ITES PROFESSOR

Nick Brokaw

Director of LTER Nick Brokaw measures the height of a Cecropia schereberiana tree, a species that grows quicly and plays an important role in the recovery of the forest after hurricanes. Most LTER research takes place near El Verde in the Luquillo Mountains and at Bisely near the Forest Service Sabana Field Station. In recent years LTER scientists have expanded their research to many other sites in Puerto Rico.

Despite two hurricanes, the Luquillo rain forest hasn’t changed much since Luquillo Long-Term Ecological Research Program (LTER) scientists began studying it in 1988. “Nineteen years is not much in the life of a forest,” says Nick Brokaw, Ph.D., director of Puerto Rico’s National Science Foundation funded Luquillo LTER program and ITES professor, noting that the Puerto Rico rain forest has existed and gradually changed for millions of years. What has changed dramatically are the Luquillo LTER program’s methods, objectives, and topics of research. They’re adapting to evolving ecological paradigms and the rapidly changing environment of Puerto Rico.

The Luquillo LTER, originally named the Luquillo Experimental Forest LTER, started out studying the ecology of the Luquillo Mountains, including animal ecology, plant ecology, and ecosystem ecology, or energy flow and nutrient cycling. “Specifically, we were looking at natural disturbance. We had a lot of studies already up and running in 1988, when in 1989 Hurricane Hugo came along and damaged the forest, so we were able to see the before and after picture, which was priceless,” says Brokaw. Hugo defoliated, delimbed, and uprooted many trees, and about 15 percent of large trees were killed. But within five years, most populations had returned to or were approaching pre-hurricane levels.

“We also started studying landslides and floods, and now we’re looking more at human disturbance. We’re looking at human impacts on the managed, built-up areas around the forest, and in our most recent phase we’re taking in the whole transect from San Juan to the Luquillo Mountains. We’ve broadened quite a bit and will begin including relevant social science, the idea being that humans are the biggest factor in changing the environment, so you better understand what motivates humans to understand why they have the effect they do on the environment.”

Historically, Puerto Rico has undergone dramatic upheavals in land use change. From the time the Spaniards arrived until around 1950 the island was steadily deforested until only five percent remained covered in forest. Then with the advent of industrialization and large migration to urban areas, secondary forests recovered about 30-35 percent of the island. “Now it seems to be leveling off because of suburbanization. This has effects on local climate. Urban heat extends to the suburbs, and this may mean less rainfall, locally. In terms of global climate change, some climate models predict that hurricanes will be stronger and perhaps more frequent. And there’s some debate about what it will do to rainfall, whether it will be dryer or wetter here.”

In response to climate model predictions, the Luquillo LTER Program launched a Canopy Trimming Experiment. Begun in 2005, the 20-year project will study the effects of increased frequency of hurricanes. The experiment mimics the two major ecological results of a hurricane by opening up the canopy and depositing debris on the forest floor. “The opening of the canopy radically changes the microclimate below because it allows more sunlight and heat ,and the deposition of debris also has an effect on the microclimate of the soil nutrient cycling. The idea is to look at the effects on microclimates, insects, invertebrates, frogs, lizards, soil moisture, soil-water nutrient concentrations, seedlings, growth, and changes in the plant community. If we can understand the impacts, we can predict what’s going to happen in the future and try to adapt.”

One of the objectives of the Luquillo LTER Program is to gather and analyze environmental data in northeastern Puerto Rico to predict changes, so that people can either manage the changes or be prepared to adapt to them. “Even if the amount of rainfall doesn’t change, it is predicted that it’s going to become more erratic with longer periods of drought and bigger floods. We’re doing some studies on how increased variability of rainfall can affect, not just water flows, but water quality.

“One thing we know for sure is that there will be change soon. The two main causes of environmental change in northeastern Puerto Rico are land use change and climate change, both local and global. The only way you can measure environmental change is to do something long term, to factor out the noise. And the only way you can understand why is to do some long-term studies and in some cases, experiments.”

Long-Term Ecological Research: the Luquillo Forest Dynamics Plot

One particular 16 hectare rectangle of El Yunque rain forest, the Luquillo Forest Dynamics Plot (LFDP), has been singled out for thorough scrutiny since 1990 because scientists have many questions about the rain forest—for example, how can so many species live together, and how does the forest respond to hurricane or human disturbance?

To help find the answers, every woody-stemmed plant in the LDFP one centimeter in diameter or more is numbered, identified, measured, and mapped. It takes scientists, technicians, and about a dozen volunteers more than a year to census the plot, and this census is repeated every five years. The data is shared with the Center for Tropical Forest Science (CTFS), a network of 18 similar long-term plot studies across the tropics. In the CTFS plots, more than three million tropical trees and 6,000 tree species are studied.

LFDP Director Jill Thompson, Ph.D. trains the technicians and student volunteers, oversees their work in the forest, and manages the information they gather. “To give you an idea of the volume of information gathered in this relatively small plot, up to 10 data items can be recorded for each stem. In the 2005 census, we looked for around 80,000 tree, shrub, and palm stems that were alive in the last census, to see what had happened to them.”

What have CTFS scientists found so far? First of all, each CTFS plot is different, with different types and numbers of species. The LFDP is unique because of its history of intensive land use and hurricane damage, which influences community dynamics and species composition. In 1998, scientists at LFDP were able to build up a picture of the forest before Hurricane Hugo hit in 1989, based on an assessment of damaged trees and data from the first census. By 1995 when the second census started, the number of large trees of most species had recovered from the hurricane damage. However, Cecropia schreberiana, a pioneer species that needs a lot of light, had about eight times more trees after Hurricane Hugo as a result of the canopy damage that let in light. The second census revealed the resistance and resilience of the forest.

CTFS researchers and LFPD scientists contributed to an article recently published in Science that showed that at local scales, some factors must promote forest diversity. “In ten CTFS forest plots if you follow a group of trees in a small patch of forest over time, as the trees grow older the number of plants of common species goes down and the biodiversity in that patch goes up,” says Thompson.

“Now we’re using species information and data from LFDP to show how the tree populations change over time after human and hurricane disturbance, and to develop a model of the forest that will help us predict what might happen in the future as the climate changes. We hope to be able to predict what might happen to the forest if we have more frequent and intense hurricanes in Puerto Rico.”

nvbrokaw@uprrp.edu
jthompson@uprrp.edu

back