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Grass vs. buckthorn: Native plants' surprising role in forest defense

New University of Minnesota research suggests that seeding native plants alongside invasive removal efforts may turn the tide in the battle against buckthorn.

Buckthorn is one of Minnesota's most widespread invasive species. Each year, millions of dollars are spent trying to remove it from the state's woodlands and forests. Most attempts offer a temporary reprieve as buckthorn quickly re-establishes dominance in the following years.

In an article recently in Ecological Engineering, researchers show that heavily seeding native grasses, like Virginia wildrye, alongside buckthorn removal efforts, greatly reduces the number and size of new buckthorn trying to reclaim the site.

They found:

• Returning buckthorn were 47% less abundant and 81% smaller in areas where grasses were established—about a 90% decrease returning from seed overall.
• Seeding is most effective in woodlands with enough light to support vigorous grass growth. Grasses do not establish well in dark forests but can form nearly impenetrable thickets in more open woodlands.
• To be effective, seeding must be paired with other management strategies that prevent larger trees from resprouting.

Buckthorn poses the biggest threat to forest restoration in the first couple of years after mature trees are removed, but this is also when the seedlings are most vulnerable to competition from native species. Establishing cover of grasses or other native species needs to happen as quickly as possible to prevent buckthorn from exploiting this critical period.

"There are no easy solutions when it comes to buckthorn, but this work clearly demonstrates that by investing in native biodiversity we can make our natural areas more resistant to invasion in a way that means less time and money combating those invaders, fewer herbicides put into our environment, and healthier ecosystems overall," said lead author Mike Schuster, a researcher in the College of Food, Agricultural and Natural Resource Sciences.

Managers interested in applying this technique should plan to seed native species in the early spring following fall or winter buckthorn removal and treat resprouting buckthorn the following summer.

"This is one way we can interrupt the seemingly endless cycle of buckthorn management that so many of our woodlands find themselves trapped within. It gives us an opportunity to choose a different path forward," said Peter Wragg, a lecturer in the Department of Forest Resources.

The research team is currently working on honing the technique by exploring the most effective and cost-efficient seed mixtures and seeding rates, as well as evaluating how seeding efficacy varies across Minnesota's diverse woodlands and forests.