Along estuaries, bays, lagoons and ocean inlets, the trees face unfavorable conditions as they hunch down over the saltwater. Characterized at times by compact vegetation that forms a green fringe, other times by a tangle of roots that serve as arched props to hold the trees sufficiently upright in the unstable mud, mangrove forests are home to a large variety of marine animals, and they help protect the coast from ocean wind and waves. In times of global warming, their ability to absorb and store carbon from the atmosphere has further raised the value of this coastal ecosystem that extends along nearly the entire Brazilian coastline, from the North to the southern reaches of the state of Santa Catarina. These mangrove forests are reacting to a rise in sea level due to climate change, as demonstrated by the group led by oceanographer Mário Soares of Rio de Janeiro State University (UERJ).
About three times each week, a team from Soares’ laboratory, the Center for Mangrove Studies (Nema), goes to the Guaratiba mangrove area 70 kilometers west of the city of Rio de Janeiro. There, along the shores of Sepetiba Bay, the researchers enter the forest and take a series of measurements in an area that has been monitored since 1998, when Soares established a permanent study area, described in 2013 by Gustavo Duque Estrada in the journal Aquatic Botany. The Guaratiba mangrove area is the only one in Brazil that has been monitored in such detail over so long a period. The research, funded by Brazil’s National Council for Scientific and Technological Development (CNPq), the Brazilian Federal Agency for the Support and Evaluation of Graduate Education (Capes), the Rio de Janeiro Research Foundation (Faperj) and other institutions, has been yielding unprecedented findings.
One of the most striking observations from those 16 years is that the forest has been advancing inland onto a flat, desert-like area. It is a hypersaline plain, a mangrove feature also referred to as an apicum or salt marsh, in which the soil is two to four times more saline than that of the mud in the mangrove forest. A few times per month, during the highest tides, the sea invades this area and does not drain off completely. The water evaporates and leaves salt in the soil, making it inhospitable even to mangrove species. Rising sea levels, however, have brought increasingly frequent flooding, and mangroves are gradually settling in. “The forest has advanced nearly 80 meters since 1998,” Soares reports.
In addition to working with their feet buried in mud that reeks of sulfur, the UERJ researchers are also using satellite images to investigate what has happened. “Images captured since 1984 confirm what we are seeing in the field,” Soares says. “The mangrove forest pulses in rhythm with climate cycles.” During wetter periods, rain washes the soil, dilutes the salt and the trees are able to inhabit the salt flats. “They are climate windows of opportunity that enable the forest to advance more rapidly,” explains Estrada, who completed his doctorate in 2013 and is now a professor at UERJ. In his opinion, one indication that the sea level is rising in that area is that at the end of a string of dry years, the mangrove forest stops advancing but does not lose ground; without the tidal flooding, the trees would die. As the forest advances inland, the water erodes the vegetation along the edge. In Guaratiba, however, this erosion has been slower than the advance of the forest, resulting in an increase in the total forest area.
Heading towards the poles
The geographic distribution of the mangroves is also expected to expand as temperatures rise. The trees in this ecosystem do not grow in low-temperature conditions, and hence, more than half of the world’s mangrove forests are found between the 10°N and 10°S latitudes. Since climate forecasts indicate that southern Brazil will become warmer by the end of the century, mangroves will likely come to occupy higher latitudes. That expectation for Brazil also applies to the Northern Hemisphere, as recently observed in the United States by Kyle Cavanaugh of the Smithsonian Institution. In a January 2014 article in the journal PNAS, his group described an expansion of the mangrove area in northern Florida, at about 30°N latitude, and attributed the change to the fact that temperatures below -4° Celsius are increasingly rare in that area. The rise in the minimum temperature, therefore, is more significant than changes in the average.
