The role of terrestrial soil nutrient supply in determining the composition and productivity of epiphyte communities has been little investigated. In a montane Hawaiian rainforest, we documented dramatic increases in the abundance and species richness of canopy epiphytes in a forest that had been fertilized annually with phosphorus (P) for 15 years; there was no response in forest that had been fertilized with nitrogen (N) or other nutrients. The response of N-fixing lichens to P fertilization was particularly strong, although mosses and non-N-fixing lichens also increased in abundance and diversity. We show that enhancement of canopy P availability is the most likely factor driving the bloom in epiphytes. These results provide strong evidence that terrestrial soil fertility may structure epiphyte communities, and in particular that the abundance of N-fixing lichens--a functionally important epiphyte group--may be particularly sensitive to ecosystem P availability.

Nitrogen‐fixing epiphytes (especially lichens with a cyanobacterial symbiont—cyanolichens) have the potential to contribute significant amounts of nitrogen (N) to montane tropical forests, which are typically low in N—but the factors controlling the abundance and distribution of epiphytic cyanolichens are poorly understood. In long‐term fertilization experiments in montane forests on a 4.l million‐yr‐old Oxisol on the island of Kauà'i and on a 152‐yr‐old lava flow on the island of Hawai'i, the epiphytic cyanolichen Pseudocyphellaria crocata increased significantly in abundance in canopies of host trees fertilized with phosphorus (P). There was no response to fertilization with N or other essential elements. Nitrogen‐fixation rates were also elevated in lichens in P‐fertilized plots at both sites. Phosphorus supply to host trees may be an important factor controlling N inputs to montane tropical forests by N‐fixing epiphytes.

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Abstract:Low phosphorus (P) supply frequently has been shown to limit the abundance and activity of nitrogen (N)-fixing organisms, potentially constraining N inputs to ecosystems. Previous research in a montane Hawaiian forest has shown that ground-level P-fertilization led to significant increases in the population size of epiphytic N-fixing lichens (cyanolichens), as well as a shift in community composition from crustose to leafy species. In this study, we ask whether these changes in the cyanolichen community have resulted in increased N inputs to the forest, and also whether the very high levels of P in the canopy of P-fertilized forest stimulate individual lichen fixation rates over those of lichens from a nearby unfertilized reference forest. We used acetylene reduction (AR) assays to measure the fixation rates of 14 cyanolichen species from P-fertilized forest, and calibrated these rates by measuring15N2fixation incorporation in four species. We found that the ratio of acetylene reduced to N fixed ranged from 2.4 ± 0.4 inPseudocyphellaria crocatato 9.3 ± 2.4 inLeptogium denticulatum. Nitrogen fixation rates in the P-fertilized forest ranged from 0.64 ± 0.05 nmol N cm−2h−1inNephroma helveticumto 3.97 ± 1.48 nmol N cm−2h−1inParmeliella nigrocincta. Fixation rates did not vary greatly among species from P-fertilized forest. We compared these P-fertilized rates to those of 10 species from the reference forest, and found that mass-based fixation rates of P-fertilized lichens were not greater than those of lichens from the unfertilized forest. Using the measured AR rates, we estimate that the P additions increase cyanolichen N inputs to the forest 30-fold, from ~0.3 kg N ha−1y−1to ~9 kg N ha−1y−1. These results suggest that P additions to this ecosystem increase N inputs primarily by increasing the abundance of cyanolichens, and that shifts in cyanolichen community composition and changes in individual fixation rate were of lesser importance in determining ecosystem N inputs.

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