Morphological plasticity following species-specific recognition and competition in two perennial grasses

Morphological characteristics and biomass allocation of two perennial grasses, Pseudoroegneria spicata (Pursh) A. Löve ssp. spicata (bluebunch wheatgrass) and Agropyron desertorum (Fisch. ex Link) Schult. (crested wheatgrass), were compared under different competition and nutrient treatments. The competitive responses of two plants grown in containers under field conditions were assessed in monocultures and mixtures in two experiments using different scales of nutrient application. In the Small-Scale Experiment, a localized fertilization was applied in the rooting zone between two plants; in the Large-Scale Experiment the entire container was supplied with nutrients. Agropyron responded more vigorously to fertilization than did Pseudoroegneria, but based on the relative performance of Agropyron in monoculture and mixture, it was not superior to Pseudoroegneria in resource competition. Pseudoroegneria was apparently able to recognize neighboring plants as either conspecifics or individuals of the other species. The responses included changes in shoot architecture, root morphology, and allocation between roots and shoots. Agropyron generally did not exhibit such morphological flexibility. In field plot plantings of 4-yr-old tussocks similar shoot differences were seen in Pseudoroegneria. There was, however, no indication of superior resource competition for Agropyron. Thus, any early advantage of Agropyron in vigorous growth of young plants in response to nutrients was apparently lost by the time the plants had reached this stage of development. Morphological and allocation flexibility of Pseudoroegneria may have compensated for slower, less vigorous growth. If species-specific recognition and morphological plasticity are common in nature, this complicates our attempts to understand mechanisms of competition.