The Luquillo (LUQ) Long-Term Ecological Research (LTER) program is named for a mountain range in Puerto Rico, which has been a tropical island territory of the United States since 1898.  These mountains sprawl across the northeastern region of Puerto Rico, an hour’s drive from the capital city of San Juan. In the 1870s, the Spanish colonial government set aside land on the upper slopes, making this one of the first forest reserves in the hemisphere. After Puerto Rico became part of the U.S., President Theodore Roosevelt established the land as a U.S. National Forest in 1903. Though small at 11,330 hectares (28,000 acres), the mountainous Luquillo area bears two official names.  El Yunque National Forest (formerly called the Caribbean National forest) is named for the tallest peak and refers to the site itself, and it is the only tropical forest in the National Forest system. Luquillo Experimental Forest (LEF) refers to the forest’s research component, the setting for LUQ. Unlike any other U.S. national forest, El Yunque’s research area encompasses the entire national forest.

From the north coast, the Luquillo Mountains rise over 1075 meters (3,525 feet) in a series of peaks cloaked in multi-hued and multi-textured greenery. Over 240 tree species inhabit the forest, ranging from majestic trees over 37 meters (120 feet) tall to gnarled shrub-like specimens. Interspersed among the trees are hundreds of types of ferns, orchids, mosses, bromeliads and lianas as well as an abundance of birds, frogs, lizards, termites and earthworms. Nine major rivers form their headwaters in these mountains. Visitors hike along the best trails on the island and cool off in the numerous streams, waterfalls, and pools. Scientists gravitate to research plots, some of which date back to the early 1900s.

The LUQ site’s lush diversity depends in large part on the forest’s climate and hydrology with warm temperatures and abundant rainfall. Mean monthly temperatures at the lowest elevations range from about 23.5º C (74.3º F) in January to 27º C (80.6º F) in September, and at the highest elevations from 17º C (62.6º F) to 20º C (68º F). Prevailing winds coming off the ocean from the east drop rain as they rise over the mountains. Thus, rainfall increases with elevation, ranging from about 3530 mm (130 inches) per year at low elevations to 4850 mm (191 inches) higher up. February through April are the drier months, but monthly rainfall is variable. The mountains’ geology  is characterized by steep slopes consisting primarily of igneous rock formed during the Cretaceous Period (145 to 66 mya) , with some intrusive materials from the later Tertiary (66 to 2.6 mya). Soils are deep, weathered, and moderately rich in nutrients. Due to rapid decomposition, little humus accumulates except in local areas at upper elevations, where decomposition seems to be inhibited by waterlogging.  Streams  in the mountains cut steep channels between sloping ridges and are filled with many boulders and series of pools and riffles. The dynamics of the streams’ ecosystems are strongly influenced by physical factors, such as the rapid change in elevation from headwaters to coastal plains, the intensity and frequency of rainfall, and the high temporal variability of stream discharge. 

Though the mountains look uniformly green from a distance, there are actually distinct ecosystems within the forest.  Scientists organize them using two major classification systems. The Holdridge Life Zones System delineates on the basis of temperature and precipitation. Four main life zones predominate in the Luquillo Mountains. Subtropical wet and subtropical rain forests are found at low and mid elevations, with lower montane rain and lower montane wet forests at higher altitudes. There is also an area of subtropical moist forest at low elevations on the southwest slope. The forests themselves are classified into four types, based on the predominant tree species. They are tabonuco forest, palo colorado forest, palm forest, and elfin woodland forest. Ascending the Luquillo Mountains through these forest types, the average tree height and diameter, number of tree species, and basal area (the cross-sectional area of tree stems) tend to decrease while stem density increases. Epiphytes become more common with elevation.

The make-up of the community of animals found in the Luquillo Mountains is greatly influenced by the mountains’ location on a small tropical island isolated from continents. The only native mammals are bats. All other mammals (cats, rats, and mongooses) are introduced. Birds include several species endemic to Puerto Rico, such as the Puerto Rican Tody, an understory insectivore, and the Puerto Rican Parrot, an endangered species. In part because of the absence of large animals, lizards and tree frogs flourish at all elevations. The distinctive characteristics of the local animal community in turn affect the structure of its food web.

The ecology of the Luquillo Mountains is not static. Over the centuries, it has been altered by the natural disturbance regime, such as hurricanes, landslides, treefalls, droughts, and floods. Human disturbances, which mainly occurred in the past, include road construction, charcoal production, clearing for pasture and crops, creation of coffee plantations, and logging. The forest dynamic over time has changed the patterns of living organisms found in different parts of the landscape in many ways.

For additional information about the setting for LUQ, see:

Brokaw, N. V. L., A. T. Crowl, A. E. Lugo, W. H. McDowell, F. N. Scatena, R. B. Waide, and M. R. Willig. 2012. A Caribbean Forest Tapestry: The Multidimensional Nature of Disturbance and Response. Oxford University Press, New York, New York.

González, G., M. R. Willig, and R. B. Waide. 2013. Ecological Gradient Analyses in a Tropical Landscape.  Ecological Bulletins 54:1-252.

Harris, N.L., A. E. Lugo, S. Brown, and T. Heartsill-Scalley (Eds). 2012. Luquillo Experimental Forest: Research history and opportunities. EFR-1. Washington, DC: U.S. Department of Agriculture. 152 p.

Odum, H. T., and R. F. Pigeon (eds.). 1970. A Tropical Rain Forest: A Study of Irradiation and Ecology at El Verde, Puerto Rico. National Technical Information Service, Springfield, Virginia.

Reagan, D. P., and R. B. Waide (eds.). 1996. The Food Web of a Tropical Rain Forest. University of Chicago Press, Chicago, Illinois.

Robinson, Kathryn.  1997. Where Dwarfs Reign: a tropical rain forest in Puerto Rico.  University of Puerto Rico Press, San Juan, Puerto Rico.