The role of pollen in forest throughfall biochemistry

Pollen from wind-pollinated forest trees, beside by dry gravitational settling, is washed out from the atmosphere by precipitation, but little is known about the effects of this pollen on throughfall biochemistry. We analysed long-term data from five UNECE ICP Forests intensive monitoring (Level II) plots in Flanders, Belgium, and nearby pollen monitoring stations. Preliminary results indicate a positive relationship between airborne pollen concentrations and both throughfall dissolved organic carbon (DOC) flux in May and biomass of fruiting/seeds in the same year. The complete opposite happens with throughfall nitrate (NO3 -) flux in May that is reduced in mast years, particularly in deciduous trees. Dissolution experiments using commercially available birch (Betula pendula L.) pollen in a 50 mg/L NO3- solution showed that pollen can remove NO3- and release DOC, nitrite, potassium, calcium, magnesium, sulphate and phosphate in throughfall samples. Pollen from wind-pollinated forest trees clearly is an important in-canopy source of nutrients during spring, which should be considered in the quality control of analytical results of for example throughfall measurements and in the calculation of nutrient budgets. The observed effects are further examined in ongoing studies at larger spatial scale using long-term data from ICP Forests Level II plots and pollen monitoring stations in 13 European countries, and throughfall samples and pollen collected in precipitation, from a subset of plots sampled during spring 2018