PARENT SESSION
Oral Session # 56: Plant Ecology IV: Plant - Water Relations II.
Presiding: D Rosenthal
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 200.

Water deficits and hydraulic conductance: Effects of water availability vs. evaporative demand.

Bhaskar, Radika^*,1, Valiente-Banuet, Alfonso², Ackerly, David¹, ¹ Stanford University, Stanford, CA² Instituto de Ecología, Universidad Nacional Autònoma de México, D.F., México, México

ABSTRACT- The influence of seasonal water deficit on plant water relations has been studied extensively in mediterranean-type climates, which are characterized by hot, dry summers and cool, wet winters. Comparisons with non-mediterranean climates provide an ideal study system to examine the consequences of water availability in the soil vs. atmospheric demand (high vapor pressure deficits) on plant water relations. In central Mexico evergreen shrub communities closely related to California chaparral experience a winter dry season followed by a warm summer with higher insolation and monsoonal rains. Thus the period of highest evaporative demand coincides with the period of highest water availability, whereas the opposite is true under mediterranean-type climates. We compared whole plant hydraulic conductance, leaf gas exchange, and water use efficiency of 13-15 dominant evergreen shrubs in two sites, one in the state of Puebla, Mexico and the other in Santa Barbara, California. The sites had similar average annual rainfall and potential evapotranspiration. Within both sites, species that experienced greater water deficits, as determined by minimum seasonal water potentials, had lower hydraulic conductance and light-saturated photosynthetic rates. Between sites, species in California had higher hydraulic conductance than species in Mexico, relative to minimum water potentials. This result may be explained by two climatic factors that distinguish the sites. The first is the concurrence of high atmospheric demand at the time of extreme soil drought in California; the second is the occurrence of freeze-thaw events during the onset of soil drought in Mexico. Both factors may select for higher hydraulic conductance in California relative to Mexico. The contribution of evolutionary divergence and phenotypic plasticity to these patterns observed in the field will be tested in future studies under common environmental conditions.

Key words: chaparral, hydraulics, non-mediterranean