Drought and changes in leaf orientation for two California chaparral shrubs: Ceanothus megacarpus and Ceanothus crassifolius.

Predawn xylem pressure potentials were measured on two California chaparral shrubs, Ceanothus megacarpus and Ceanothus crassifolius, throughout the winter and spring growing season and into the summer drought. On the days xylem pressure potentials were measured, leaf orientation measurements were made on a population of marked leaves from the same shrubs. Predawn xylem pressure potentials decreased from -0.1 MPa in both species to -7.8 and -6.6 MPa in C. megacarpus and C. crassifolius, respectively, between May and August, 1981. Leaf inclinations became more vertical during this period with the greatest change observed in C. crassifolius. This change in leaf inclination was reversible, and, in the late winter and early spring, one year old leaves became more horizontal. Leaf azimuths were random and did not change seasonally. Simulations of solar radiation interception indicated that the increase in leaf inclination associated with summer drought reduced the absorption of solar radiation in August by 6% for C. megacarpus and 20% for C. crassifolius. Standard leaf energy budget calculations suggest that steep leaf inclinations would result in slightly lower leaf temperatures and transpiration rates under summer conditions.