Vertical distributions of plant nutrients and heavy metals in salix viminalis stems and their implications for sampling

We studied the vertical distribution of elements in the woody biomass of two willow stands and discuss its implications concerning the reliability of the stratified sampling method used when whole-shoot sampling is not feasible. Five-centimeter pieces from two- (sandy soil) and three- (clay soil) year-old shoots of Salix viminalis (clone 78183) were sampled at 1 m vertical intervals in late March-early April 1993 in central Sweden. The stand on clay was sampled again 2 years later. Concentrations of P, K, Ca, Mg, S, Mn, Zn, Cu, Ni and Cd increased significantly with height, which was assumed to be mainly a consequence of increasing bark proportions. The increase was least consistent for Cd. Differences in concentration gradients between elements were ascribed in part to their differential redistribution in the tissues. With respect to vertical concentration gradients along the shoot the elements ranked in the same order in the 2-, 3- and 5-year-old shoots. For example, Ni increased most from the lowest to the highest sampling level (sixfold increase in the 5-year-old shoots), followed by P and Cu (fourfold). Concentrations of Ca, Mn and Zn doubled, and Cd increased by 20%. It was shown that for the 3-year-old shoots a slight shift in the sampling point location could result in a 10% change in the measured shoot Ni concentration. In the older shoots, where concentration gradients were mainly found in the upper parts but the bulk of the biomass was in the lower shoot parts, a shift in sampling point location would probably be of less importance.