Growth response of Halimium halimifolium at four sites with different soil water availability regimes in two contrasted hydrological cycles

The relationship between water availability and plant growth response in Halimium halimifolium (L.) Willk throughout two years with contrasted precipitation (300 and 850 mm) was examined by measuring vegetative growth and midday leaf water potential in four sandy soil sites with different water availability in Doñana National Park, Spain. H. halimifolium, Cistaceae is mostly restricted to sandy substrates close to coastal Mediterranean areas. At Doñana National Park this species is the main component of stable sand vegetation. Vegetative growth started in March, ending in July. The maximum shoot elongation rate (110 mm/year) and number of branches (8.3 branches/year) occurred in plants growing in the most hygrophytic site (MN) in both the wet and dry years. Plants at this site exhibited higher midday water potentials throughout the year. In contrast, the minimum shoot elongation rate (40 mm/year) and midday water potential (–4.0 MPa) occurred in Monte Intermedio plants (MI) in the dry year, although the water table was shallower than in Monte Blanco (MB). In the wet year the minimum shoot and branch elongation were recorded in MB. The number of leaves per branch for a single main shoot was higher (55 leaves/branch) in the driest area (MB), but these leaves had the smallest area. ANOVA showed significant differences in growth response and midday water potential between the four sites. A stepwise multiple linear regression showed that midday water potential, water table depth and monthly average temperature were the variables most closely associated with shoot elongation rate. We conclude that under severe dryness, the expected patterns of greater growth in sites with better water supply may differ from compared with the expected growth pattern in normal conditions.