Estimation and relevance of bark proportion in a willow stand

We studied bark proportion of a willow (Salix viminalis) plantation established to produce biomass for energy, the vertical distribution of elements (N, P, K, Mg, Ca, Co, Cu, Zn, As, Si, Cd, Cr, Pb, Hg and Ni) in bark and in wood of the shoots and the content of elements in the standing biomass. The study is based on 5-year-old shoots (clone 77-683) from a 12-year-old plantation. The bark proportion of the whole willow stand was 19% (9tDMha-1). The bark proportion of single shoots was constant after they had reached the size of 20 mm at stem diameter at 55 cm height. Compared to wood, bark had significantly higher concentrations of N, P, K, Mg, Ca, Cd, Pb, Co and Zn. The nutrient element (N, P, K, Mg) concentrations in the bark and in the wood of the current year shoot fractions (twigs) were significantly higher compared to the bark and the wood of other fractions. The accumulation of heavy metals occurred due to the accumulation of tree biomass and not due to the increase of heavy metal (Cd, Cr, Pb and Hg) concentrations in plant tissues over time. In summary, different management regimes give a possibility to influence shoot size frequency distribution of the crop and the chemical composition of biomass. For minimizing element removals from the soil and corrosion processes in power plants, energy willow stands should be managed in a way that promotes low bark proportion and thereby as little nutrient removal from the site by biomass harvest as possible.