WATER RELATIONS OF SEEDLINGS AND ADULTS OF 2 FYNBOS PROTEA SPECIES INRELATION TO THEIR DISTRIBUTION PATTERNS

1. To test whether differences in water relations were responsible for differences in species distributions across a landscape, we investiga ted patterns of water use (water potential, transpiration) of Protea s usannae (associated with deep soil) and Protea compacta (associated wi th shallow soil) in the field (mature plants) and in the laboratory (s eedlings). 2. Adults of P. susannae had more extensive root systems an d less conservative water use than those of P. compacta, suggesting gr eater access to water. However, no substantial differences in water po tential or transpiration were found, for either species, between plant s in deep and shallow soils. 3. Germination, seedling growth and water relations were compared under controlled-environment conditions. Prot ea compacta seedlings were much larger with greater root depth and tra nspiration rate (per plant). The consequence of this was a drying out of the soil profile leading to greater dependence on the water table. Protea susannae seedlings were initially conservative in water use but , after 30 weeks, had a similar rate of transpiration (per plant) to P . compacta seedlings, despite a much smaller leaf area. 4. Both specie s were found to experience switches in water-use strategy between seed ling and adult stages: from conservative to water spending in P. susan nae and from water spending to conservative in P. compacta. These swit ches, together with differences in water relations, particularly at th e seedling stage, suggest that habitat specialization could play a cri tical role in determining these species distributions.