Separated from the southeastern Sierra de Cayey subrange by the Caguas Valley, it is concentrated from west to east in the municipalities of Rio Grande, Luquillo, and Naguabo in the northeastern region of the island.

[3] Easterly winds off the Atlantic Ocean rise and cool as they pass over the mountains, and the ensuing heavy precipitation brings an annual rainfall of 5,000 mm (200 in) on the ridge.

[7] Common trees of the Sierra de Luquillo include Cyathea arborea, Prestoea montana, Cecropia peltata, and Ocotea species while Weinmannia pinnata, Brunellia comocladifolia, and Podocarpus coriaceus are found in the cloud forests of the highest peaks.

[9] A research study found that between 1936 and 1988, there were 46 landslides associated with heavy rain in the upper regions of the mountains, and these created gaps that allowed the seeds of pioneering tree species to germinate and ferns such as Dicranopteris pectitnata to proliferate.

[10] The forest recovers more quickly after a natural disaster, such as a tropical cyclone, than it does after human activities such as logging, charcoal burning, coffee production or agriculture.