Cadmium variability in leaves of a Salix fragilis: simulation and implications for leaf sampling

Plant analysis is a valuable tool to evaluate the pollution level. However, leaf sampling is complicated because of the high variability within the cr own. To investigate the variability of cadmium (Cd) in the leaves of a tree , we sampled one Salix fragilis L. at 292 locations, each with a volume of 0.3 x 0.3 x 0.3 m. The Cd concentration was found to be normally distribute d within a range from 2.4 to 10.6 mg.kg(1) dry mass (DM), with an average o f 6.3 mg.kg(1) DM. A trend was found with high values in the lower parts of the crown and low Cd concentrations at the top. After removal of this tren d the residuals showed a clear spatial structure modelled by a variogram. T he Cd distribution in the leaves of the entire tree was predicted by sequen tial indicator simulation. These results were used to evaluate the current sampling strategy for tree leaves, i.e., sampling sun leaves of the upper t hird of the tree crown. The latter procedure was found to yield biased esti mates of the average Cd concentration as well as the risk of exceeding a co ntamination threshold. An alternative sampling procedure is proposed. This procedure investigates whether a trend is present. Once the height where sa mpling will result in a correct statement of the tree's pollution is locate d, the rest of the stand could be sampled at this height.