The effects of light availability, prey capture, and their interaction on pitcher plant morphology

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Michael Segala, Department of Biology, Texas Christian University, Fort Worth, TX and John D. Horner, Department of Biology, TCU, Fort Worth, TX

Background/Question/Methods
Carnivorous plants inhabit nutrient-poor environments and supplement nutrient acquisition by capturing and digesting insect prey. Carnivorous adaptations have been hypothesized to be beneficial only in environments with high water and light availability. We hypothesized that plant morphology would change in response to resource availability, exhibiting traits that increase carnivory when light is abundant and exhibiting traits that increase photosynthesis when light is limited. In a field manipulation in Leon County, Texas, we examined the effects of feeding, shading, and their interaction on the morphology of the pitcher plant, Sarracenia alata. We employed a two-factor, cross-classified design, with shading (two levels, shaded and unshaded) and prey capture (two levels, fed and unfed) as factors. Eighty plants were haphazardly assigned to one of four treatments: (1) unshaded and fed (control); (2) shaded and fed; (3) unshaded and unfed; and (4) shaded and unfed.
Results/Conclusions
When light availability was reduced, plants produced pitchers that had smaller diameters, which is reflective of a photosynthetic morphology. Unfed plants exhibited reduced growth (produced fewer pitchers and had lower sum of pitcher heights). There was a significant interaction effect on estimated seasonal aboveground biomass: shading had no effect on the mass of unfed plants, but shading reduced the mass of fed plants. As the season progressed, competing vegetation reduced light availability to all pitchers. Plants in all treatments began to produce pitchers that were blade-like with a small, non-functional opening and a widened keel. This morphology would maximize light capture at the expense of prey capture. This experiment provides support for a theoretical model that suggests that carnivorous traits are only beneficial under conditions of high light availability. It also emphasizes the importance of periodic burns of carnivorous plant bogs to remove vegetation, thereby reducing light competition.