A multi-nutrient budget helps to understand masting regulation in beech forests along a fertility gradient

Abstract. Forest ecosystem fertility and tree nutrition strongly depend on efficient biogeochemical nutrient cycles. Mast events require large amounts of carbon and nutrients and may therefore affect cycling efficiency. This study aimed to quantify the impact of episodic mast events on multiple nutrient fluxes under contrasting fertility conditions.

We conducted a multi-nutrient study (K, Mg, P, S and N) over an 12-year period (2012–2023) in three mature European beech (Fagus sylvatica L.) forests along a fertility gradient (S1>S2>S3). Nutrient fluxes associated with aboveground biomass production (wood, leaves and reproductive organs), canopy leaching, and leaf nutrient resorption were quantified. Mast effects were assessed by comparing mast years (MY) and non-mast years (NMY).

Across ecosystems, total aboveground biomass production increased by 19% on average during MY due to reproductive organ formation, while wood and leaf production remained unchanged. MY significantly enhanced nutrient fluxes via litterfall but also via canopy leaching particularly for K (+46%, +9 kg ha⁻¹ yr⁻¹), with impacts on seasonal dynamics. In contrast, leaf nutrient resorption was globally not affected by MY. The total nutrient content in aboveground biomass production (NCB ; sum of all the measured fluxes) increased for all nutrients during MY compared to NMY, with the strongest responses observed for Mg (+53%), K (+44%) and P (+43%), and more moderate effects for S (+24%) and N (+9%). Overall, our results highlight that canopy leaching and resorption, which remain understudied, are essential for accurately estimating nutrient budgets in aboveground biomass production.

A clear fertility effect was observed, with higher fruit production and increasing K, Mg and P requirements from S3 to S1. Despite elevated nutrient demands, MY did not induce nutrient limitation or reduce vegetative growth, indicating that current soil and tree nutrient reserves accross the 3 ecosystems are sufficient to buffer reproductive costs under present climatic conditions. This study highlights the importance of a multi-nutrient perspective and proposes a novel indicator comparing MY nutrient budgets with available nutrient reserves to better assess nutritional constraints associated with masting.