Geology, Soils and Climate; how they affect the El Yunque National Forest’s Ecosystems.
By Alan Mowbray
El Yunque National Forest
Puerto Rico is part of a large volcanic island complex that occurs between the junction of the American and Caribbean tectonic plates. These islands were shaped during the formation of the Caribbean plate and the separation of North and South America.
The rocks underlying the Luquillo Mountains were formed by active volcanoes at or near sea level during the Cretaceous and Tertiary geological periods. Debris from these volcanoes was deposited in deep water after being moved by submarine landslides and currents. Later tectonic activity uplifted this debris and it became the dominant structure of the Luquillo mountains. The Luquillo Mountains are composed mainly of igneous rock formed in the Cretaceous period, with some intrusive materials from the Tertiary.
Luquillo mountain soils were formed in residual volcanic ash, and are moderately rich in nutrients. Little stable organic matter accumulates, due to rapid decomposition, except in local areas at upper elevations, where decomposition is inhibited by water saturation of the soil. Except for phosphorous, nutrients in the soil are typically greater than those found above ground.
Most soils in the upper Luquillo Mountains are typically a red or yellowish color, due to their high concentrations of iron and aluminum oxides and hydroxides. In addition they often contain quartz and kaolin, and small amounts of other minerals and organic matter. Soil profiles vary as a function of topography. Steep slopes, cliffs and stream beds may have less soil depth than flatter areas.
Puerto Rico lies in the path of the trade winds that constantly move eastward across the Atlantic Ocean and the Caribbean Sea after they form off the coast of Africa. Because they lie at the northeastern end of the island, the Luquillo Mountains are the first land mass that the moisture laden trade winds encounter. As a result, these mountains are the wettest area on the island. Wonderful map of annual rainfall in Puerto Rico. Average rainfall ranges from 96 inches at lower elevations to over 175 inches at higher elevations. Maximum rainfall occurs from May to November, although intense rainfall may occur at any time during the year. These intense rains can measure as much as 2.5 inches (6.35 centimeters) in one hour.
The forest’s average temperature ranges from 80°F at lower elevations to 64°F at the higher elevations. El Yunque is typically 10° F cooler than nearby coastal areas. The forest’s maximum and minimum temperatures are 90º F and 52° F .
El Yunque’s elevations range from 575 feet (175 meters) at the forest’s northern boundary to 3,533 feet (1,076 meters) at El Toro, the forest’s highest peak. The terrain ranges from gentle slopes in the lower elevations to steep, almost vertical rock-faced cliffs at higher elevations.
What does all of this mean? Out of necessity, some of the trees, plants and shrubs of the forest have adapted to the varying geological, topographic and climatic conditions of the forest’s four vegetation zones.
As an example, the Mountain Sierra Palm (Prestoea montana); grows exposed “buttress roots” to stabilize the tree by helping it cling to steep cliff edges, unstable soils and river banks occurring in the forest’s 5,000 acre Sierra Palm zone found above 1,400 feet elevation. Other trees, plants and shrubs (and some animals) have adapted in special ways to accommodate the forest’s differing conditions.
When you next visit El Yunque, and view its vast biological diversity while you are hiking, discover for yourself how some plants and animals have had to adapt to accommodate their environment or habitat! . . . . . . abridged version of an article written by Alan Mowbray.
The Dwarf or 'Cloud' Forest is a unique example of adaptation to extreme conditions of wind and rain.