Sometimes it is useful for scientists to study something over a long period of time. The Forest Service created 81 experimental forests and ranges with this idea in mind (figure 4). The Forest Service started setting up these experimental areas in 1908. Most of the experimental areas are at least 50 years old. These areas are useful for scientists because they provide scientists with the opportunity to observe and conduct experiments in an area over a long period of time.

When scientists are able to gather information over several years, they can gain a better idea of what is happening in the area. For example, think about a time in school when a student does not give his or her best effort with schoolwork. Then the student has several years in which he or she gives his or her best effort. If the student were given a choice to be evaluated over the short time period when they didn’t give their best effort or the longer time period, the student would want to be judged on the schoolwork done over the longer period of time. The longer time period would provide better information about the student. Similarly, scientists gain a deeper understanding of what is happening when they can review several years of data. Scientists are also able to identify trends in the data this way. In this article, you will learn about an experimental forest located in Puerto Rico (figure 5).

A watershed is the area that drains to a common waterway,
such as a stream, lake, estuary, wetland, aquifer, or even the
ocean (figure 6). Understanding the activities that occur within watersheds and the location of the watershed is important. The activities and location can affect the watershed’s water quality.

In this research, scientists studied two watersheds in Puerto Rico. The watersheds are located in tropical forests. Tropical forests are unique because they are located near the Equator (figure 7).

These forests have a high degree of biodiversity and are characterized by a lack of a winter and the occurrence of only two seasons: a wet season and a dry season. In particular, the drainage areas for the watersheds the scientists studied are located in a tabonuco (tä bə nü kō) forest. This type of forest has tall trees and very little light reaches the forest floor. The tabonuco tree (Dacryodes excelsa) dominates the tabonuco forest. The tabonuco tree can grow up to 100 feet tall (figures 8a and 8b).

The scientists chose to study two of the Bisley Experimental Watersheds in the Luquillo Experimental Forest (figure 9).

Luquillo Experimental Forest shares its boundaries with El Yunque (ū(ng) kā) National Forest. Because the watersheds are located within the Luquillo Experimental Forest, the scientists could observe and record how things changed over a long period of time. For this research, the scientists studied particulate organic matter (POM).

POM consists of small pieces of twigs, leaves, soil, and other items found in the water (figure 10).

POM measurements are important for several reasons. For example, POM is a source of food and energy for many animals. POM keeps some environmental pollutants out of the water by binding heavy metals and pesticides. POM also serves other roles in an ecosystem such as being an important part of the food web. POM is a part of the decomposition process of the food web (figure 11).

In this research, the scientists examined the amount and types of a particular type of POM called coarse particulate organic matter (CPOM). CPOM is the pieces of organic material that falls into or is carried into the river. CPOM is generally greater than 1 millimeter (mm) in size. Examples of CPOM are leaves, sticks, and other pieces of plant material. Because CPOM is an important component of an ecosystem and food webs, the scientists wanted to figure out how the amount and quality of CPOM changed over a period of time.

The scientists collected data from the two watersheds from 1987 to 2005. The scientists collected mean annual rainfall, size of watershed area, stream channel length, as well as dates of hurricanes and other natural disturbances, such
as drought (figure 12).

The scientists wanted to know how much CPOM was being exported from the watersheds. After CPOM is in the water, it undergoes some amount of decomposition. The amount that actually leaves the watershed is the amount that is exported. From 1987 to 2005, scientists collected CPOM that was trapped in metal mesh traps that they placed in the streams.

The scientists collected CPOM every other Tuesday and following large storms. The metal mesh traps were placed across the stream channels (figure 13).

Scientists tested several mesh traps and sizes from 1987 to 1989. These test traps broke during periods of high water flow. Since 1989, however, the traps the scientists designed did not break. These traps recorded accurate measurements of CPOM by collecting all the CPOM going into the trap.

The scientists also completed chemical analysis on some of the samples to see what kinds of elements were in the CPOM. Elements are important to both living and nonliving things. For example, approximately 96 percent of the human body is made up of four elements: carbon, oxygen, hydrogen, and nitrogen. In this study, the scientists measured elements such as carbon, nitrogen, phosphorus, potassium, and calcium in the CPOM.

During the 18-year study period, the scientists found several things. The highest CPOM export through the watershed occurred before and during Hurricane Hugo. The scientists found that other tropical storms and hurricanes after Hurricane Hugo did not create as much CPOM export as during the time of Hurricane Hugo.

Relatively high exports of CPOM were recorded from 1995 to 2000 (figure 15). The greatest export of leaves (which are part of CPOM) occurred during April, May, and September. These months have the most intense rain showers and the greatest number of plants with falling leaves. During this time, the leaves are mostly falling because of storms, wind, and rain, and they are still green when they fall. Therefore, these leaves have different nutrients and different chemical composition than leaves that fall in the dry season from natural aging.

Low CPOM export occurred during 1990–1991, 1993–1994, and 2000–2002. The lowest amounts of CPOM export were during times of drought. The months of December, January, and February also show low CPOM exports.

Watershed 1 had higher values of potassium than watershed 2. Watershed 2 had higher levels of calcium, magnesium, aluminum, and iron. The scientists found no difference between the two watersheds for levels of nitrogen, phosphorus, and manganese. The scientists thought that the types of leaves and the amount of decay that the leaves had undergone may have affected the elements they found in the watersheds.

Hurricane Hugo (figure 16), which made landfall in September 1989, created some of the highest stormtide heights that had ever been recorded on the east coast of the United States. Hugo was a category 5 hurricane in the Atlantic Ocean and then became a category 4 storm that went through the Caribbean. Hurricane Hugo had maximum sustained winds of 135 to 140 miles per hour. During that time, Hugo was the strongest storm to hit the United States within a 20-year period, and it created approximately $7 billion ($7,000,000,000) in damage (figure 17). To learn more, visit http://www.weather.gov/chs/HurricaneHugo-Sep1989.

The Saffir-Simpson Hurricane Wind Scale is a 1-to-5 rating based on a hurricane’s sustained wind speed (figure 18). The National Oceanic and Atmospheric Administration explains that “this scale estimates potential property damage. Hurricanes reaching Category 3 and higher are considered major hurricanes because of their potential for significant loss of life and damage. Category 1 and 2 storms are still dangerous, however, and require preventative measures.”

The export of CPOM generally followed a seasonal pattern that was a combination of the monthly rainfall pattern and seasonal patterns for leaves falling from plants and trees. The scientists found that one of the greatest events of CPOM export was during Hurricane Hugo. The scientists also found that tropical storms and hurricanes after Hurricane Hugo did not create as much CPOM as during the time of Hurricane Hugo.

The scientists believe that this finding has to do with the fact that, during Hurricane Hugo, the forest was more mature. A more mature forest means that it is an older forest with larger trees and more growth. After Hurricane Hugo came through, parts of the forest were destroyed and fewer large trees remained. When tropical storms or other hurricanes came through the area after Hurricane Hugo, less organic material was available to fall and export into the streams.
Therefore, the frequency and intensity of hurricanes and tropical storms, along with the age of the forest, had a measurable effect on the export of CPOM through the
watersheds.