Previous research in phosphorus-fertilized Hawaiian montane forests has demonstrated that cyanolichens respond to elevated canopy phosphorus (P) availability by increasing in abundance and diversity; a similar but more muted response is seen for chlorolichens and mosses. In this study, I ask whether P also controls cyanolichen abundance in five unfertilized Hawaiian forests, four on the island of Kauai and one on Hawaii. In three of the four Kauai sites, trees which hosted abundant cyanolichens had significantly higher foliar P content than trees with sparse cyanolichen growth. In the Hawaii site, both foliar P and bark P content were significantly higher on high-cyanolichen cover trees (0.074% vs. 0.053% for leaves, p < 0.001, and 0.085% vs. 0.035% for bark, p < 0.001). Chlorolichen and bryophyte abundance were best predicted by cyanolichen abundance at three out of four of the Kauai sites. These results suggest that host tree P content may be an important factor controlling the abundance of cyanolichens in unfertilized Hawaiian forests, and that cyanolichens may facilitate colonization by chlorolichens and mosses.