Nitrogen-fixing lichens (cyanolichens) may be important sources of fixed nitrogen (N) to N-limited forests in the montane tropics and temperate zone, but little is known about the controls of their abundance and diversity. In this dissertation, I investigate the role of P in controlling canopy cyanolichen abundance, diversity, and ecosystem importance in fertilized and unfertilized montane forests in the Hawaiian Islands. My initial study showed that the cyanolichen Pseudocyphellaria crocata responds to ground-level P-fertilization by increasing in abundance and nitrogenase activity in fertilization experiments on young soils on Hawaii and old soils on Kauai, suggesting that P availability strongly influences the ecosystem importance of this one species. To gain an understanding of the effect of enhanced P availability on the entire epiphyte community, I measured epiphyte community composition in 5 fertilizer treatments (control, +N, +NP, +P, and +P and nutrients besides N) in the Kauai fertilization experiment, and found that elevated P availability in the canopy increased both the species richness and abundance of cyanolichens and green algal lichens (chlorolichens) as well as the species richness of mosses. To investigate whether P is important for cyanolichens in unfertilized forests, I measured epiphyte communities and host tree P content across a range of montane forests in Kauai and found that trees hosting abundant cyanolichens had significantly higher P content than trees with low cyanolichen cover, indicating that P availability is indeed important in controlling cyanolichen abundance outside of fertilized forests. Finally, I quantified the ecosystem importance of cyanolichens in P-fertilized forest in the Kauai fertilization experiment by measuring the nitrogenase activity of 14 cyanolichen species and scaling up to a yearly N input to the forest. I estimate that between 6 and 9 kg of N are contributed to the P-fertilized forest per hectare each year by the cyanolichens, a number comparable to all other N inputs to the forest combined. These results suggest that cyanolichen abundance in forests may be determined by forest P status, and that enhanced cyanolichen abundance in P-rich but N-limited forests may eventually bring N supply into alignment with P supply
