Scientists often make comparisons as a part of their research. Many research questions ask about the similarities and differences between things. For example, the scientists in this research wanted to compare mangrove forests growing in different countries and conditions.

You are probably familiar with experimentation in science. In experimentation, one variable is exposed to a treatment in one situation. The same variable is not exposed to the treatment in another situation, called the control. Scientists then compare the experimental case with the control.

In nonexperimental research, scientists observe and measure one or more variables in natural settings that differ in one or more ways.
In this research, for example, the scientists compared the same variables in mangrove forests located close to the ocean and away from the ocean, among other conditions.

Pretend that you want to compare the taste of different pizzas. In one comparison, you order pizza with the same list of toppings from two different restaurants. In another comparison, you make two similar pizzas at home, but you add pineapples to one of the pizzas. Otherwise, your homemade pizzas are the same. Which of these two comparisons is experimental? Which is more like the comparisons made in this research? Why?

For a moment, think about a time when you may have yelled loudly at your friend. Maybe you were angry because your friend did not invite you to play a game with them. Despite your feelings, yelling was probably not the best thing to do.

On the other hand, let us say your friend was just about to step in front of an oncoming bicycle. You yelled to warn your friend of danger. In this situation, yelling was the best thing to do.

In this research, the scientists studied something that was helpful for an ecosystem. In other ecosystems, this same thing can do harm. The scientists studied the accumulation of sediment in mangrove forests that grow along ocean coastlines (figure 1).

Sediment consists of organic particles and minerals, such as sand and the remains of plants and animals. It contains solid materials that have been worn down by water and wind. When sediment moves into water bodies, it can be carried to a new location and deposited.

In some ecosystems, such as streams, rivers, or even coral reefs, too much sediment can cause cloudy water and may contain pollutants. In mangrove forests, however, healthy amounts of sediment are necessary to sustain the mangrove ecosystem.

Mangroves are flooded by ocean tides 1-2 times a day depending on where they are located. As the water floods the mangroves, the roots and trunks slow the water down and any sediment that is suspended in the water settles out.

Sediment helps mangrove forests in many ways. Over the past century, sediment has helped mangrove forests to adjust to rising sea levels. When the sea level rises, the surface of the ocean gets higher. As sediment is deposited in mangrove forests, the forest floor also rises in height (figure 2). The mangrove forest can then maintain the height of its forest floor relative to the water height of the ocean.

Mangrove forests are found in tropical coastal areas worldwide (figure 3). Mangrove trees live in the intertidal zone. The intertidal zone is the area between low tide and high tide on or near the coast (figure 4).

Mangrove forests provide many ecosystem services. Mangrove forests reduce the impact of powerful waves generated by storms, such as typhoons and hurricanes. These storms can impact human and natural coastal communities.

Mangrove forests provide places for fish, turtles, crabs, birds, amphibians, and many other animals to breed, raise their young, and live. Even Bengal tigers live in mangrove forests; one of the last preserves of the endangered species is in the Sundarbans. The Sundarbans is the largest continuous mangrove ecosystem in the world and stretches through parts of India and Bangladesh.

Mangrove forests improve water quality by filtering water and removing pollutants. Mangroves also remove massive amounts of carbon dioxide from the atmosphere and store it their trunks, branches, and roots, helping to protect against climate change (figures 5 and 6). Mangroves store more carbon than any other forested ecosystem on Earth!

Figure 5. Mangrove forests have a variety of complex root systems. These roots anchor the trees in flooded locations, provide habitat for many different organisms, filter pollutants out of the water, and store massive amounts of carbon.

USDA Forest Service photos by Rich MacKenzie.

Rising sea level is one of the biggest threats to healthy mangrove ecosystems. Some research has suggested that mangroves can keep up with rising sea level through the accumulation of sediment on the mangrove forest floor and belowground root growth (see figure 2 in Thinking About the Environment above). When mangroves keep up with rising sea levels, the height of their forest floor rises at the same rate as the sea level. When sediments and roots accumulate and add height to the forest floor, it is called vertical accretion (ǝ krē shǝn).

However, as the rate of sea level rise continues to increase over time, scientists wonder if vertical accretion in mangrove forests can keep up with rising sea level. The scientists in this study wanted to compare the accumulation of sediment on the forest floor, vertical accretion, and the amount of belowground carbon accumulation in different mangrove forests (figure 7).

Human activities, such as harvesting too many trees, and natural processes such as hurricanes, can result in deforestation of mangroves. When trees are removed, large amounts of the carbon that are stored within them may be released back to the atmosphere (figure 8). Increasing levels of carbon in the atmosphere is changing our climate. Therefore, protecting ecosystems that store carbon, such as mangrove forests, is important.

Mangrove forests with several different mangrove tree species have more diverse root structures than mangrove forests with only one species. More diverse root structures may contain more carbon and may trap and hold more sediment than less diverse root structures.

Mangroves impacted by bridges, roads, and other human structures may trap and hold less sediment than mangroves that are not impacted. These human-built structures may restrict the flow of water and sediment from rivers or the ocean into mangrove forests.

