Drought Relationships and Distribution of Two Mediterranean‐Climate California Plant Communities

The summer drought—deciduous coastal sage in Southern California near Los Angeles has a bimodal, coastal and interior, distribution at low elevations (sea level to 300 m), whereas the evergreen sclerophyllous chaparral is developed at higher elevations. Winter rainfall is 300—350 mm at the low elevations and 500—600 mm at the higher altitudes. Photosynthesis and transpiration in response to artificial drying were examined in field—collected leaves of species from the two vegetation types. The fully hydrated mesomorphic leaves of the coastal sage species had initially higher photosynthesis and transpiration rates which declined rapidly during the first 20—30 min of drying, presumably because of hydroactive stomatal closure. At zero net photosynthesis cuticular loss of water was high, nearly 25% of the maximum rate for these species. In the chaparral species photosynthesis and transpiration rates were one—fourth to one—third those of the coastal sage species. Decline of rates with drying time was not nearly as rapid. Net photosynthesis continued at 20—30% of maximum after 70—80 min with lower cuticular losses of water vapor. During cool spring periods with sufficient soil moisture, high transpiration and assimilation rates allow for rapid growth, flowering, and fruiting in plants of the coastal sage community. They are drought—evading by virtue of their summer deciduous habit and are better adapted to prolonged summer drought in areas of low rainfall. Chaparral species, at slightly higher altitudes with higher precipitation and with deeper and more extensive root systems and xeromorphic leaf characteristics, are less sensitive to conditions of high evaporative demand and can maintain net assimilation, although low at times, throughout the long summer drought period.