Evidence for increased sensitivity to nutrient and water stress in a fast-growing hybrid willow compared with a natural willow clone

The hypothesis that fast-growing breeds of willow (Salix spp.) are more sen sitive to nutrient and water stress and less efficient in nutrient- and wat er-use than slower-growing natural willow clones was tested. Cuttings of a natural clone of S. viminalis L. collected in Sweden (L78183) and a hybrid clone of S. schwerinii E. Wolf. x S. viminalis L. ("Tora") were grown outdo ors in pots under various experimental conditions in a full-factorial desig n. The experimental conditions included three fertilization, two irrigation and two temperature regimes. Classical growth analysis techniques, based o n an initial and a final harvest, were used as a screening method, together with calculation of intrinsic water-use efficiency (foliar carbon isotope ratio; delta C-13). In addition, nitrogen-use efficiency was calculated as the product of nitrogen productivity and mean residence time of nitrogen on an annual basis. There were significant differences in plant structural pa rameters (leaf area ratio, specific leaf area) and water-use efficiency bet ween the clones. Furthermore, several clone x treatment interaction effects on various growth parameters indicated that the clones adapted to specific environments in different ways. "Tora" plants produced up to 25% more shoo t biomass than plants of the natural clone in response to high rates of fer tilization and irrigation, whereas clone ranking was reversed in most other treatments. The results support the hypothesis that fast-growing hybrids a re more sensitive to nutrient and water stress than slower-growing natural clones. The hypothesis that natural clones have higher resource-use efficie ncy than fast-growing hybrids was supported with respect to water, but not nitrogen.