Changes in crown leaching composition induced by a sudden increase in atmospheric deposition. A case study in South-Western Bulgaria

Abstract

Rain and throughfall chemistry has been monitored for five years (1987–1991) in three adjacent stands (Norway spruce, Silver fir and Scots pine) situated at an altitude of 1500 m above sea level in the Rhodopes mountains (South-Western Bulgaria). Throughfall collectors have been set up near the stem, below the opening between the crowns, and halfway between the stem and the border of the tree crown. Pollutant concentration in bulk precipitation strongly increased during the period of investigation. Because base cation deposition increased together with sulfate deposition, the increase in proton deposition remained moderate. The increase in throughfall concentration was parallel for different tree species. The time course of mineral concentration in throughfall was approximately similar to that of rainfall but the inter-event variability in concentration was reduced. Net throughfall fluxes of Ca, Mg and K increased during the study period whereas net throughfall fluxes of Cl, Na and S remained more stable. Although the wet deposition of protons increased, the proton input in net throughfall decreased or remained stable in relation to the increase in base cation concentration. This suggests a low dry deposition in the study region and conversely an increased net leaching of base cations. However, the seasonal variations in net throughfall as well as the time course of the difference between the concentrations of throughfall samples collected at different distances from the stems indicates that most of the increase in the net throughfall of Ca and Mg was due to dry deposition. The difficulties associated with the use of ions such as Na as deposition tracers for other base cations is emphasized.