Dominant tree pollen shapes nutrient levels in European forest throughfall

Every spring, Europe’s forests are showered with far more than rain. Wind-pollinated trees release staggering quantities of pollen into the atmosphere, and much of it eventually settles on leaves, branches and the forest floor. New research now shows that this pollen is not merely biological debris — it is a measurable, taxon-specific driver of nutrient fluxes in the water that drips through forest canopies, a flow scientists call throughfall. The findings, published in Environmental Monitoring and Assessment, provide the first continental-scale evidence directly linking pollen deposition to forest water chemistry, and they suggest that decades of forest monitoring may have been quietly misattributing a slice of nutrient input to other sources.

The study was carried out by a large international team led by Elena Gottardini of the Fondazione Edmund Mach in Italy, working within the ICP Forests programme, a pan-European network that monitors air pollution effects on forests. The researchers analysed throughfall samples collected in 2018 from 60 intensive monitoring plots — known as Level II plots — across eight countries: Belgium, Finland, France, Germany, Italy, Norway, Switzerland and the United Kingdom. The plots ranged in altitude from 14 to 1,900 metres above sea level and were dominated by six tree genera: Abies, Fagus, Larix, Picea, Pinus and Quercus. In total, 196 throughfall samples were collected between February and July, covering the early vegetative season when most European trees flower.

The innovation of the study lies in its methodology. Rather than inferring pollen inputs from nearby aerobiological samplers, the team examined the very same water samples that were being chemically analysed. A known quantity of water from each sample was filtered through a 5-micrometre cellulose nitrate mesh, and a calibrated number of 10-micrometre polystyrene-divinylbenzene microspheres — roughly 155,000 per sample — were added as a counting standard. The filters were then dissolved in acetone, centrifuged, and the resulting pellet embedded in fuchsin jelly on microscope slides. Trained analysts identified and counted pollen grains under 400-times magnification, stopping when about 1 percent of the added microspheres had been counted. From these counts, the team calculated pollen deposition rates in grains per square metre per day, correcting for filtered volume, throughfall quantity and sampling duration.

The results were striking. Fifty-three pollen taxa were identified across the samples, with woody species accounting for 98 percent of all grains. Four genera — Pinus, Picea, Fagus and Quercus — together made up 91.4 percent of the total. Pine pollen alone represented more than half of everything collected, a dominance the researchers attribute to the buoyant air sacs on pine grains, which allow them to travel enormous distances from both local and distant sources. In beech, spruce and pine stands, the pollen of the dominant genus itself prevailed, making up 49.7, 59.1 and 84.3 percent of the local pollen spectrum respectively. In oak stands, oak pollen ranked second at 26.9 percent, behind pine. Data from fir and larch stands were excluded because sampling there did not align with those genera’s flowering periods.

By combining the measured pollen curves with published flowering phenology, the team defined main pollen seasons for each genus. Beech and oak flowering spanned April and May, with beech peaking in the last week of April and oak showing a double peak at weeks 19 and 23, likely reflecting multiple oak species flowering at slightly different times across the wide latitudinal and altitudinal spread of plots. Spruce pollen peaked around weeks 19 to 20, while pine peaked at week 19 with a secondary pulse at week 22. Overall, 57 percent of the throughfall samples fell within their stand’s flowering period for at least half of their collection window, providing a robust temporal overlap between pollen deposition and water chemistry measurements.

The chemical analysis, conducted at the Research Institute for Nature and Forest laboratory in Belgium, quantified major ions including potassium, calcium, magnesium, sodium, chloride, phosphate, sulphate, nitrate, nitrite and ammonium using ion chromatography, alongside total dissolved nitrogen and non-purgeable organic carbon measured on a TOC/TN analyser. Linear mixed-effects models then tested whether pollen deposition rates could predict the flux of these compounds, with plot identity included as a random effect and autocorrelation between repeated samples explicitly modelled.

The answer was a clear yes. Every statistically significant association between pollen and throughfall chemistry was positive, meaning more pollen corresponded to more nutrients reaching the forest floor. Potassium emerged as the star player: its flux was significantly explained by pollen from all four main genera, with pollen accounting for an average of 33 percent of the variance. This makes biochemical sense, because potassium is one of the predominant ions in pollen grains, and pollen washed from flowers and crowns by rain or deposited dry is a direct source of the ion during the flowering season — an idea first proposed decades ago but never confirmed at this scale. The strongest single relationships involved oak pollen, which explained 77.8 percent of the variance in ammonium fluxes and 55 percent in total dissolved nitrogen.

Broadleaved and coniferous pollen left different chemical fingerprints. Beech and oak pollen together significantly explained fluxes of non-purgeable organic carbon, dissolved organic nitrogen, total dissolved nitrogen, ammonium and magnesium, while the combined pollen of pine and spruce was linked with every measured compound except phosphate. Intriguingly, broadleaved pollen showed a weak negative association with nitrate fluxes. The authors urge caution in interpreting this pattern, since several non-exclusive mechanisms could be at play. Laboratory experiments have shown that pollen and its associated microbiota can transform extracellular nitrate into nitrite and nitrous oxide, an effect pronounced in beech and oak but weak or absent in pine and spruce. Canopy uptake of nitrogen and coincidentally lower atmospheric deposition during broadleaf flowering are also possible explanations that this single-year study cannot disentangle.

The dissolved organic carbon carried in throughfall also differed by forest type. Specific ultraviolet absorbance at 254 nanometres, a proxy for aromatic and recalcitrant carbon, was highest in spruce stands, whose needles release lignin derivatives and tannins. The humification index was significantly higher in oak throughfall, reflecting more decomposed, humic organic matter. Most revealing was the fluorescence index, which distinguishes microbial from plant-derived organic matter: beech and oak samples had significantly higher values, with medians near 1.9, than pine and spruce samples, which sat near 1.7. Values above 1.9 typically indicate microbially processed carbon, and the elevated readings under broadleaves suggest microbes play a larger role in transforming pollen-derived carbon in those forests — hinting that pollen inputs feed not just trees but entire soil microbial food webs.

For the authors, the implications reach well beyond academic curiosity. Routine throughfall monitoring has historically ignored pollen as a variable, meaning that chemical enrichment previously attributed solely to canopy leaching or atmospheric deposition may in fact be partly pollenic. As climate change pushes flowering earlier and increases airborne pollen amounts across arboreal taxa, these hidden inputs could grow. Masting behaviour in beech and oak — the episodic, synchronized production of enormous seed and pollen crops — introduces strong year-to-year variability that any nutrient budget must now reckon with.

The researchers acknowledge the limitations of a one-year, correlative dataset and call for multi-year studies incorporating canopy architecture, stand age, phenology and insect infestations, as well as stable isotope tracing or controlled exclusion experiments to nail down mechanisms. Still, the message is clear: pollen is not just about reproduction, or even allergies. It is a seasonal fertiliser delivered from the sky, and forests have been banking on it all along. Incorporating pollen surveys into long-term forest observation networks, the team argues, will be essential for accurate nutrient budgets in a changing climate.