Effects of Prey Availability on Sarracenia Physiology at Harvard Forest 2005

Allometric relationships exist between maximal mass-based net photosynthetic rates, leaf mass per unit area, and foliar Nitrogen (N) and Phosphorus (P) content, which hold across a diverse spectrum of over 2500 plant species worldwide. Carnivorous plants depart from this spectrum because they dedicate substantial leaf area to capturing prey, from which they derive N and P under very nutrient-limiting situations. We conducted a manipulative feeding experiment to test whether morphological and physiological allometric relationships of carnivorous plants when nutrients are not limiting are more similar to allometric relationships of non-carnivorous plants. We examined the effects of prey availability on photosynthetic rate (Amass), chlorophyll fluorescence, growth, architecture, and foliar nutrient and chlorophyll content of ten pitcher plant (Sarracenia) species. We tested the hypothesis that increased prey availability would stimulate Amass of one or more leaves, increase photosynthetic N- and P-use efficiencies (PNUEN, PNUEP), increase relative biomass allocation to photosynthetically efficient, non-predatory phyllodes rather than pitchers, increase overall plant biomass, and reduce stress-related chlorophyll fluorescence. This is the first multi-species, controlled feeding experiment using realistic prey treatments, measuring these physiological parameters directly, and elucidating mechanisms of nutrient stoichiometry and allometry in carnivorous plants. Increased prey availability increased chlorophyll content, Amass and photosystem efficiency (the Fv/Fm ratio) across the 10 Sarracenia species. These increases were most evident in younger leaves, as older leaves rapidly translocated nutrients to newer, growing tissues. Better-fed plants produced a significantly higher proportion of phyllodes than controls. Higher prey availability was associated with lower N:P ratios, and a shift from P- to N-limitation. PNUEP was significantly enhanced by supplementary feeding, whilst PNUEN was not. Overall biomass and root:shoot ratios were unaffected by feedings. Feeding shifted allometric relationships of P relative to Amass, N, and LMA from outside the third bivariate quartile to within the 50th bivariate percentile of the relationships found for non-carnivorous plants; other allometric relationships were unaffected. Carnivorous plants can plastically shift phosphorus allometry when nutrients are plentiful, but are less flexible in terms of nitrogen and other ecophysiological parameters.