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.

Microsite utilization by Bromus tectorum L. and Poa pratensis L. in a meadow steppe community.

Emergence of Bromus tectorum L. and Poa pratensis L., two dominant alien grasses in eastern Washington, from 24 microsites was monitored in spring and autumn 1979. Spring emergence of B. tectorum reached a maximum on north-facing slopes of artificially constructed pocket gopher mounds and burrows while spring emergence of P. pratensis was maximized on both north-and southfacing slopes of mounds. Earliest autumn emergence of B. tectorum seedlings was in fissures of 0.5 cm, 1 cm, and 2 cm depth while earliest emergence of P. pratensis was 24 days later in fissures of 0.5 cm, 1 cm, 2 cm, and 5 cm depth. All B. tectorum seedlings had emerged within 44 days after first emergence on north-facing slopes of artificially constructed mounds. Maximum emergence of P. pratensis (37%) seedlings occurred on the same north-facing positions of mound slopes and also on the north-facing slope of the burrow, 24 days after first emergence. Both species exhibit broad microsite tolerance and are found on similar but temporally separated microsites.