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Why forests are different?

Biologists identify mechanisms that fuel the competition amongst trees and differentiate the forests of São Paulo

EDUARDO CESARA ‘caraguatá’ (Bromelia balanceae) from the Cerradão: the brightest and driest of the fur study areasEDUARDO CESAR

It is not just jaguars that fight for space. Plants also establish their territory, usually with subtlety: a leaf that falls can put other species to flight. “Did you see that, around here, there are almost only guarantãs?”, asks Flaviana Souza, a botanists from the Forestry Institute, in a reserve of the Atlantic Rain Forest in the municipality of Gália, in the southwest of the state of São Paulo. Here, the guarantãs, or Esenbeckia leiocarpa – the species preferred by the song-thrushes to make their nests -, with their straight trunks of up to 60 centimeters in diameter and 20 meters in height, create a certain homogeneity in the midst of the twisted, branchy trees in the neighborhood. In the laboratory, Flaviana saw that the leaves of the guarantã release substances that delay the germination of the seeds and cause rotting in other species.

But the domination of the guarantã has limits. “The forest is remaking itself from end to end, like a crazy quilt”, comments Geraldo Franco, a botanist from the Forestry Institute. In some stretches, a tangle of lianas imposes itself, growing on the trees and between them. Maria Teresa Toniato, another researcher from the Forestry Institute, avers that she did not lose her good humor even once when she faced this web of climbing plants, along with biologist Tiago Barreto, to reencounter and measure, once again, each one of the 13,053 trees listed four years before, when a group of biologists, agronomists and forestry engineers let itself be taken by a daring task: to discover how and why the forests of São Paulo are different amongst themselves.

Even though the gigantism of the Amazon puts Brazil’s other forests in the shade, a more attentive look will find, in the territory of São Paulo, a varied vegetation, which ranges from the exuberant forests of the coast to the dry woods of the interior, with a right to rich nuances between one extreme and the other. And it isn’t small: São Paulo houses 15% of what remains of the Brazilian Atlantic Rain Forest, where the density of exclusive plants and animals is equivalent to the Amazon. The area covered by the remnants of natural vegetation, corresponding to 13.9% of the total area of the state, is practically the same as that covered by the sugarcane plantations.

The task of discovering the reasons for the differences between the forests of São Paulo brought together specialists in soil, vegetation and light, captained by Ricardo Rodrigues, Vinícius Souza and Sérgius Gandolfi, professors from the Luiz de Queiroz College of Agriculture (Esalq), of the University of São Paulo (USP), and Alexandre Oliveira, from the Biosciences Institute, also at USP. At the start, they verified the scope and the limits of the oldest project for monitoring forests, which occupies 50 hectares of an island in the Panama canal – there, for 25 years, the growth of about 300 thousand trees has been accompanied. It was a good model, adopted in 13 countries, but this group preferred to follow new trails.

They demarcated four smaller areas, but which could be compared: a sample of the Atlantic Rain Forest from the coast, one from the Cerradão (forested savanna),  one from the salt marshes, and one of the Atlantic Rain Forest in the Serra do Mar (Sea Ridge). Each area has 10.24 hectares and represents the main kinds of natural vegetation in the state. Another innovation was to investigate the influence of the soil, water and light on the plants.

“It is the first time that one has such a large sampling of the São Paulo forests, examined with the same methodology, integrating specialists from different fields of knowledge”, comments Oliveira. “With time, we hope to compare the conclusions of this work with those obtained in similar projects under way in Brazil and in other countries.” Each forest revealed itself as a unique organism, endowed with deep differences in the way it worked.

The diversity and distribution of species and, on a broader plane, the very physiognomy of each kind of vegetation depend on very specific combinations of sparser or more abundant rainfall, higher or lower temperatures, more intense or more tenuous light, and on soil more fertile or poorer in nutrients, capable of storing water for a longer or shorter time. One of the results of this combination is that each forest houses unique sets of species of trees. Only five species – of the 537 that the researchers found after identifying the genus and species of 64,004 trees – manage to adapt themselves to the four environments, giving little importance to the quantity of nutrients in the soil, water or light.

So much work for what? In order to understand how a forest reorganizes itself, differentiates itself, and reacts to local or global phenomena like climatic changes, which can only be evaluated by means of long-term measurements. “If the temperature of the planet continues to go up, first of all the species on the tops of the mountain ranges should disappear, as they are more sensitive to climatic variations”, comments Rodrigues, the coordinator-general of this project, which began with 15 members and now brings 104 together. “Whereas the forests from the interior, more accustomed to drought, ought to adapt more easily.”

