Different Phenotypic Response of Halimium halimifolium in Relation to Groundwater Availability

Halimium halimifolium (Cistaceae) is a shrub occurring on sandy substrates in the Mediterranean region and is the principal element of the sandy vegetation in Doñana National Park (S.W. Spain). The annual cycles of leaf water potential, stomatal resistance and shoot growth were studied in permanently marked H. halimifolium plants. Three sites with different water availability at the stabilized sands of the Park -- Monte Blanco (MB, top of the dunes), Monte Negro (MN, depressions) and Monte Intermedio (MI, slopes of dune ridges) -- were chosen. The study was carried out during a period of extreme drought. MN individuals showed the greatest growth, both in main shoot and in ramets, less negative water potential values and lower stomatal resistance values than the other two areas, because individuals' roots could reach the water table all year long. In winter there were no differences in predawn, midday and sunset water potential or in stomatal resistance values in the three areas. In summer, MI individuals showed the most negative water potentials and the highest stomatal resistance, although they occupied an intermediate position in the water table depth gradient. Under drought conditions MI individuals are the most affected, because they are acclimated to use ground water during the growth period, while MB individuals, that never reach the water table, are acclimated to use found surface water and are, thus, better adapted to water scarcity.