Nitrogen economy of the purple pitcher plant, Sarracenia purpurea L., in the Gulf of Mexico coastal plain
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Description
This work addresses a focal question in ecological strategy theory, investigating the importance of carnivory to plant nutrient economy under differing conditions of substrate nutrient availability. Cost/benefit models predict that allocation to carnivory will increase as substrate nutrients decrease. I investigated allocation patterns of Sarracenia purpurea and relative contributions of nitrogen sources to the plant's nutrient budget, in two Gulf of Mexico coastal wet pine savannas differing in soil N availability Soil net N mineralization, measured by buried bag incubations, and inorganic N in precipitation were significantly greater at a site in Mississippi than at a similar site in Florida, as were N-fixation rates in leaf (pitcher) cavities, estimated by acetylene reduction. The potential seasonal contribution of N-fixation to the N budget of S. purpurea was estimated to equal 4--5% of the N content of mature plants. Ages of plants in this study were not known, but because S. purpurea is a long-lived perennial, cumulative contribution of associative N-fixation during a plant's lifetime could be substantial Indicative of increased allocation to prey-capture, mature pitchers at the more N limited FL site had relatively smaller keels and larger mouths, and FL plants had proportionally larger root systems, but higher tissue C:N ratios, indicative of greater N limitation relative to MS plants. Contrary to expectations, MS plants acquired a significantly greater proportion of N from prey (33--51%) than FL plants (22--37%), estimated in situ using 15N natural abundance data in a mixing model. MS pitchers, reflecting relative relief from N constraints, had larger keels and smaller mouths, indicative of an allocation shift toward light-capture. Pitcher morphology of experimentally shaded plants reflected a dramatic allocation shift toward light-capture. I suggest that changes in the light environment interact with differences in soil (and prey) nutrient availability to shape plastic responses by S. purpurea and perhaps other carnivorous plants at both the level of individual leaf morphology and whole-plant level biomass allocation patterns Results of this work indicate that S. purpurea exhibits a striking degree of plasticity facilitating its tolerance of low nutrient environments, and that carnivory supplies a substantial proportion of its nitrogen nutrition