Autecological traits of Halimium halimifolium in contrasting habitats under a Mediterranean type climate -: A review

Halimium halimifolium (L.) WILLK. is a woody Cistaceae species occurring locally in the Western Mediterranean. At the Donana National Park (S Atlantic coast of Spain), Halimium halimifolium is the main component of stable sand vegetation. It grows in a range of environmental conditions from flood-prone depressions to mobile dunes, but it is most abundant, and dominates the scrub composition on the slopes of fixed dune ridges with a water table depth of 2 to 4 m. The species exhibits not only morphological modifications (hairy leaves and twigs), but also structural and ecophysiological adaptations. The response of H. halimifolium to stress conditions (leaf water potential, leaf diffusion resistance, and plant growth) have been studied in the field, throughout an annual cycle in four populations growing in different environments, three dune types differing in soil moisture, and one dune slack. Results showed that plants from the hygrophytic area (MN) had the less negative water potentials, the lowest stomatal resistances and the biggest vegetative growth, together with the highest Leaf Area Index, canopy light extinction, the largest leaf area and the lowest sclerophyll index. Plants from the most xerophytic area (MB) had the lowest Leaf Area Index, the smallest leaf area and the highest sclerophyll index. Even though in dune slopes (MI) water table was shallower than in MB, plants in the latter had more negative water potentials and bigger vegetative growth. Water potential values only exhibited significant differences among the four populations at the end of the spring period and over the summer. These results are discussed in relation to the climatic conditions of the study period (3 years of a drought cycle). Plants from the mobile dune system showed features which were intermediate between MN and MI plants.