Significance: Physiological decline in large, old trees could involve diminished reproductive effort, but evidence is lacking. A global analysis finds that fecundity decline in large trees is pervasive, consistent with physiological decline, thus providing new evidence for tree senescence. For the 20% of species not showing fecundity declines, this lack of evidence was linked not to specific species groups, but rather to lack of large trees in the data. Large trees remain important for their structural attributes, but results highlight the critical role of intermediate trees for their contribution to reproduction.

The relationships that control seed production in trees are fundamental to understanding the evolution of forest species and their capacity to recover from increasing losses to drought, fire, and harvest. A synthesis of fecundity data from 714 species worldwide allowed us to examine hypotheses that are central to quantifying reproduction, a foundation for assessing fitness in forest trees. Four major findings emerged. First, seed production is not constrained by a strict trade-off between seed size and numbers. Instead, seed numbers vary over ten orders of magnitude, with species that invest in large seeds producing more seeds than expected from the 1:1 trade-off. Second, gymnosperms have lower seed production than angiosperms, potentially due to their extra investments in protective woody cones. Third, nutrient-demanding species, indicated by high foliar phosphorus concentrations, have low seed production. Finally, sensitivity of individual species to soil fertility varies widely, limiting the response of community seed production to fertility gradients. In combination, these findings can inform models of forest response that need to incorporate reproductive potential.