Throughfall/StemflowThroughfall/StemflowPrecipitation that passes through a vegetation canopy reaches the soil surface as either canopy throughfall or stemflow. As the water penetrates the canopy and flows down the stems, nutrients may be leached from or taken up by the plant tissue. Thus, nutrient concentrations may either increase or decrease relative to the amount input by atmospheric deposition (e.g., wetfall and dryfall). The difference in nutrient flux between total deposition and stemflow plus throughfall is termed net canopy exchange:
Net canopy exchange = Total deposition (wet+dry) - (Throughfall + Stemflow)
In general, nitrate and ammonium are strongly retained while chloride, potassium, calcium and magnesium are released from the canopy. Canopy processes such as cation exchange of base cations for protons (H+) often have a strong pH buffering effect that partially consumes acidity contained in acidic deposition. Here is an example of net canopy exchange for two ecosystems with contrasting fluxes of atmospheric deposition.
