduced by more frequent burns, however, trees and shrubs still continued to encroach on the areas, albeit more slowly. The brushfires seemed neither to completely prevent nor reverse the advance of woody vegetation.

According to Dodds, climate change may also be affecting the growth rate of woody plants in these riparian areas. Grasses are much better than trees at conserving water, but this means that they need less carbon dioxide for respiration, a key process in plant growth. In theory, as levels of carbon dioxide rise in the atmosphere, trees can gather all they need and gain a growth advantage over grasses.

“The tallgrass prairie is almost nonexistent on the globe,” said Veach to KSU News. “In order for us to preserve tallgrass prairie, we need to look at woody encroachment because it has been an issue. Things like no fire or differences in climate change may allow woody plant species to competitively take over grasslands.”

The study can be found in its entirety in the journal PLOS One.