Environmental limitations of photosynthesis on a California evergreen shrub

The photosynthetic response of Heteromeles arbutifolia, a California evergreen shrub, was measured in natural habitats representing both the climatic center of distribution and the southern warm arid extreme of this species. At all sites the seasonal carbon gain was limited primarily by available water, secondarily by photoperiod, and only slightly by temperature. Thus, the warmer winter temperatures at the southern habitat would not result in a carbon gain sufficient to offset losses due to the longer drought there. It is thus postulated that the competitive advantage of evergreens over drought deciduous types is often limited at these arid habitats.