In forests where they are abundant, epiphytic lichens and bryophytes can greatly modify nutrient cycles, canopy microclimates, and habitats of other arboreal organisms. Nitrogen fixation by cyanolichens is the most obvious contribution of epiphytes to forest nutrient cycles. Most of the N fixed by cyanolichens is used for thallus growth and reproduction (Ahmadjian 1993). This N is ultimately released to the ecosystem via litterfall and decomposition of thalli (Rhoades 1983; Guzman et al. 1990; McCune and Daly 1994; Esseen and Renhorn 1998b). Some of the newly fixed inorganic nitrogenous compounds (e.g., NH4+) in cyanolichens are leached by rainfall (Millbank 1982). The enriched throughfall solutions are a source of N for other organisms, including epiphytic chlorolichens and bryophytes (Pike 1978). Bacteria living on their thalli also quickly absorb inorganic N leached from cyanolichens and are, in turn, consumed by arthropods (Carroll 1980). Thus, cyanolichens subsidize food webs in the forest canopy. The total N contribution of epiphytic cyanolichens depends on their stand-level biomass and local patterns of temperature and precipitation (Antoine 2004). In some forests, N fixed by cyanolichens represents a major ecosystem-level input (Sollins et al. 1980). For example, some old-growth Pseudotsuga-Tsuga forests support over three tons of cyanolichens per hectare ( Neitlich 1993), and these epiphytes may contribute over 16 kg of N per hectare annually ( Antoine 2004). Such high N inputs may actually boost ecosystem productivity in these forests, but critical experiments testing this hypothesis have yet to be conducted.