Comparative Gas Exchange of <i>Ulmus crassifolia</i> (Cedar Elm, Ulmaceae) and <i>Ungnadia speciosa</i> (Mexican Buckey, Sapindaceae) at Ambient and Elevated Levels of Light, CO₂ and Temperature

Abstract

Ulmus crassifolia Nutt. (Cedar elm, Ulmaceae) is a tree found in central and east Texas, northern Mexico, east to Florida, and north to southern Missouri and Oklahoma. Ungnadia speciosa Endl. (Mexican-buckeye, Sapindaceae) is a shrub or small tree found in woodlands and savannas of central and western Texas, southern New Mexico and northern Mexico. In central Texas, both species are found in Juniperus ashei / Quercus virginiana woodlands or sa vannas or also at low density in inter-canopy grassland gaps or patches. Environmental conditions in this area are stressful because of shallow soils, high summer temperatures, and inconsistent low rainfall. Currently, both species have a low density in these areas, and Ulmus crassifolia is usually a tree , while Ungnadia speciosa is a woody understory shrub. This study suggests U. cras sifolia and U. speciosa are tolerant or intermediate species , with juveniles starting in shade. Maximum photosynthetic rate ( A max ), dark respiration (R d ), intercellular CO 2 , light saturation ( L sp ) and water use efficiency significantly increased when light levels and CO 2 concentrations were elevated for both spe cies , but not when temperatures were elevated. Stomatal conductance de creased when the CO 2 concentration doubled , but there were few e ffect s from elevated temperature. These findings suggest that U. speciosa and U. crassifolia should be more common and imply that they will have a higher density in a future high CO 2 environment.