PARENT SESSION
Organized Oral Session 11: Hydraulic limitations in vascular plants
Organizer(s): F Berninger, L Lauzon, and E Nikinmaa
Monday, August 8, 1:30 PM - 5:00 PM, Meeting Room 511 C, Level 5, Palais des congrès de Montréal

Hydraulic constraints on transpiration in two size classes of Larrea tridentata.

Medeiros, Juliana¹, Pockman, Will¹, ¹ Department of Biology, University of New Mexico, Albuquerque, New Mexico, USA

ABSTRACT- We assessed the role of hydraulic limits in different size classes; we tested three hypotheses on small and large plants in a population of Larrea tridentata: 1) during times of high resource availability 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 (E), 3) small plants more closely push the hydraulic limits on E than large plants. We made excavations, measured shoot growth, gas exchange, predawn soil and plant (_PD) water potential, leaf specific hydraulic conductance (k_l) and hydraulic conductivity (k_h) in the field from March 2002 –August 2003. A model determined the maximum transpiration rate (E_crit) for each size based on xylem vulnerability curves and soil texture analysis. Small plants exhibited significantly higher midday E and photosynthesis than large plants during the 2002 monsoon season, when_PD was high. E_crit determined by the model was higher for small plants than for large, especially at high _PD, and small plants had higher saturated k_l than large plants. In contrast, at low_PD, the hydraulic limits and k_l were very similar for the two sizes. Seasonal changes in A_R:A_L were suggested by excavations, and the model indicated that these changes could effectively increase hydraulic limits on rates of water loss in small plants. There were no significant differences between small and large plants in safety margins from E_crit, loss of k_l or k_h, indicating small plants do not more closely push hydraulic limits on E than large plants.

Key words: hydraulic limits, Larrea tridentata, water relations, warm desert