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 relia bility of the stratified sampling method used when whole-shoot samplin g is not feasible. Five-centimeter pieces from two- (sandy soil) and t hree- (clay soil) year-old shoots of Salix viminalis (clone 78183) wer e 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. Con centrations of P, K, Ca, Mg, S, Mn, Zn, Cu, Ni and Cd increased signif icantly with height, which was assumed to be mainly a consequence of i ncreasing 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 resp ect to vertical concentration gradients along the shoot the elements r anked in the same order in the 2-, 3- and 5-year-old shoots. For examp le, Ni increased most from the lowest to the highest sampling level (s ixfold increase in the 5-year-old shoots). followed by P and Cu (fourf old). 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 sa mpling point location could result in a 10% change in the measured sho ot Ni concentration. In the older shoots, where concentration gradient s 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 pr obably be of less importance. (C) 1998 Published by Elsevier Science L td.