Canopy modification of atmospheric deposition in oligotrophic Quercus pyrenaica forests of an unpolluted region (Central-Western Spain)

The aim of this study was to examine bulk precipitation, throughfall, and stemflow in order to determine the processes involved in changes in solution chemistry throughout the canopy (wash-off of dry-deposited solutes and canopy leaching), and the amount of atmospheric deposition in an oligotrophic, deciduous forest ecosystem of Quercus pyrenaica. The work was conducted over three years in the 'Sierra de Gata' mountains (Central Western Spain), a remote area with a low degree of anthropogenic pollution. Four permanent plots were selected following a rainfall gradient (annual mean precipitation ranges from 720 to 1580 mm yr-1). The results confirm the absence of acid or polluted deposition from anthropogenic sources at these sites, with a weakly acid precipitation (pH 5.4 - 5.6), which has a low level of solutes, specially of SO42- and NO3-. Throughfall was slightly less acid than or equally acid to the incident rain; however, stemflow was more acidic than rain, possibly because of its enrichment organic acids by leaching. Throughfall and stemflow were specially enriched in Mn, K, and P, and to a lesser extent, in Mg and dissolved organic C (DOC) from tree organs. Cation enrichment in solutions flowing through forest canopies came mainly from canopy exchange (CE), especially Mn2+, K+, and Mg2+. By contrast, inorganic anions (Cl-, NO3-, and SO42-) came from atmospheric sources (wash-off), although there was a slight CE of chloride. Part of the ammonium and nitrate from precipitation and dry deposition were absorbed by the canopy, resulting in a negative net balance. CE reflected the limited-nutrient status of the forests studied, with low CE rates and appreciable canopy absorption rates. The low acidity of the atmospheric deposition also contributed to the low CE. Depositions from bulk precipitation was higher than dry deposition (DD), as is typical of remote areas, but DD also represented an important input of nutrients to these forests, with over 30 % of the total atmospheric deposition, on average. Overall, both kind of depositions could substantially contribute to the annual nutrient demand for canopy growth.