No seed size–number trade-off in European beech: climate governs investment per seed

Mast-seeding trees can vary seed output by orders of magnitude among years, but it remains unclear whether high seed production comes at the cost of reduced per-seed investment, as predicted by fixed-budget allocation models. We quantified individual seed production with seed mass in European beech across 2,792 trees and 123 populations spanning the species’ European range and quantified seed protein and lipid content in a subset of 35 populations. Seed mass showed a positive association with seed production, with seeds from high-seeding years being 14% heavier than those from low-seeding years, providing no evidence for a seed size–number trade-off and instead supporting variable reproductive allocation. In contrast, seed protein content decreased by 31% with increasing seed production, whereas lipid content increased (by 14%), indicating that nitrogen becomes constraining at high reproductive output while carbon-based provisioning is maintained. Climate further structured provisioning: seed mass and protein content were the lowest at climatic range margins, being 28% and 32% lower, respectively, than at the center of the climatic range. Together, these results show that European beech can increase seed output without reducing per-seed biomass, but that nitrogen limitation emerges under high seed production and that climatic constraints reduce both seed size and protein content toward range edges. This identifies a pathway that may strengthen regeneration bottlenecks at both trailing and leading margins, especially as climate warming intensifies.