Scientists usually begin their careers studying a topic that captures their interest. As a scientist continues to study the topic, she or he becomes an expert in that topic. This study, for example, is Dr. Hughes’ seventh research project about an invasive tree species growing in Hawai’i and the Pacific Islands. After years of researching a range of questions about this tree species, Dr. Hughes is known as an expert in this tree species.

Dr. Jane Goodall studied chimpanzees for 60 years and became the world’s foremost expert on chimpanzees. Another scientist who is known as an expert is Dr. Anthony Fauci. Dr. Fauci has been studying infectious diseases since 1968. Not every scientific expert is well known, however, and scientists can become experts at any age. As a scientist digs deeper into their chosen topic, they continue to learn more details about that topic. In what topic would you like to become an expert?

Across the United States, Federal agencies manage lands and waters on behalf of American citizens. In many areas, invasive species have spread onto Federal lands and into Federal waters, causing problems for native ecosystems. An invasive species is any plant, animal, or organism that is not native to the ecosystem it is in and is likely to cause harm to the environment, the economy, or human health.

Invasive species are so harmful to native ecosystems that every Federal land-managing agency has a policy to identify and remove as many invasive species as possible. These agencies also work to restore native ecosystems following invasive species removal.

In this research, the scientists tested the effect of a method that was used to eliminate an invasive, nonnative tree. This tree had invaded the National Park of American Samoa. American Samoa is a group of islands within the Micronesia/Polynesia hotspot. (Learn more about this hotspot in the “About” tab for this article.) American Samoa is a territory of the United States, and its citizens speak both Samoan and English (figure 1).

The National Park of American Samoa is spread across the islands of American Samoa (figure 2). The largest area of national park is on the island of Tutuila (figure 3). Since 2001, forest managers have been working to eliminate a nonnative, invasive tree species from Tutuila. The tree species was introduced to a nearby island in the early 1800s and spread to Tutuila Island in the early 1900s.

Eliminating this invasive tree species from the national park is a priority for park managers. The National Park of American Samoa is located within the Pacific Ocean’s Polynesia/Micronesia biodiversity hotspot. This invasive tree grows quickly and dominates the park’s native ecosystem.

The invasive tree species is Falcataria moluccana (fǝl kǝ te rē ǝ mǝ lǝ kä nǝ) (figure 4). This tree species is known as tamaligi (tǝ mä lǝ jē) in Samoa. Tamaligi is a fast-growing, invasive tree that discourages the growth of native trees and plants and encourages the growth of other invasive plants.

Tamaligi trees are nitrogen fixers. A nitrogen fixing plant or tree forms a mutually beneficial relationship with a bacterium that lives inside the roots of the plant. The bacteria absorb nitrogen from the air and convert it into a form that can be used by the plant for nutrition. In exchange, the plant supplies, or “feeds,” the bacteria with carbon for energy. This kind of relationship is known as a symbiotic relationship. This symbiotic relationship supports the tamaligi tree’s rapid growth and encourages the growth of other invasive plants. Whenever the trees shed their leaves, or when the tree dies and the wood and roots begin to decay, the nitrogen in the tree goes into the soil. There, the nitrogen becomes available to other trees and plants.

Nitrogen is an important element used by plants for photosynthesis. Because tamaligi is a nitrogen fixer, it enriches degraded forest soils. This soil enrichment supports tamaligi growth in all forest soils.

Tamaligi trees had invaded the national park ecosystem. This invasion limited the growth of endemic species and threatened the biodiversity of the national park. Although other invasive tree species are found in the national park, they do not dominate the forests like the fast-growing tamaligi trees.

The scientists in this study were interested in two questions: (1) How are tamaligi trees impacting the native forests in the national park? and (2) How are native forests responding to the removal of tamaligi trees?

Recall that the scientists wanted to understand how the tamaligi trees’ presence and their removal affected the native forests of American Samoa. Forest managers had begun to kill tamaligi trees in 2001 (figure 5). The managers killed tamaligi trees in specific areas during 2001, 2003, 2006, and 2007. This killing of tamaligi trees over a period of years provided a unique opportunity for the scientists to study their questions.

