Use of artificial trees to assess dry deposition in spruce stands

In order to quantify the contribution of dry deposition to net throughfall composition in spruce, plastic Christmas trees were set up next to real spruce trees of approximately similar height in clearings of the Strengbach catchment (Vosges Mountains, France). Bulk precipitation and throughfall chemistry under the artificial trees, the real spruce trees, and in 3 spruce stands aged 15, 35 and 90 yr were monitored for 8.5 months. Fluxes of base cations in net throughfall below plastic trees were, on average, 1.5 times higher (Ca, Mg), 1.3 times higher (Na) or close (K, Mn) to those in bulk precipitation. Ratios in net throughfall between the fluxes of Ca, Mg, K and Mn, on the one hand, and Na on the other hand, were considered as representative of the composition of dry deposition. Assuming that foliar leaching of Na was negligible, the ratio between Na fluxes in net throughfall under living and plastic canopies was used as a particle deposition index. Using this index, particulate deposition was computed for the real spruce trees in the clearings as well as in the 3 closed stands. Results confirmed that the Ca and Mg flux in net throughfall was mainly supplied by dry deposition whereas K and Mn originated from foliar leaching. The increase in net throughfall fluxes of Ca and Mg with stand age appeared mainly related to an increase in dry deposition.