Luquillo Experimental Forest LTER Site

Network News Fall 1988, Vol. 4 No. 1
Top Stories

Periodic visits by hurricanes and tropical depressions in the Caribbean and their interaction with steep topography offer an opportunity to study long-term responses of complex ecosystems to natural disturbances. Researchers from the University of Puerto Rico, the Institute of Tropical Forestry, and 15 other institutions are conducting long-term studies of the relationship between disturbance regime and forest structure in the newly-designated Luquillo (loo-key-yo) Experimental Forest LTER site in Puerto Rico. The objectives of these studies are:

  1. To investigate the relative importance of different types of disturbance within the four life zones constituting the landscape of the Luquillo Experimental Forest (LEF)
  2. To analyze the importance of the biota in restoring ecosystem productivity after different types of disturbance within representative watersheds in one of these life zones

The LEF, encompassing elevations between 100-1070 m in the Luquillo Mountains in eastern Puerto Rico, is a National Forest and a Biosphere Reserve and contains several research natural areas within its 11,231 hectares.

Four life zones occur in the LEF:

  • Subtropical wet forest
  • Subtropical rain forest
  • Lower montane wet forest
  • Lower montane rain forest

Four major vegetation types occupy these life zones. The dominant tree below 600 m is the tabonuco (Dacrvodes excelsp), which is best developed on protected, well- drained ridges. Above the average cloud condensation level (600 m), palo colorado (Cyrilla racemiflora) is the dominant tree except in areas of steep slope and poorly drained soils, where the palm Prestoep montana occurs in nearly pure stands. The dwarf forest occupies ridge lines and is composed of dense stands of short, small diameter trees and shrubs which are almost continually exposed to winds and clouds. Both the palm and dwarf forests are dominated by only a few plant species. Tree height, number of species, basal area, average tree dbh, and complexity index decrease with elevation and stem density increases.

The history of disturbance in the LEF is well known through measurements of long-term observation plots established by the U.S. Forest Service in the 1940s, although ecosystem responses have not been well studied for each type of disturbance. The major forms of disturbance in this humid tropical forest are natural treefalls, landslides, hurricanes, and selective cutting. The integral role of disturbance in the LEF has been emphasized by computer simulation of actual relative abundances of tree species, using a forest dynamics model that incorporated treefalls and hurricanes.

Components of the LTER program include examination of:

  1. Pattern, frequency, and intensity of disturbance in the LEF (e.g., treefalls, landslides, and hurricanes)
  2. Environmental properties that are expected to vary with disturbance size, age, and origin (e.g., light, nutrient availability, moisture, temperature, and soil organic matter)
  3. Biological properties that are expected to vary with environmental properties (e.g., species composition, growth, nutrient dynamics, reproductive success, carbon fixation, and food web structure)
  4. System properties that emerge from the effects of disturbance pattern and frequency on the mutual interaction of abiotic environment and biota (e.g., nutrient cycling, phases of recovery, rates of recovery, and displacement from and return to steady state)

Tropical forestry research in Puerto Rico has already produced observations that suggest certain patterns of biotic response to both large- and small-scale phenomena. For example, the estimated age of large colorado trees (Cyrilla racemiflora) and palms (Prestoea montana) can be related to hurricanes in 1867 and 1932, respectively. Studies show that frequently measured forest parameters such as biomass, tree density, number of tree species, basal area, wood volume, wood density, above ground primary productivity, and complexity index will change in predictable patterns over periods of 40 years following a hurricane.

Variation in annual rainfall that is unrelated to hurricane events can also elicit biotic responses since periods with more frequent and higher rainfall increase the probability of landslides and can result in a doubling of annual stream runoff. Because hydrologic fluxes are critically important to many ecosystem processes, infrequent but large fluctuations in rainfall will have significant effects on ecosystem functions such as organic matter export, nutrient cycling, and productivity. Long-term records are necessary to measure and understand such infrequent events. Co-principal investigators for the Luquillo Experimental Forest LTER are Bob Waide (Center for Energy and Environment Research, University of Puerto Rico) and Ariel Lugo (Institute of Tropical Forestry).

Further information on the program or on accommodations at the El Verde Field Station can be obtained from:
Robert B. Waide, CEER, GPO Box 3682, San Juan, PR 00936.