Michał Bogdziewicz is a PhD student at Adam Mickiewicz University in Poznań, supervised by Rafał Zwolak (AMU, as well) and Elizabeth Crone (Tufts University, USA). Michal’s work involves the study of different aspects of mast seeding consequences on ecosystems: the connection between masting and Lyme disease and the effects of masting on animal populations. The main goal of his PhD is to evaluate how nitrogen affects the reproduction of mast seeding trees. The effects of nitrogen addition on different aspects of oak reproductive ecology were analysed in his recently published paper in Journal of Ecology titled ‘Effects of nitrogen deposition on reproduction in a masting tree: benefits of higher seed production are trumped by negative biotic interactions’.
Numerous studies show that anthropogenic global changes (e.g. global warming, nitrogen deposition) are driving increases in plant seed production. However, it is not clear whether this increase translates into higher plant recruitment. To test this, we measured how nitrogen addition affects the seed crop of red oaks. We had this unique opportunity because of the long-term nitrogen supplementation experiment at Harvard Forest, USA, which has been going for more than 25 years. We also measured how N-addition affects the interactions of oaks with their most important seed predators (weevils, Curculio sp.) and seed dispersers (rodents) in Eastern USA, and assessed the germination potential of acorns produced by N-fertilized oaks.
We found that nitrogen fertilization increases the production of acorns by oaks up to 9-fold. Simultaneously, however, it increased seed predation by weevils, most likely because increased seed production allowed the buildup of insect population. This is particularly interesting as it may mean that global changes that drive increase in seed production in masting plants also disrupt predator satiation. Many researchers who have studied patterns of seed production in masting plants in changing environments warn that the direct positive effects of global changes might disappear if biotic interactions of plants are also altered. In particular, increased seed production in non-mast years might disrupt the regulation of seed predators. Our results provide evidence that this might be the case. Furthermore, weevil-infested acorns produced by N-fertilized oaks were more likely to be killed (i.e. have they embryo depredated by larvae). Thus, chronic nitrogen addition disrupts both predator satiation mechanisms at the population level, and at the seed level. But that is not the end of the bad news for oaks. Nitrogen deposition decreased seed caching by rodents and lowered the probability of acorn germination. These two effects seem to follow from the altered quality of acorns.
These combined effects trumped the positive, nitrogen-mediated effect of increased seed production on oak reproduction. Our results stress the importance of considering biotic interactions when evaluating the effects of global changes on plant communities. Nitrogen deposition in the environment is ubiquitous and accelerating. Thus, similar effects as found in our model system might operate in others, resulting in alternations of tree communities.
Adam Mickiewicz University in Poznań