Grassland Prairies are Under Threat from Growing Trees

Grassland prairies may be under a new and unusual threat. It's not from animals; it's not from humans. Instead, the threat comes from trees and shrubs. Now, scientists are investigating streams in order to prevent tallgrass prairies from morphing into shrublands and forests.

Grasslands in North America and around the world are rapidly disappearing. Woody plants, like trees, are expanding and turning grasslands into forest ecosystems. It's likely that trees are experiencing a surge in growth due to there being more carbon dioxide in the atmosphere. While grasses are better at conserving water and efficiently using carbon dioxide, the increased concentration makes it easier for trees and gives them a growing advantage over grasses. In particular, this change from grassland to woodland can impact stream hydrology and biogeochemistry.

In order to better understand these changes, the researchers studied 25 years of aerial photography, observing the expansion of trees and shrubs in riparian areas, which include areas within 30 meters of streambeds. More specifically, the scientists focused on three factors that affect grassland streams: burn intervals, grazers and the historical presence of woody vegetation.

So what did they find? It turns out that burn intervals predicted the rate of woody vegetation expansion. Burning every one to two years 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," said Allison Veach, one of the researchers, in a news release. "Woody plant encroachment may not be prevented by fire alone."

The findings reveal that by burning, it's possible to maintain the tallgrass prairie to some extent. Yet additional actions are also needed in order to preserve this ecosystem.

"The tallgrass prairie is almost nonexistent on the globe," said Veach. "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 findings are published in the journal PLOS One.