Polylepis trees. Forest of Polylepis sp. trees growing on a mountain pass in Ecuador, at an altitude of 4,000 metres. Typically these trees grow in monospecific (one species) forests which may have been more widespread before the advent of cutting and grazing, which began in pre-Inca times. Growing above the tree line at 3,500 metres Polylepis sp. are among the highest altitude growing trees. The flaky bark may provide thermal insulation or it may be a device for shedding epiphytes when these become burdensome.