Variation in growth responses to availability of water in Cistus albidus populations from different habitats.

Seeds of Cistus albidus L. plants from three populations that are exposed to differing temperature and precipitation in Almería province of south-east Spain, were collected and grown together in a factorial experiment with two irrigation treatments. The aim was to determine whether populations from different habitats differed when exposed to common conditions, or differed in the plasticity of their response to availability of water. Significant differences in growth of branches and in leaf dimensions were found between treatments, indicating phenotypic plasticity. There was also significant variation between populations in growth of branches and leaf dimensions, with a population from a location that is intermediate in terms of precipitation and temperature showing the greatest growth of branches and production of leaves under the well-watered treatment. This population is from a semi-arid climate, where precipitation is unpredictable, and selection may have occurred to favour rapid growth when water is available. This population had the narrowest leaves under both treatments, and the lowest leaf mass in the well-watered treatment. It also maintained the same mass per leaf under the two treatments, whereas the others showed an increased mass of leaves with increased availability of water. Thus, populations differed both in their manner of allocating resources and their response to availability of water.