EXOTIC GRASSES IN CALIFORNIA

Exotic species have spread rapidly throughout the world with modern civilization and advanced forms of transportation. In California, exotic annual grasses and forbs have supplanted native perennial grasses over  nearly a quarter of the state. At an August meeting of the Ecological Society of American a team of scholars from Santa Barbara  and one from Minnesota reported on what they have found out about why these exotic grasses have been so successful.

Three hypotheses were investigated. First, the exotics might dominate resources such as sunlight and water; second, they might be winning out because the sparse natives produce too few seeds to retake the fields; and third, grasses, whether exotic or native, are not easily dislodged once they have become established. Field experiments were done near the coast in southern California. They found that mixtures of exotics, including black mustard, ripgut brome and soft chess, used less water, nitrogen and sunlight than a mixture of five native grasses, including California brome and purple needlegrass. They also found that the natives were able to retake patches of exotic grasses if the researchers planted more native seeds. Seed abundance was the key. Trying to re-establish native grasslands won’t be easy and more research is certainly needed. After all, areas of exotic annual grasses usually contain a seed bank of tens of thousands of seeds per square meter – a formidable impediment to native perennial grasses. [excerpted from Science(September 6, 2002), pp. 1635 and 1637]

Mike Kuhn

9-11-02