Scientists have theorized that invasive plants such as downy brome (cheatgrass) can outpace native species by changing soil so that they have access to more nutrients and can grow faster and stronger. This soil “engineering” is only one way plants deal with competition, but it may be an important one.
The authors of an article published in the current issue of Invasive Plant Science and Management undertook a case study of the invasive downy brome, one of the most problematic weeds in North America, to determine whether it alters soil to boost its growth potential and perhaps even increase its invasiveness. They tested a hypothesis that soil already infested with downy brome would act as a better growth medium for the weed than other soils.
One theory for why exotic plants grow stronger and more quickly in their host environment rather than in their native environment is that weeds alter the soil to their advantage. To study this, the authors compared the plant–soil relationships and growth of downy brome. They grew it in soil from the northern Great Basin of California that had and had not already been invaded by the plant. They then sampled and compared the root mass and soil at three depths and the plant biomass.
After one season, downy brome grown in invaded soil had 250% more biomass and almost twice the root mass of plants grown in the non-invaded soil. By the second growing season, biomass in the invaded soil was still almost double the amount in the non-invaded soil, while root mass had decreased and was similar between invaded and non-invaded soils.
The authors also found that downy brome became more competitive when it received more nutrients, particularly nitrogen, phosphorus, and manganese. They found that the plant increased these nutrients in the soil for its own benefit.
The authors concluded that downy brome actively increases its growth potential. They speculated that the plant can take over areas filled with supposedly resistant plants by increasing and then exhausting the soil nutrients, causing downy brome to thrive and native plants to struggle. Further studies are needed to determine if results are similar for other areas and other plants.
Full text of the article “Soil engineering facilitates downy brome (Bromus tectorum) growth—a case study,” Invasive Plant Science and Management, Vol. 6, No. 0, 2013, is now available.