PARENT SESSION
Contributed Oral Session 140: Forest Photosynthesis and Water Relations
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 520 B, Level 5, Palais des congrès de Montréal

Hydraulic strategies of Metrosideros polymorpha at high and low precipitation.

Cornwell, William^*,1, Bhaskar, Radika¹, Sack, Lawren², Cordell, Susan³, ¹ Dept. of Biological Sciences, Stanford, CA, USA² Department of Botany, Honolulu, HI, USA³ USDA Forest Service, Hilo, HI, USA

ABSTRACT- Metrosideros polymorpha (ohi'a lehua, Myrtaceae) experiences a huge range of precipitation across its distribution in Hawai'i--from more than 5000 mm / yr to less than 500 mm / yr. What aspects of intraspecific variation in hydraulic architecture explain its success under both high and low rainfall? We sampled populations of M. polymorpha on two different aged lava flows on both the wet and the dry sites of the island of Hawai'i, while holding both elevation and mean annual temperature constant. Rainfall and vapor pressure deficit differed greatly between the wet and the dry sites. We measured leaf and stem traits that reflect water use and transport as well as carbon balance at each site. We found that across the aridity contrast, populations of M. polymorpha have remarkably consistent pre-dawn water potentials. This suggests variation in rooting depth--deep roots in dry sites--that maintains similar access to water despite extreme differences in rainfall. Across sites we found no difference in the xylem water potential at which 50% of stem conductance is lost. Leaf specific conductance was much higher at dry sites, which could allow greater stomatal conductance without a drop in leaf water potential. We also found greater nitrogen per unit leaf area and less depleted ¹³C values at the dry sites. This is consistent with increased water use efficiency. Overall, these results suggest that populations of M. polymorpha maintain similar water potentials across a striking range of precipitation--and so avoid soil water deficit in dry sites. In addition a rooting strategy that ensures access to soil water, high stem conductance, and high nitrogen per leaf area at dry sites appear to represent a whole plant strategy to lessen the effect of high vapor pressure deficit on carbon gain.

Key words: leaf specific conductance, xylem vulnerability, ohi'a, water use efficiency