Temperature-Dependent Physiology of Poa secunda, a Cool Season Grass Native to the Great Basin,United States

Poa secunda Presl. is one of the few native perennial bunchgrasses in the Intermountain West to persist and co-occur with the invasive annual Bromus tectorum L. following widespread overgrazing and frequent wildfires. To identify potential mechanisms responsible for the co-occurrence of P. secunda with B. tectorum, respiration rates ((R_{operatorname{CO} _2 })) of eight populations were measured at 10, 20, and 30°C on laboratory-grown plants by infrared gas analysis. In addition, (R_{operatorname{CO} _2 }) and metabolic heat rates (q) of nine field-grown populations were measured at 10 and 20°C using calorimetry on eight dates over a growing season to compare temperature-dependent physiology of P. secunda with previous published patterns for B. tectorum. Laboratory respiration rates of P. secunda populations suggest considerable intraspecific variation in physiological response to temperature. Changes in slope for (R_{operatorname{CO} _2 }) and q over the growing season were steeper at 20 than at 10°C , suggesting that P. secunda populations are more capable of maintaining steady rates of metabolism at low than at high temperatures. However, growth rates of P. secunda were relatively lower than those for B. tectorum at 10°C. Calculations of growth rates and efficiency of converting substrate carbon into biomass of P. secunda consistently remained positive, while those for B. tectorum rapidly declined at temperatures above 10°C. These data suggest that P. secunda co-occurrence with B. tectorum over a broad range of plant communities in the Intermountain West may be partially explained by having a similar ability to maintain positive and stable growth rate at low temperature. In addition, the greater ability of P. secunda to maintain growth rates and metabolic efficiency at higher temperatures than B. tectorum may allow this perennial grass to compensate for the greater relative growth rates of B. tectorum at low temperature.