Forest allocation of net primary productivity (NPP) to reproduction is poorly quantified globally, despite its critical role in forest regeneration and a well-supported trade-off with allocation to growth. Here, we present the first global synthesis of a biometric proxy for forest reproductive allocation (RA) across environmental and stand age gradients. We find that ecosystem-scale RA increases ~ 60% from boreal to tropical forests. Nonlinear relationships with climate metrics are important, explaining nearly 14% of variation, but are not the sole predictors of RA. The influence of forest age is comparable to climate in terms of effect size, and metrics of soil fertility also showed small but significant relationships with RA. These results provide strong evidence that RA at the ecosystem scale is mediated by climate, forest age, and soil conditions, and is not a globally fixed fraction of positive NPP, as assumed by most vegetation and ecosystem models.