Transposed to Brazil, this situation would be the equivalent of finding these forests along the coast of the southern state of Rio Grande do Sul near Porto Alegre, but at present the mangroves are far from that point and do not appear to have advanced latitudinally. In a 2012 paper in the journal Estuarine, Coastal and Shelf Science, Soares and his team identified the Santo Antônio Lagoon in the municipality of Laguna, Santa Catarina State, 100 kilometers south of Florianópolis, as the southern limit of the Brazilian mangrove forests. This was the same location noted by Yara Schaeffer-Novelli of the Oceanographic Institute at the University of (IO-USP), based on observations made in 1979.
Celebration over the expansion of the mangrove forest is not just madness on the part of someone who took pleasure in braving mosquitoes in the mud. The research of the Nema group and of groups in other countries shows that this ecosystem has significant capacity to absorb atmospheric carbon. “Since 2003 we have had permanent plots dedicated to analyzing carbon sequestration,” Soares says. His group developed several mathematical models—the first of which he created while working on his PhD, which he defended in 1997—for estimating the amount of carbon stored in the trees of each typical mangrove species. “If anyone wants to do a study specifically on the leaves, we have a model; if someone wants to analyze the trees as a whole, we have one for that too,” he notes. Using these models, one need only take a few basic measurements to get an estimate of the plant’s biomass and how much carbon is stored within.
One as-yet-unpublished study by the UERJ group shows that, looking at just the trunks and leaves of the trees, mangroves have a carbon storage capacity slightly smaller than that of other tropical forests. The total amount is just not significant because the area of the coastal ecosystem is much smaller (slightly more than one million hectares) than that of the Amazon region (approximately 500 times larger) or the Atlantic Forest. “But if we take into account the roots and the sediment, the mangroves win on a per-unit-of-area basis,” Soares notes, citing preliminary data. To obtain these data, the group dug cube-shaped holes in the mud in Guaratiba, removing 10-centimeter layers from each. “It takes a group of 15 people four days to do the job,” he says. The roots are then removed, dried and weighed to estimate their biomass. Cylinders of mud are also collected to measure the amount of carbon stored in the sediment. We will see the results in the next few years.
Ana Paula CamposWhat is learned about the mangroves at Sepetiba Bay can be used to understand what is happening in other regions. “There are analyses that we validated in Guaratiba, and we can apply them to other studies,” the oceanographer explains. Adaptations will be needed because the height of the forests and their carbon storage capacity are growing in the direction of the Equator. The group has now begun to apply these models to mangroves along virtually the entire length of the Brazilian coastline—from Florianópolis in Santa Catarina to São Caetano de Odivelas in the state of Pará. The Nema team travels a great deal, but Soares is also seeking partnerships. One important new development is participation in the National Institute of Science and Technology for Tropical Marine Environments (INCT-AmbTropic), which was established in 2012. Based at the Federal University of Bahia (UFBA), it focuses on studying how the Brazilian coastline responds to climate changes. Soares shares the coordination of the Mangroves Working Group with geologist Marcelo Cohen of the Federal University of Pará (UFPA), which whom he has begun a collaboration.
From past to present
“We’re working together to combine our two scales of approach,” Soares says. Cohen specializes in the study of what has happened to the mangroves in the past 10,000 years—the Holocene geologic period. “In order to be sure of the effect of each variable on the existence of the mangroves, it is essential to understand how they have evolved in the past 100, 1,000 and 10,000 years,” the geologist explains. He is doing a series of studies in the North, the Northeast and the Southeast, in part as a partner with physicist Luiz Carlos Pessenda of USP, who is heading up a project to study the characteristics of the coastline of the state of Espírito Santo in those far-off times.
Near the end of an intense glacial period 20,000 years ago, the sea level was about 100 meters lower than it is today, and it began to rise. “Between 7,000 and 5,000 years ago, the sea level was close to the present-day level, enabling the mangrove forests to start expanding,” Cohen explains. Despite the existence of global patterns, the local scale must be examined as well in order to understand how the specific conditions—involving river flows, sediment dynamics and tectonic movements—influenced the coastal vegetation. In the Amazon Basin, according to Cohen, there was a period of lesser rainfall between approximately 10,000 and 4,000 years ago. During that period, marine influence moved upriver because of the rising sea level and lower outflow from the rivers, and the mangroves moved back. With the increased rainfall over the past 4,000 years, salinity levels in the estuaries have fallen and the mangroves have receded to areas with greater marine influence.
