CONTRASTING WATER-USE PATTERNS AMONG SIZE AND LIFE-HISTORY CLASSES OF A SEMIARID SHRUB

1. Ecophysiological characteristics of plants from different size and life-history classes were investigated in a field population of Chrysothamnus nauseosus. 2. Juvenile plants (> 1-year-old but pre-reproductive) had higher rates of photosynthesis, stomatal conductance and transpiration than reproductive adults, even though pre-dawn xylem pressure potentials of juveniles were slightly lower. Juveniles were also less water-use efficient than adults based on instantaneous gas exchange (photosynthesis/transpiration) and carbon isotope discrimination (DELTA). 3. A comparison of DELTA-values indicated a gradient of water-use efficiency that was correlated with size: Seedlings < Juveniles = Small Adults < Large Adults. Reproductive status did not account for any variation independent of size. 4. Small establishing plants may experience short-term environmental conditions and long-term selective pressures different from those of larger reproductive plants. 5. The pattern of smaller plants having higher rates of gas exchange and less efficient use of water in the absence of higher pre-dawn xylem pressure potentials suggests a developmental progression in ecophysiological parameters with increasing size, in addition to the environmentally induced variation that has been documented for these characters for many species.