Drought adaptations in two Californian evergreen sclerophylls.

A field study was initiated to determine the patterns of water stress imposition and stomatal resistance to gas exchange in representative species of 2 evergreen sclerophyllous communities. In concurrent experiments plant water potential, temperature and vapor pressure gradient were varied to determine the relative importance of morphological and physiological parameters in delaying onset of water stress during drought periods.In general, stomatal and photosynthetic responses to water stress were similiar in both species. Both were able to fix carbon even when leaf water potentials dropped as low as-25 bars. Stomatal movements were positively correlated with soil water potential rather than to leaf water potential. However, water stress developed much more rapidly in Arbutus menziesii, a plant of more northerly distribution, than in Heteromeles arbutifolia where they occur on adjacent sites. Morphological parameters were primarily responsible for the very different patterns of water stress imposition. Consequently, Arbutus is limited to areas of shorter drought duration than is Heteromeles and this is reflected in their differing distributions.