The Brazil Nut (Lecythidaceae) Family: the Most Successful Clan on the Rio Negro in Brazil

Manaus - Brazil’s Rio Negro (Negro River) contains an unusually high percentage of species of the Lecythidaceae family, popularly known as the Brazil nut family. Not only does this great diversity of tree species within one family suggest a rich variety of plant life in general; it also signifies a healthy, well-preserved ecosystem. A favorable environment and limited human impact set the conditions for the rainforest to flourish on the Negro River. Botanical explorers in the 19th and 20th century such Richard Spruce, Richard Schultes, Margaret Mee, Ghillean Prance and Scott Mori have all suggested that the forests of the Negro River represent the richest plant ecosystem in the world.

Some species of Lecythidaceae are exploited for timber, but most instead provide secondary tropical forest products. The most famous examples are the edible seeds of the Brazil nut or “castanha,” Bertholletia excelsa, and the paradise nut or “sapucaia,” Lecythis zabucajo. Until the end of the Second World War Brazil nuts were the second largest export from the Brazilian Amazon after rubber; today they are number one. Brazil nut harvests of more than 100,000 tons have been registered every year since before 1970. Twenty percent of the heavily farmed department of Madre de Dios is covered by forests rich in Brazil nuts. The lesser-populated Negro River probably contains even more. Supplies however are not unlimited. If the harvest is to continue apace or increase, large extractive reserves must be established in areas already rich in Brazil nuts.

Fruits of most species of Lecythidaceae are generally thick; the outer layers pulpy, the inner ones ligneous and hard. These indehiscent fruits develop a fissure from which the seed falls. The seeds are large and ligneous without an endosperm. At maturity the large, round, woody fruits of castanha and sapucaia fall to the ground with the seeds inside. The seeds, which themselves have a bony testa inside the larger shell, are removed from the capsule and (naturally) dispersed by rodents, especially agoutis (Dasyprocta spp.) and squirrels. These rodents may be the only ones with teeth strong enough to gnaw through the extremely woody pericarp. They eat some of the fruits and cache others for later use. Forgotten seeds eventually germinate and thus the species is propagated.

Brazil nuts are harvested almost entirely from wild trees - collected tree-by-tree just as latex rubber once was along lonely forest trails called “estradas”- during the five to six month rainy season. The fruits, which weigh from between 0.5 and 2.5 kilograms and contain 10 to 25 seeds, are gathered as soon after they fall as possible to minimize the effects of insect and fungal attack. On average a single tree may produce anywhere from 63 to 216 capsules per year, a substantial quantity by any census and strong argument for the sustainable management of this species and preservation of the rainforest in which it thrives.

Roots, wood, leaves, fruits and seeds of Lecythidaceae are also known to be used extensively in the field of traditional medicines though the pharmacological properties of most species have yet to be studied. Chemical and pharmaceutical research of Lecythidaceae would help predict resource exploitation over time with more certainty. Furthermore it is necessary to study the physiology and the ecology of the germination of the seeds as well as the dynamic of the population. The absence of this information presents a barrier to devising better plans for sustainable management of this important family of tropical trees.

Why on the Negro River do we find so many species of the same plant family? Why do we find so many species of so many plant families in the Amazon rainforest? For one thing the Amazon, a tropical forest located on the earth’s Equator, is characterized by abundant light, copious amounts of water, and warm temperatures year round. To the surprise of many, the Amazon also contains a wide diversity of habitats due to great variations in topography and soil. Climatic changes over time have also played a role; the expansion and contraction of the forest has resulted in isolated clusters of unique forests in which endemic species have evolved. Geological events such as the appearance and disappearance of lakes and rivers have encouraged species diversification. Extensive interaction between many different plants and animals has resulted in even more species of plant - and animals. Diversity breeds diversity. This is evolution. Nowhere in the Amazon is evolution more evident than on the Negro River.

In the family of Lecythidaceae bees, bats and beetles of many species serve as pollinators. Seeds are dispersed by a variety of means: gravity, wind, water and animals. Castanha do Pará (Bertholletia excelsa) is pollinated by bees of the genera Bombus, Centris, Epicharis and Xylocopa; after the fruit has fallen to the ground its seeds are consumed and, more importantly, dispersed by agoutis and squirrels. Sapucaia (Lecythis pisonis) is even more specialized; it is pollinated only by female mamangaba bees (Xylocapa frontalis) and dispersed by bats that feed on the pulp of the seeds found inside its large fruit.

Like the proverbial chicken and the egg it is not always clear which came first, the plant or the pollinator. But no one can doubt the vital role of a plant’s pollinator is defining it biogeography. For example three species of Jarana (Lecythis barnebyi, L. poiteaui and L. brancoensis) are all pollinated by bats. L. barnebyi is endemic to the lower Negro River and lives in much of the same forest as L. poiteaui whose habitat extends north to the Guianas. L. brancoensis is endemic to the northern "cerrado" (dry, shrubby forests) of Roraima state only. The parent species is most likely L. poiteaui, which has with a large habitat that overlaps that of the other two species, the specialist L. brancoensis and the co-habitator L. barnebyi. The likely origin of L. poiteaui is the ancient forest of the Guiana Shield. As conditions dried and the forest disappeared L. poiteaui evolved locally into L. brancoensis and moved south evolving into L. barnebyi. The common factor between these three species of Jarana is the bat that pollinates them, and only them.

It should be obvious that the success of a plant family such as Lecythidaceae can only be guaranteed by the preservation of the rainforest as a whole. And the success of Lecythidaceae guarantees the success of many forms of life in the forest too: pollinators such as bats and bees; dispersers such as pacas, squirrels, agoutis and other rodents; animals that feed upon its fruit such as monkeys; and the myriad of insects, plants, fungi and micro-organisms that accompany the lifecycle of this amazing family of trees from birth to death to rebirth again. Lecythidaceae truly is worthy of the crown of most successful tree family in the Amazon rainforest, and symbol of the most botanically-rich river found there, the Negro River.

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