Classic plant nutrient competition work has focused on below-ground dynamics, either examining root structure and function or partitioning of soil resources. Most epiphytic plants do not conform to this theoretical framework because many have evolved elaborate nutrient acquisition mechanisms in above-ground organs as well as in roots. Some epiphytic nutrient acquisition mechanisms can be considered competition avoidance strategies since many of these morphologies avoid interactions with other plants. For example, atmospheric bromeliads have absorptive peltate trichomes covering their foliar surface which obtain nutrients from wet and dry deposition, precipitation and throughfall while their roots function as holdfasts. This approach makes the plant independent of the phorophyte, avoids root competition and exploits a nutrient resource directly that many other plants cannot. Elaboration of leaves and stems for nutrient capture aboveground should increase the diversity of nutrient sources that the epiphytic plant community can use (e.g., throughfall, canopy soil organic matter) and may ultimately facilitate and maintain the high morphological and taxonomic diversity of canopy communities. We investigated if various functional groups not only partition nutrient sources but also N form by measuring N preference using excised root uptake methods in over 200 individuals from the ferns, orchids and bromeliads. We then related uptake rates of ammonium, nitrate and amino acids to the nutrient availability of source nutrients. Because nutrient availability in nutrient sources varies throughout the year, we also fertilized individual species for one month and remeasured their nutrient uptake rates.
Results/Conclusions
We found that there was variation in preference for N species with bromeliads preferentially using amino acids while ferns and orchids preferentially used ammonium. All epiphyte groups had minimal nitrate uptake reflecting the low nitrate concentrations in nutrient sources. These data indicate that epiphytes are partitioning N species and sources thus reducing competition and likely facilitating the high species richness found in the rainforest canopy. When individuals were fertilized for one month, we found that the uptake rates of ammonium, nitrate and amino acids for many species increased significantly indicating that epiphytes are able to respond to temporal changes in nutrient availability.