Pollen influences throughfall deposition of nitrogen in beech forests
INBO investigated the impact of tree pollen on throughfall deposition in forests. Throughfall deposition is the amount of nutrients and pollutants that reaches the forest floor annually via precipitation after passing through the canopy. With this study, we hoped to find an explanation for some regularly recurring anomalies in analysis results during spring. For instance, we saw strikingly low concentrations of nitrate (NO3-) and peaks of nitrite (NO2-) and carbon.
The study used long-term monitoring data from forests across Europe. These forests largely consist of oak, beech, pine and Norway spruce. Five experimental areas in Flanders are part of this ICP Forests Level II monitoring network: Wijnendalebos, Gewestbos Ravels, De Inslag in Brasschaat, Aelmoeseneiebos in Gontrode and Zoniënwoud. The data were analysed in combination with airborne pollen concentrations measured in towns near these forests. Sciensano monitors these concentrations in Flanders as a function of pollen-related allergies.
The link between the two data sets confirmed that pollen is an important source of potassium and organic compounds (carbon and nitrogen) in throughfall water. That input represents 4-10% of the annual throughfall deposition of these elements. Pollen from beech also appears to add ammonium nitrogen to the fall-through water, while, to our great surprise, they remove a limited amount of nitrate from the water. It is currently unclear whether pollen itself removes nitrate or the micro-organisms that naturally occur on the pollen. Exactly how this works, we now want to investigate further. This will be done with a lab experiment in collaboration with UGent.
Depositions of inorganic nitrogen are still above the critical load for forests in many Flemish forests. This research gives us more insight into the processes that help determine deposition under the canopy of forests.
Image above: inflorescences of beech with pollen (photo: Arne Verstraeten)