Competition between Bromus tectorum L. and Poa pratensis L.: the role of light.

Bromus tectorum L. dominates sites of large-scale disturbance, while Poa pratensis L. dominates the sites of small-scale disturbance in the Festuca/Symphoricarpos habitat type in eastern Washington (USA). The role of incident irradiation in influencing these distributions was examined using field and glasshouse experiments. Glasshouse grown swards of B. tectorum growing beneath an established canopy of P. pratensis displayed larger biomass and higher survival when exposed to supplemental light versus controls. Neither mean plant height nor the skewness in the individual biomass distribution were significantly different between the supplemental light and control plots. Maximum net photosynthesis for P. pratensis and B. tectorum was 11.5 and 14.87 mg CO2·dm-2·h-1, respectively. For Poa, light utilization efficiency was greater, although light compensation point was lower than that exhibited by Bromus. These photosynthetic characteristics along with the seasonal pattern of light transmission through the meadow steppe canopy may largely account for the successful establishment of P. pratensis in small sites of disturbance with little light, while restricting the establishment of B. tectorum to patches with more irradiance.