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Ecology

An unexpected forest

Richer in terms of its biodiversity, the Atlantic Rainforest is poorer than the Amazon Region when it comes to nitrogen

EDUARDO CESARNorthern coast of the state of São Paulo: a 35 million year-old forestEDUARDO CESAR

Biologists and agronomists have concluded that the Atlantic Rainforest – at least that part of it in the state of São Paulo’s northern coast – must have a different, maybe unique and currently unknown way of obtaining, using and releasing nutrients to grow and sustain themselves. The soil of the forests that cover the hillsides of Ubatuba and São Luís do Paraitinga are even shallower and poorer than those of the Amazon region in terms of nitrogen, which is an essential nutrient for plants, as are water and light. There is no way at present to explain how a forest so poor in nitrogen can be more exuberant than that of the Amazon region in terms of its variety of plant and animal species.

This study, begun in 2003, has shown that we still know very little about the forest that is closest to the country’s largest cities, by contrast with the Amazon region, which began to be examined at least four centuries ago by European naturalists. “The fact that we know more about the Amazon forest than we do about the Atlantic Rainforest, which is much closer to us, is disturbing,” notes Luiz Antonio Martinelli, a researcher at the University of São Paulo (USP) and one of the coordinators of the group consisting of soil, plant and environmental experts, in an effort to build a broad and integrated picture of Brazil’s Atlantic Rainforest.

Botanists were the first people to notice that nitrogen was not as abundant in the Atlantic rainforest as it is the Amazon region. As the basis for the whole study, almost 15 students and research assistants explored the 14 areas to be studied – each of which measures one hectare (10,000 sq.m) of forest with the most preserved vegetation possible at three ranges of altitude (from 5 to 50 meters, from 50 to 500 meters, and from 500 to 1,200 meters above sea level) in the towns of Ubatuba and São Luís do Paraitinga. Under the coordination of Simone Vieira, an agricultural engineer at USP, and Luciana Alves, a biologist at the Institute of Botany, they were given the task of locating and marking with small metallic plates every single tree that was at least 4.8 centimeters in diameter, even those that were still growing. This totaled 28,000 trees.

The botanists found that the representatives of the botanical family of legumes such as jatobá, Brazilian ironwood and jacaranda were not as abundant there as they are in the Amazon region. The trees of the legume family are important for the whole forest because, more than other species, they draw nitrogen from the atmosphere via the association between the roots of the plants and groups of bacteria of the Rhizobium genus. Initially they use the nitrogen for their own benefit and afterwards they distribute it to other plants, when the leaves fall and nitrogen spreads through the soil and into the rivers.

While the botanists examined the plants, Luiz Felippe Salemi and Juliano Daniel Groppo collected rain water. Then they examined it in Piracicaba using equipment at the Center for Nuclear Energy in Agriculture (Cena), under Martinelli’s guidance, and discovered that there was very little nitrogen in the rain water and river water, in the soil and in the leaves of the trees. “We thought that the equipment was faulty,” says Martinelli. Little by little they came to the conclusion that the Atlantic Rainforest must work in a totally different way from how they had imagined, as it has perhaps half the nitrogen level found in the Amazon region – which has a level considered low – although more than in the European woodlands, which are very modest in terms of their biodiversity relative to the forests of the Americas.

Martinelli does not yet know if the lack of nitrogen is one of the reasons why the Atlantic Rainforest is one of the world’s oldest, having been around for about 35 million years, or whether the Atlantic Rainforest has lived so long because it has always had a little nitrogen. He believes that comparisons with a very different natural environment, such as the Cerrado (savanna), whose plants have adapted to the shortage of nutrients and water, could help explain why the coastal tropical forest shelters such biological wealth, in terms of its diversity of animal and plant species; if we look at epiphytes such as orchids and bromeliads, the Atlantic Rainforest has three times as many per square meter as the Amazon rainforest, which is much richer in nitrogen.

Trees in a hurry
At least now the contrasts are clear, and not just between Brazil’s largest forests but also within the Atlantic rainforest. Low altitude forests grow and live – and operate – differently from those at higher altitudes, as if they were totally different organisms. The soil of the forests that are 5 to 50 meters above sea level are shallow (they do not exceed 30 centimeters) and are even poorer in nutrients than those found on land at altitudes of between 800 and 1,200 meters. According to the results obtained to date, in the low altitude forests the plants seem to be in a hurry and absorb nitrogen directly from the leaves that drop onto the soil – without waiting for this valuable nutrient to mix in with the earth and form the thick layer of organic soil material found in high altitudes forests, where the nitrogen circulates more slowly. In the forests closest to the beach, the amount of rainfall equals half of that falling on mountain top forests, which are enshrouded in fog for at least 200 days a year. At the highest altitudes the trees are thicker-bodied and denser than in the low altitude forests. “You can no longer say with confidence that the Atlantic Rainforest works one way or the other,” declares Joly.

Simone Vieira and Luciana Alves also estimated the biomass – the quantity of carbon stored mainly in the Atlantic Rainforest’s trees, palm trees and ferns. They collected information regarding the diameter, height and density of the wood for almost 30 thousand trees and came to the conclusion that the biomass in the Atlantic Rainforest’s vegetation may vary from 80 tons of carbon per hectare in the forests closest to the sea to 120 tons per hectare in the forests on the hillsides and mountain tops. “This is a far greater amount of carbon than we had been expecting,” declares Simone. This biomass suggests that the Atlantic Rainforest has a high capacity for storing organic carbon, although the mechanisms whereby this is achieved remain a mystery, because the quantity of nitrogen that the forest receives should not allow the trees to grow to such a girth.

The biomass estimate indicated that each hectare of deforested Atlantic forest implies an emission of at least 100 tons of carbon, which is similar to the minimum emission range found in the Amazon Forest (the burning of one hectare of Amazon Forest, depending on the density and the composition, implies an emission of anything between 100 and 200 tons of carbon). “It took us five centuries to find out the biomass of the Atlantic Rainforest, which is a basic piece of information,” recognizes Martinelli. His indignation is mixed with the joy of having discovered such a huge entity close at hand, yet unknown for so long and that certainly holds many other surprises in store.

The Project
Floristic composition, structure and workings of the dense ombrophile forest of the Picinguaba and Santa Virginia centers in the Serra do Mar Park (nº 12/51509-8); Type: Thematic Project – Biota FAPESP Program;
Coordinators: Carlos Alfredo Joly (Unicamp) and Luiz Antonio Martinelli (Cena-USP); Investment: R$ 2,576,067.24 (FAPESP)

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