Establishing a link between pollen dispersal, seed production and throughfall dissolved organic carbon (DOC) flux in temperate forests

Throughfall dissolved organic carbon (DOC) concentrations and fluxes exhibit strong seasonality, with one or even two peaks, in spring and autumn. Several factors are believed to cause the spring peak (in May), including leaching from opening buds and unfolding leaves, pollen dispersal and the activity of phytophagous insects. However, it is difficult to separate the effects of these factors acting simultaneously. In order to shed light on the underlying sources of DOC fluxes we did an exploratory analysis of long-term data (1999‒2017) on biomass of litterfall fractions and throughfall DOC flux in May in five Level II plots in Flanders, i.e. two coniferous (Pinus sylvestris L., Pinus nigra ssp. laricio var. Corsicana Loud.) and three deciduous (two Fagus sylvatica L. and one mixed Fagus sylvatica L. - Quercus robur L.) forests. The total biomass of fruiting/seeds derived from litterfall traps was positively related to throughfall DOC flux in May only in the deciduous plots, with high DOC peaks observed particularly in mast years. In a next stage, we will further explore this relationship at a larger spatial scale, including also airborne pollen concentrations from sampling locations in several European countries including Flanders, Italy and the UK, to test the hypothesis that pollen dispersal is the main driver of the spring peak in throughfall DOC both in coniferous and deciduous (Norway spruce, Silver fir, Scots pine, oak and beech) forests. Successful mast fruiting in coniferous and deciduous stands likely requires preceding abundant pollen dispersal, triggering a peak in throughfall DOC in May. This carbon (C) input should be considered as canopy generated, rather than as C input with atmospheric deposition when calculating C balances for forest stands. In coniferous forests it was more difficult to demonstrate this relationship due to more constant release of pollen.