In the Southeast about 5,000 years ago, the sea level exceeded today’s levels and produced a large number of estuaries with conditions suitable for mangrove expansion. When the sea level dropped, deltas consisting predominantly of sandy sediment formed in those areas, making them less favorable to the survival of mangrove forests.
To open windows into these different time scales, Cohen’s group is combining a variety of techniques. Sediment cylinders can reveal pollen from plants that lived thousand of years ago, which enables scientists to reconstruct the environment in which the sediments accumulated and the surrounding vegetation over time. Pessenda’s team at the Center for Nuclear Energy in Agriculture (Cena) at USP in Piracicaba is analyzing isotopes of nitrogen and carbon to characterize the organic matter and estimate the age of the sedimentary deposits.
To put together a picture of what has occurred in recent decades, Cohen is also using remote sensing, which has revealed erosion of mangrove areas and migration of sandbars onto the mud deposits, pushing the forest to higher areas.
Aerial and satellite images are an essential resource for broader assessments of vegetation coverage. Such images were the tool that enabled the American group to detect the northward expansion of the mangroves in Florida. At the Federal University of São Paulo (Unifesp), biologist Marília Lignon is using remote sensing data to monitor the natural impacts and human activities in mangrove forests in different areas. Having looked down, she is not abandoning the view from up close. “Each scale has its particular features, and one can enhance the other,” she comments.
She recently directed the mapping of the transition areas between mangrove and restinga forests in the state of São Paulo. “They easily support human occupation and offer beautiful scenic beauty,” she says. These flat, sandy areas are easily occupied and may function as an escape route for the mangrove forest in the face of climate change. Gustavo Duque Estrada reports that this has occurred in the area of Sepetiba Bay, where Companhia Siderúrgica do Atlântico built a steel plant. “Changes in sea level were not taken into consideration during the planning, and that installation makes the mangrove forest highly vulnerable to a scenario involving a rising sea level.”
Taken collectively, these studies by the Brazilian groups draw attention to the need to consider the importance of transition zones in studies aimed at long-term conservation of mangrove forests and their ability to protect the coastline and the air. According to physicist Joseph Harari of IO-USP, the rise in sea level along the Brazilian coast is near the global average, about three centimeters per decade. It is impossible to make a general prediction about what may occur in the near future as a result of climate changes, which include both local and global factors. But one thing is certain: the mangrove forest will not stand still.
1. Interdisciplinary paleoenvironmental studies in the Espírito Santo State coast (nº 2011/00995-7); Grant mechanism Thematic project – PFPMCG; Principal investigator Luiz Carlos Ruiz Pessenda – Cena/USP; Investment R$1,008,962.77 (FAPESP).
2. Mangroves of São Paulo State: spatial and temporal analysis (1979 – 2009) (nº 2009/05507-0); Grant mechanism Post-doctoral research grant. Grant recipient Marília Cunha Lignon – Unifesp; Investment R$ 138,069.95 (FAPESP).
COHEN, M. C. L. et al. Impact of sea-level and climatic changes on the Amazon coastal wetlands during the late Holocene. Vegetation History and Archaeobotany. V. 18, No. 6, p. 425-39. Nov. 2009.
ESTRADA, G. C. D. et al. Analysis of the structural variability of mangrove forests through the physiographic types approach. Aquatic Botany. V. 111, p. 135-43. Nov. 2013.
FRANÇA, M. C. et al. Mangrove vegetation changes on Holocene Terraces of the Doce River, Southeastern Brazil. Catena. V. 110, No. 1, p. 59-69. Nov. 2013.
SOARES, M. L. G. et al. Southern limit of the Western South Atlantic mangroves: Assessment of the potential effects of global warming from a biogeographical perspective. Estuarine, Coastal and Shelf Science. V. 111, No. 1, p. 44-53. April 2012.