According to him, the prospect of an extinction of the plants of the Atlantic Rain Forest is worrying, in first place, because it renders useless the effort of creating and maintaining the majority of these areas of environmental preservation, located along the coast. It would be an attack on the green heart of São Paulo, represented by the stretches of the Atlantic Rain Forest that have escaped the urban and agricultural expansion, precisely for occupying mountainous terrain, and that help to keep the temperature mild for the 18 million inhabitants of Greater São Paulo.

Alert to the future, but strongly rooted in the present, the team from São Paulo is sowing more adequate practices of conserving and restoring natural spaces, in view of the accumulated knowledge on the interactions between the species and of each one of them with the soil, water and light. According to Rodrigues, restoration will be important, especially in the interior of São Paulo, where the natural vegetation is very fragmented, by virtue of the continuous expansion of the plantations and of the pastures. He himself is using this knowledge to recover forests close to rivers in sugarcane mills: in five years, his team has already managed to repopulate 2,500 hectares of riverine woodlands.

Also germinating are a few hypotheses about the mechanisms for survival proper to each kind of forest. Why don’t species from the interior reach the coast? “Because they cannot manage to live with so little light”, replies Sergius Gandolfi, a professor from Esalq, who scattered dozens of light sensors to understand how variations in luminosity can favor or make difficult the survival of the plants. And why do those from the coast not reach the interior? This time, who has the answer is Miguel Cooper, also a researcher at Esalq, who coordinated the studies of water in the soil. “Because they do not know how to live with little water”. The information gathered up until now indicates that, as the soil becomes more fertile, some species grow more than others, the density of trees drops, and the competition for light increases, since the trees that grow most impose themselves in the forest, overshadowing and eliminating the others.

Fertile soil, variable water and light
It is mainly the fertility of the soil that makes the Atlantic Rain Forest of the interior a lush scenery. In this area for study, integrated with the Caetetus Ecological Station, which belongs to Gália, a municipality in the southwest of the state of São Paulo with almost 8 thousand inhabitants, there is a mixture of trees thick and thin, tall and short – the tallest have about 30 meters, half the size of the more stretched out ones in the Amazon Forest. “One of the largest trees I found here was a Gallesia integrifolia, a ‘pau-d’alho’ (garlic wood), which emits a strong smell of garlic, sensed meters away, and has a trunk a meter and a half in diameter”, says Franco.

For the plants, there is no lack of nutrients or water, because the soil retains the rain that falls between November and January. “It is if it rained the whole year and the roots could always count on a stock of water”, says Cooper. But there are not only gains: according to Rodrigues, it was because of the richness of this soil, in addition to the flat landscape, that many forests of this kind were felled to give way to pastures, coffee, sugarcane, or soybeans. Another peculiarity is that roughly half of the trees lose their leaves during the dry season, from June to August. It is when the forest, being illuminated, practices mechanisms of renewal of its very own. Gandolfi and Flaviana saw that the trees that lose their leaves work like a seasonal clearing, letting through the light that for three months bathes the trees that until then had been fighting to grow in the shadow of the others.

The clearings created by the fall of the leaves and, throughout the year, by the wind, bring the light that feeds the net of lianas and maintains the high diversity of this forest, where 151 species live together, represented by 13,053 trees. It was to find out how the forest is remaking itself that Maria Teresa and Barreto went after each one of them. Over six months, they noted how many died and measured, identified and mapped those that grew to the point of attaining the minimum 15.7 centimeters of circumference required to be incorporated into the survey.

Poor soil, little water and much light
Although it is nearby, some 80 kilometers away, the Cerradão (forested savanna) of the Assis Ecological Station, in Assis, a municipality of almost 90 thousand inhabitants, also in the southwest of São Paulo, is very different – and now it is known why. In this forest, which constitutes the forest form of the Cerrado (savanna), many trees grow, because there is plenty of light – it is the brightest and driest environment of the four. It was where the greatest density of trees was found: in 10 hectares, 23,495, almost double the quantity found in the other areas, although the diversity is lower, only 122 species. But the trees rarely grow more than 15 meters, because of the soil, poor in nutrients. And for being sandy, the soil lets the rain water drain rapidly and dries quickly.

