These mountain rainforests are characterised by a single-layered tree canopy, with epiphytic ferns, bryophytes, lichens, orchids, and bromeliads draping tree branches. Grasses are rare or absent. At high altitude forest structure becomes less complex, with dwarf tree forms. Although rainfall is abundant (up to 6000 mm per year), productivity is limited by cool temperatures, wind exposure and shallow soils, although under the canopy a moist shady microclimate provides stable habitat for a frog, bird, plant and invertebrate species that are found nowhere else.