The work done by Michał Bogdziewicz and Rafał Zwolak (both from Department of Systematic Zoology, Institute of Environmental Biology) in cooperation with Elizabeth E. Crone (Tufts University) and Michael A. Steele (Wilkes University): on the effects of chronic nitrogen fertilization on plant reproduction has been accepted in Journal of Ecology.
Anthropogenic global changes such as global warming and nitrogen deposition are often reported to cause increased seed production in plants. Yet, can we assume that these increases actually translate into better plant recruitment? Results of our most recent study caution that this does not have to be the case. We used a long-term (>25 years) nitrogen addition experiment in Harvard Forest, USA, to measure how anthropogenic nitrogen affects seed crop of red oaks and their interactions with the most important seed predators (weevils) and dispersers (rodents). We also assessed the influence of nitrogen on the probability of acorn germination. We found that nitrogen deposition greatly increases the production of acorns: it was up to 9-times higher in nitrogen addition than in control plots! Simultaneously, however, nitrogen increased seed predation by weevils, most likely because high seed production allowed for the built-up of insect population. Furthermore, nitrogen addition resulted in considerably lower probability of acorn germination and negatively affected acorn dispersal by rodents. All these negative effects trumped the positive influence of nitrogen on seed production. These findings highlight the importance of measuring multiple aspects of plant reproductive biology when assessing its response to anthropogenic changes.