Fruit production is a strong sink for plant resources that competes with other life-history functions, such as vegetative growth. This is referred to as the ``costs of reproduction''. In masting species, it is generally assumed that the production of a large crop of seeds depletes stores of resources and that these take more than one year to replenish; this is accepted, theoretically, as the proximate mechanism of mast seeding (resource budget model). However, direct evidence of resource depletion in masting trees is very rare. In this seminar, I will present our research on carbon and nitrogen sources for seed production using stable isotopic labeling. In addition, I will assess reproduction-related reduction in resource storage at individual level by monitoring seasonal and inter- annual variations in carbon and nitrogen concentrations in various tissues after a full masting event.