The Potential for Dominance by Stipa pulchra in a California Grassland

Abstract

Abstragt: In a California grassland where Stipa pulchra, a perennial bunchgrass, has regained dominance, Avena fatua is dramatically reduced in importance as compared to surrounding annual grasslands without S. pulchra. This study examined the potential of S. pulchra to reduce the importance of A. fatua in the perennial grassland compared to the annual grassland. Soil moisture was monitored frequently over two growing seasons and was not limiting to growth of A. fatua in the perennial grassland. Soil nutrients were examined by chemical analysis of field soils and by fertility experiments in greenhouse studies. Available nitrogen occurred at concentrations limiting to growth of A. fatua in both grasslands, but it occurred in lowest concentrations in the annual grassland where Avena abounded. Animal activity and light reduction did not support a hypothesis for reduction in abundance of A. fatua. The allelopathic potential of aqueous leachates of S. pulchra straw was experimentally tested on A. fatua, and demonstrated a slight toxicity as evidenced by the growth of A. fatua. Root exudates of S. pulchra reduced the shoot growth of A. fatua. This was especially pronounced when the two species were grown together. The importance of the allelopathic reactions to the pattern in the field is related to the phenology of the two species. A synergistic effect of allelopathic reaction and limitation of available nitrogen is suggested as one potential mechanism by which A. fatua is reduced in grasslands dominated by S. pulchra.