These characteristics of the soil help one to understand why the scenario is relatively uniform. The trees of the Cerradão, whose trunks show similar  thicknesses, without great variations, in general have few leaves and ramifications, as if they didn’t want to call attention. They are even discrete in dying: they die upright. They lose their leaves, the branches fall, and afterwards they are slowly consumed by termites – different to the trees from the Atlantic Rain Forest or from the Amazon, which, when dying, fall noisily, taking others with them: that is when the clearings form and the opportunity arises for other species to germinate, thus explaining, in good measure, the high diversity of species in the tropical forests.

Recognizing trees and paths in the Cerradão calls for an eye as sharp as that of Giselda Durigan, a researcher from the Forestry Institute who began to explore the forests of this region when she was still a girl. It is she who shows the most common species like the balsam copal tree (Copaíba) (Copaifera langsdorffii), with its stout trunk covered with lichens and with gray and dark red patches, and which accounts for 27% of the trees identified in this study area. The analysis of the species of trees that grow in this Cerradão revealed a curious phenomenon, which suggests how one kind of forest can be transformed into another: some species that are indifferent to drought or to humidity begin to predominate, which also live in the Atlantic Rain Forests of the coast or of the interior, such as the balsam copal tree itself, the “canela-do-cerrado’ [cinnamon of the Cerrado] (Ocotea corymbosa) and the “limão-bravo”[wild lemon] (Siparuna guianensis). “The Cerradão may be a transition form from the Cerrado to the Atlantic forest”, Giselda ponders, “or from the Atlantic Rain Forest to the Cerrado, if the temperature of the planet increases”.

Fertile soil, much water and little light
If in the interior water is rare or, at the least, irregular, in the other two study areas, both close to the coast, what there is no lack of is humidity. “It has rained all the times I have come here”, comments Cooper, out-of-breath, under a cold and incessant rain, which he climbs the slopes of the Atlantic Rain Forest reserve that makes up the Carlos Botelho State Park, shared by the towns of Sete Barras, Eldorado and São Miguel Arcanjo.

The Atlantic forest of the coast is the most impressive, with tree trunks covered with bromeliads, and the richest of the four areas studied, housing 220 species of trees, although it is also the least populated: in the study area, there were only 10,582 individuals sufficiently sturdy to enter into the survey. It is also the darkest. Gandolfi saw that the leaves closest to the surface of the soil receive only 1% of the light that reaches the top of the forest.

Poor soil, little light and much water
This exuberance contrasts with the discrete stature of the other research area that is also to be found on the coast, about 80 kilometers away: the salt marsh of the Cardoso Island State Park, in Cananéia, one of the oldest Brazilian settlements, today with about 23 thousand inhabitants, in the far south of the state of São Paulo. When Daniela Sampaio reached the island for the first time, in May 2001, having recently come out of the biology course, she just saw a green mass. Four years later, after having identified 16,890 trees from 177 different species, she walks though the salt marsh as if she were in her garden, avoiding with naturalness the spines of the palm trees and of the bromeliads, whose leaves half a meter long sprout from the soil like lances.

Little by little, as the survey that she is doing is added to the studies of the other specialists of the group, she was able to understand why this forest is like it is. The trees rarely grow over 15 meters in height, because of the soil that is poor in nutrients, as in the Cerradão. But the sandy soil has another problem: it is always soaked. In the lowest areas, the water table emerges and forms streams of copper-colored water – the color is due to the concentration of iron – which move slowly between the banks of white sand like a sheet of paper. But Cooper, a tall, strong, 40-year old Argentinean, recommends: it is better to keep far away from this water. He stirs it up with his boot and asks whether the other members of the expedition whether they can feel the smell – a smell of bad eggs, a result of the sulfur accumulated in the slowly decomposing leaves and trunks. “Much worse”, says he, “would be to deforest and to drain off this water”. The sulfur, reacting with oxygen, can be transformed into sulfate, which, when it combines with water, forms sulfuric acid, which can leave the soil sterile.

The Project
Diversity, dynamics and conservation of trees in forests of the state of São Paulo: studies in permanent portions (nº 99/09635-0); Modality Thematic Project connected with the Biota/FAPESP Program; Coordinator Ricardo Ribeiro Rodrigues -Esalq/USP; Investiment  R$ 1,785,067.39 (FAPESP)