Phenology shifts in response to your neighbor: How plant-plant competition and parasitism alter reproductive fitness

Background/Question/Methods

In plants, phenological and reproductive consequences of pairwise plant interactions have rarely been studied relative to fitness consequences of pairwise competitive interactions. Parasitism is a ubiquitous plant-plant species interaction that is often overlooked despite as the fact parasitic plants are keystone species in many systems. Therefore, the effects of plant parasites on host fitness may reveal mechanisms by which parasitic plants structure ecosystems. Here, we hypothesize that biotic interactions such as intraspecific competition, interspecific competition, and parasitism have differing effects on the timing of reproductive events and on fitness outcomes of focal host plants. To test this hypothesis, we conducted an experiment with 284Chamaecrista fasciculata grown as focal individuals in pots alone or with one of three potential antagonists: Chamaecrista fasciculata (intraspecific competitor), Koeleria macrantha (interspecific competitor), and Agalinis tenuifolia (parasite). We collected detailed flowering phenology data, conducted pollinator observations, quantified seed set, and collected above and belowground biomass. We measured 1) flowering phenology, 2) reproductive fitness, and 3) above and belowground biomass in focal plants.

Results/Conclusions

Reproductive phenology and all measures of fitness of focal plants differed among three treatments intraspecific competitor, interspecific competitor, or parasite. Preliminary results, using a combination of linear models, bootstrap resampling approaches, indicate that Chamaecrista fasciculata growing under intraspecific competition had more synchronous first flowering dates than C. fasciculata grown alone; however, there is not much evidence that C. fasciculata grown under any of the antagonism treatments differed in first flowering dates. Some evidence exists that duration of flowering differs among treatments. These results may be interesting because only 33% of C. fasciculata growing under either competition treatment flowered twice within the growing season while approximately 67% of C. fasciculata growing alone or under the parasitism treatment flowered twice within the growing season. Additionally, the antagonism treatment was not a good indicator of seed set; however, mean seed set was highest for plants growing with a parasite and lowest growing with an interspecific competitor. Mean C. fasciculata biomass was highest growing under parasitism and lowest growing under intraspecific competition. These results may show that species interactions can shape reproductive fitness through changes to biomass and phenology.