Why don’t carnivorous pitcher plants occur in nutrient-rich soils? The relative importance of competition and hypoxia intolerance
The carnivorous syndrome in plants has long been hypothesized to be an adaptation for thriving in nutrient-poor soils and more recently as an adaptation for obtaining nutrients in wetland soils. Two alternative hypotheses have been proposed to explain why carnivorous plants are largely absent from habitats with nutrient-rich soils. The competitive inferiority hypothesis states that carnivory puts plants at a competitive disadvantage to non-carnivorous plants in nutrient-rich soils. The hypoxia intolerance hypothesis states that avoiding substrate hypoxia vis-à-vis producing shallow roots and obtaining nutrients from prey above ground in oligotrophic wetlands is inadequate in eutrophic wetlands with highly reduced soils. To test these two alternative hypotheses, we excavated the carnivorous pitcher plant Sarracenia alata from its native bog habitat and planted it into pots containing nutrient-poor bog soil or nutrient-rich marsh soil. We then planted the pots into either bog or marsh (the latter of which was dominated by taller and more productive non-carnivorous vegetation) and removed neighbors from half of the plots to reduce competition. We assessed the relative growth rate of pitcher plants non-destructively over a growing season and took soil redox potential measurements to determine if marsh soil was more hypoxic than was bog soil.
Our results provide no support for the competitive inferiority hypothesis and partial support for the hypoxia intolerance hypothesis. Neighbor removal had no effect on pitcher plant growth at either site or within either soil type (F = 0.02, p = 0.903). Relative growth rate of pitcher plant transplants was lowest at the marsh site (within both soil types) and was highest in the pine savanna soil and site, resulting in a significant site x soil type interaction (F = 13.58, p < 0.001). Soil redox potential was significantly lower at the marsh site than at the bog site (F = 107.88, p < 0.001), suggesting that hypoxia tolerance by pitcher plants was not adequate at the marsh site. Additional measurements of soil redox potential, nutrient availability, and other soil variables currently await analysis. Once completed, we will use multiple regression to determine if low soil redox potential is more important than nutrient toxicity and other variables in reducing growth to a greater extent in marsh soil than in bog soil. The experimental results presented here, combined with the forthcoming regression results, could force a re-examination of widely held assumptions about the benefits and costs of botanical carnivory.