Creating vegetation barriers to block the spread of invasive species

The notorious annual grass called, medusahead (Taeniatherum caput-medusae), drives land managers in western North American absolutely crazy. Infesting almost a million acres from California to Texas, the highly invasive grass crowds out native vegetation, degrades wildlife habitat, reduces grazing capacity, and increases wildfire risk.

A new study in the journal Biological Invasions shows that land managers may be able to contain highly invasive annual plants like medusahead by establishing barriers of perennial bunchgrasses to block the spread.

Kirk Davies and fellow researchers tested the idea on plots in the foothills of Steen Mountain in Oregon. They planted the perrenial grass Agropyron desertorum (also known as 'desert wheatgrass') in 6 meter-wide bands along the invading fronts of medusahead. They found that medusahead cover was 42 times greater and 47 times denser in unprotected plant communities compared to areas protected by the vegetation barrier.

They also measured nutrients and found that potassium and ammonium concentrations were reduced where desert wheatgrass was seeded.  Together, these results indicate the vegetative barrier increased biotic resistance of plant communities.

Medusahead outcompetes native plants by initiating growth earlier and acquiring more resources. Agrophyron desertorum  also initiates growth early and is competitive for soil resources. The researchers also suspect that vegetation barrier helps block seed dispersal thus impeding the spread of medusahead.

The vegetation barrier does have some risks of its own. Desert wheatgrass is also a non-native plant that can potentially outcompete native vegetation. However, as a perennial bunchgrass, Agrophyron desertorum provides  similar wildlife habitat and ecosystem functions as native plants and poses much less of a risk than medusahead.

Furthermore, land managers could potentially use a native bunchgrass instead - the researchers chose desert wheatgrass because it has superior competitive characteristics to native bunchgrasses and its seed is cheaper to obtain.

While the study shows that this weed control method has promise, the authors advise that it should be used in combination with a number of approaches. They note that some medusahead was able to make it beyond the vegetation boundary.

They write,

"This suggests that the effectiveness of competitive vegetation barriers could be improved by making them wider and incorporating an early detection and eradication program for satellite populations that establish beyond competitive vegetation barriers."

--Reviewed by Rob Goldstein

Davies, K., Nafus, A., & Sheley, R. (2010). Non-native competitive perennial grass impedes the spread of an invasive annual grass Biological Invasions DOI: 10.1007/s10530-010-9710-2