Carbon gain and hydraulic limits on water use differ between size classes of Larrea tridentata

Juvenile woody desert perennials often exhibit higher rates of transpiration than co-occurring adults, which may facilitate establishment by increasing carbon gain when water is plentiful, but increase the risk of hydraulic failure during drought. We tested three hypotheses regarding the role of hydraulic limits in two size classes of Larrea tridentata: 1) small plants grow faster and use more water per unit leaf area than large plants, 2) small and large plants have similar hydraulic limits on transpiration, and therefore, 3) small plants more closely approach hydraulic limits on transpiration than large plants. We used a hydraulic model, parameterized with plant and soil characteristics from field measurements of a single population, to predict maximum transpiration rate (Ecrit) for each size class and compared Ecrit with measured transpiration over fluctuations in predawn plant water potential (ΨPD). At high ΨPD small plants had higher Ecrit, gas exchange and growth than large plants, and maintained larger safety margins from Ecrit than large plants. At low ΨPD, all measured and modeled characters were similar for the two sizes. Our data indicate that higher transpiration in small plants facilitates increased carbon gain when water is available without increasing the risk of hydraulic failure during drought.