National Park of American Samoa staff had kept careful records and maps of when and where tamaligi trees were killed. The scientists, therefore, could compare native forests with forests recovering from tamaligi invasion over time. To do this, they established five forest study sites on the northwest side of Tutuila Island. Most of the study sites were within the boundary of the national park (figure 6).

The scientists established 5 study plots within each of the 5 study sites, for a total of 25 study plots (figure 7). One of the five study plots within each study site contained forest that had never had any tamaligi trees. The other four study plots within each study site contained the remains of tamaligi trees that had been killed by girdling.

Within each of the 25 study plots, the scientists identified 3 smaller areas, including subplots and quadrats. The scientists then collected information about the trees and plants they found in each plot, subplot, and quadrat (table 1).

The scientists entered the measured diameter at breast height (or DBH) of the trees and vines into mathematical equations, or formulas. These equations were used in a computer program to estimate the amount of biomass, or living matter, contained in each study plot.

The scientists then estimated the density of the trees and plants in each study plot by counting them (figure 8). The scientists also counted the number of different tree species in each study plot. Finally, they collected soil samples to take back to the laboratory where they calculated the amount of inorganic nitrogen available to plants and trees in the soil. Inorganic nitrogen is nitrogen from sources that are not or never were living. Recall that tamaligi trees are nitrogen-fixers. This means that when they were living, they were working with microorganisms to fix and use the nitrogen from the air.

After their research, the scientists knew the following information for each of the 25 study plots:

• Whether or not the plot had contained tamaligi trees,
• What year the tamaligi trees were girdled (and therefore how many years the tamaligi trees had been dead),
• How much living plant material was in each plot,
• Which tree species were found in each plot,
• How many different tree and vine species were in each plot,
• How many trees were growing in each plot
• How much inorganic nitrogen was available in the soil of each plot.

The scientists compared this information to determine what impact the tamaligi trees had on native forests and how native forests responded after the tamaligi trees were killed.

The total biomass was about equal in forest study plots with and without tamaligi trees. However, the amount of living plant material by species in forests with tamaligi trees was different when compared with native forests (table 2).

Right after the tamaligi trees had been killed, biomass was reduced in those study plots. However, new native trees of different species grew rapidly in the plots where tamaligi trees had been killed. After 8 years, the amount of total biomass in the plots in which tamaligi trees had been killed was again about equal to native study plots. Native species accounted for almost all new plant growth (figure 9). Other nonnative species did not expand in the study plots where tamaligi trees had been removed. After 8 years of growth, the variety of native tree species was about equal in all plots.

Immediately following tamaligi removal, inorganic soil nitrogen was three to five times higher in those plots as compared with native forest plots. After 3 years, however, the amount of inorganic nitrogen in the soil was about equal for all study plots.

The girdling and killing of tamaligi trees in the national park was successful. The scientists believe that several factors contributed to the effort’s success.

First, enough funding was made available for national park field crews to girdle and kill over 6,000 mature tamaligi trees. Girdling easily killed tamaligi trees.

Second, the tamaligi removal effort was supported by the local community. This support included the involvement of local citizens on removal field crews. Frequent communication between local villagers, other American Samoan citizens, and the National Park of American Samoa also supported the effort’s success.

Third, native Samoan trees had evolved to grow rapidly following forest clearing caused by Samoa’s frequent cyclones and tropical storms with high winds. When tamaligi trees died, they created forest clearings like the impact of these weather events can do. Native Samoan trees, therefore, rapidly grew into the areas where the tamiligi trees had been, and these native trees reproduced in the cleared areas as they had evolved to do.

Fourth, because tamaligi trees cannot tolerate shade, their seeds and seedlings could not germinate or grow under the shade of rapidly growing native trees.

Fifth, the inorganic nitrogen left in the soil following the death of tamaligi trees provided an important resource that was used by native trees to support their growth for the first 3 years.

Natural conditions, along with human action, created a unique opportunity to control the spread of invasive tamaligi trees and restore the Samoan native forest.