Combined measurements of microscopic leaf wetness and dry-deposited inorganic compounds in a spruce forest

Abstract

Hygroscopic particulate salts on leaf surfaces may facilitate the formation of microscopic leaf wetness, wax degradation, and enhanced of trace gas exchange in forest environments. In order to investigate the interaction of microscopic leaf wetness and dry-deposited inorganic compounds, a field campaign that combined measurements using electrical conductance sensors and leaf washing techniques was carried out in a coniferous forest in Germany during the summer period. Within the canopy, electrical conductivity was directly and continuously measured across a needle surface exposed to the air and covered with branches. Inorganic ion concentrations of washed samples at the top and middle of the canopy were measured and used to estimate leaf-level dry deposition rates of inorganic compounds. The results of both electric conductance and washed ion concentrations indicated the presence of deposited salts on needles within the canopy. Sulfate (SO42−) and nitrate ions (NO3−) were dominant among soluble mass compounds in washed samples. A higher deposition rate of oxidized nitrogen (measured as NO3−) was estimated at the top of the canopy (0.59 μmol m−2 h−1) than in the middle (0.15 μmol m−2 h−1), indicating that dry deposition of oxidized nitrogen mainly occurs near the canopy top. The deliquescent behavior of deposited hygroscopic salts was suggested by a hysteresis effect depending on increasing or decreasing relative humidity in our leaf wetness measurements. The results indicate that deposition of atmospheric inorganic compounds on needles and leaves can facilitate the formation of leaf wetness on the vegetation surface.