Effects of nitrogen deposition on reproduction in a masting tree: benefits of higher seed production are trumped by negative biotic interactions

Relatively little is known about the effects of anthropogenic environmental changes on reproductive ecology of trees. Yet, recruitment is a primary determinant of the long-term dynamics of plant populations in changing environments. We used the Long-Term Ecological Research site at Harvard Forest to evaluate the effects of chronic (over 25years) nitrogen fertilization on reproductive ecology of red oaks (Quercus rubra). Oaks growing in fertilized plots produced 4-9 times more acorns than control trees. However, nitrogen deposition simultaneously affected oaks' biotic interactions. It increased pre-dispersal seed predation by insects (primarily weevils, Curculio spp.) on fertilized plots, most likely as the result of the disruption of predator satiation. In addition, infestation by weevils was more likely to result in embryo destruction in fertilized than in control acorns. Furthermore, the proportion of acorns dispersed and cached by rodents decreased on fertilized plots. Finally, germination of fertilized acorns was lower than control acorns, even after controlling for the effects of weevils and rodents. Inclusion of the altered biotic interactions reversed the final picture of the effects of long-term nitrogen fertilization on oak reproduction: the positive effects on acorn quantity were trumped by the nitrogen-mediated changes in biotic interactions. Synthesis. Our results stress the importance of considering indirect effects and consumer interactions when evaluating the effects of environmental change on plant population dynamics. Long-term nitrogen fertilization has a strong potential to decrease the recruitment of masting trees. Given the ubiquitous increase in the anthropogenic nitrogen deposition, processes similar to those found in our system might operate in others, resulting in a widespread alteration in trees' recruitment dynamics.