The rate of vertical accretion may also be impacted by the amount of sediment carried into a mangrove forest by tides or by inland rivers. Therefore, the proximity of a mangrove forest to the ocean may affect the amount of sediment that is deposited on the mangrove forest floor, vertical accretion, and the accumulation of underground carbon
(figure 9).

When mangrove forests are cut down, aboveground and belowground root growth stops. As trees are removed, the carbon is also removed. In addition, water and soil temperatures increase due to more sunlight. Warmer water and soil cause faster decomposition of roots, and the forest floor starts to collapse.

To stop or reverse carbon loss in tropical coastal ecosystems, new mangrove forests must grow. Mangrove forests may grow back naturally on their own over time, or people can purposely plant new mangrove forests. The scientists were interested in the differences between these two kinds of regrown mangroves.

The scientists in this study wanted to identify which mangrove forests were keeping up with sea level rise. To identify these mangroves, the scientists compared mangrove forests growing under the different conditions: mangroves with one species of tree versus many, mangroves impacted by human-built structures and those without, mangroves growing at different distances from the ocean, and mangroves that have regrown naturally or have been replanted.

The scientists studied mangrove forests on Babeldaob (bä bǝl daub) Island in the Republic of Palau (pǝ laů) and in Vietnam (figure 10). The scientists established six mangrove study sites in Palau and two in Vietnam (figure 11).

The scientists studied mangrove forests with different conditions and characteristics (table 1). Unlike the mangrove forests in Palau, the mangrove forests in Vietnam had been harmed between 1961 and 1971 by a chemical used to eliminate forest cover. Much of Vietnam’s southern mangrove forests were destroyed or severely damaged by this chemical use. Therefore, the scientists studied mangrove forests in Vietnam that were either purposely regrown or grew back naturally.

Within each of the study site locations, the scientists established study plots to measure mangrove forest variables, including the density of the trees, the types of tree species, and the amount of wood in the forest (table 2 and figure 12).

Based on the data collected in each study plot, the scientists estimated these values for the whole mangrove forest. The scientists also collected sample cores of the mangrove forest soil from these study plots. After each core was pulled, the scientists cut the core into sections that were either 2 or 4 centimeters long. These sections were labeled and taken to the laboratory for analysis.

These scientists are collecting soil samples in mangrove forests. Look carefully at the picture on the right; you can see the scientists measuring and cutting the soil sample into sections.

The scientists calculated sedimentation rate per year, vertical accretion rate per year, and how much carbon had accumulated in the soil every year (table 3). They calculated this information from the core samples of sediment. Sedimentation rate is calculated from the amount of sediment deposited each year. The vertical accretion rate is calculated from the height of the forest floor each year. These two values (sedimentation rate and vertical accretion rate) are related but are not necessarily the same. Carbon accumulation is the additional amount of carbon stored in the soil over a year’s time.

The scientists then used computer programs to compare the sediment sample core values with the different mangrove tree variables. Recall that these mangrove forest variables included an estimate of the density of the trees, the types of tree species, and the amount of wood in the mangrove forest.

The scientists also compared these values with the different conditions existing in mangrove forests of Palau and Vietnam (see table 1 above).

The scientists then compared the vertical accretion rates with the amount of measured sea level rise in Palau and Vietnam. The amount of sea level rise is measured by tide gauge instruments.

Sedimentation and vertical accretion rates were almost five times higher in mangrove forests of Vietnam than in Palau. Carbon accumulation rates in the soil were greater in mangroves located in Vietnam than in Palau, but the difference was not significant. In scientific research, the word “significant” has a special meaning. A significant finding is one that is not likely due to an error in measurement or to chance. In this research, the measured difference in carbon accumulation between mangroves in Palau and Vietnam may have occurred by chance or by a measurement error.

More diverse mangrove forests and forests with more trees experienced greater rates of sedimentation and vertical accretion (table 4).

Using instruments that measure the altitude, or height, of the ocean surface, an Australian government organization tracks sea level in Palau. This organization reported that the sea level in Palau rose 9 millimeters (mm) per year from 1993 to 2010. Compare this to a tide gauge in Palau. This gauge measured a 1.5 millimeters per year rise from 1969 to 2000. In Vietnam, coastal tide gauges measured a sea level rise of 3.1 millimeters per year.

The scientists converted vertical accretion rates to millimeters per year for Palau and Vietnam. They then compared the reported sea level rise per year with the calculated vertical accretion per year. The scientists found that both mangrove forests in Vietnam are keeping up with sea level rise. If the sea level is rising 9 millimeters per year in Palau, only 1 of the 11 mangroves in Palau is keeping up with sea level rise. If the sea level is rising 1.5 millimeters per year in Palau, all 11 mangroves are keeping up with sea level rise.

Depending on which measure of sea level rise is used for Palau, mangroves in Palau and Vietnam appear to be keeping up with sea level rise. As the rate of sea level rise is expected to increase, however, it is unknown if mangrove forests will continue to keep up in the future.

One of the most important differences between mangrove forests in Vietnam and Palau is the diversity of tree species. Vietnamese mangrove forests were more diverse with 35 tree species as compared to Palau’s 18 tree species. The scientists found that more diverse mangrove forests experienced greater rates of sedimentation and vertical accretion rates. This finding is important to consider as governments have been recently replacing once diverse mangrove forests with mangrove forests of a single tree species.