Comparative Gas Exchange of Juniperus ashei (Ashe Juniper, Cupressaceae) at Ambient and Elevated Levels of Light, CO 2 Concentration and Temperature with Potential Influences on Community Structure

Abstract

Juniperus ashei (Ashe juniper, Cupressaceae) is a shrub or small tree found in woodlands and savannas of central and western Texas, northern Mexico, Missouri, Arkansas and Oklahoma. Environmental conditions where this species is found are stressful due to mostly shallow soils, high summer temperatures, low and inconsistent rainfall. Juniperus ashei has increased in density from the distant past not because of elevated CO 2 or increased temperature, but lower fire frequency and reduced competition with prairie grasses because of heavy grazing. This study demonstrates that J. ashei functions as a sun species and juveniles beginning growth in canopy shade most likely will not be recruited into the canopy. Juniperus ashei is a sun species because the A max (maximum photosynthetic rate) is high and increased significantly (45%) with increased CO 2 concentrations to 17.20 µmol CO 2 ∙m − 2 ∙s − 1 at the highest CO 2 concentration tested (780 µL/L) but did not change with increased temperature. Dark respiration (R d ) doubled when temperature increased but did not change with CO 2 concentrations. Intercellular CO 2 increased as the CO 2 concentration increased but not with temperature. Light saturation (L sp ) and water use efficiency (WUE) increased significantly when light levels and CO 2 concentrations were elevated but not when temperatures were elevated. Stomatal conductance decreased when the CO 2 concentration increased, but not by elevated temperature. There was a significant interaction for WUE that increased with increased light level and CO 2 concentration, but decreased overall with temperature. This suggests J. ashei juveniles in canopy shade will be more stressed at high CO 2 and elevated temperatures in low light reducing recruitment into the canopy.