1. Changes in canopy area and canopy age structure of the leguminous shrub Retama sphaerocarpa were estimated non-destructively by monitoring growth and survivorship of cladodes on marked shoots.
2. Gas exchange of cladodes of different age classes was measured by infra-red gas analysis during a period of 18 months. The effects of seasonal and year-to-year variation in the area and age structure of the canopy on daily carbon assimilation were assessed.
3. New canopy area produced by spring growth was highly variable between years and also affected the age structure of canopies because of differential survivorship of older cohorts of cladodes.
4. During the first year (1993), predawn water potential of cladodes and, therefore, their maximum diffusive conductance (0·35–0·45 mol m–2 s–1) and photosynthetic rate (20–26 μmol CO2 m–2 s–1), changed little with the season because the deep-rooted shrubs had access to permanent water sources. Lack of rainfall in the winter of 1993/1994 constrained predawn water potential and rates of gas exchange in the following growing season when daily maxima of A declined continuously and fell to a minimum of 10 μmol CO2 m–2 s–1 in July 1994.
5. Nitrogen concentration and specific cladode area, but not nitrogen content, were significantly different between annual cohorts. Mainly as a result of secondary diameter increment of cladodes, specific cladode area and nitrogen concentration and therefore photosynthetic rate generally declined with increasing age of cladodes, but the differences in the photosynthetic rate between cohorts were insignificant during the cool season.
6. Variation in canopy area caused variation in daily carbon assimilation by 30–36% within years and by 37–52% between years. Seasonal and between-year variation in the age structure of canopies, however, only accounted for changes in daily carbon assimilation of ± 2%. Canopy age structure thus appears to be an insignificant variable affecting carbon gain in R. sphaerocarpa.