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South America’s white-sand forests: poorly known and under threat

  • Neotropical white-sand forests are unique rainforests found throughout tropical South America, often occurring as “habitat islands.”
  • Scientists were surprised to discover that only 23 percent of plant species in western Amaonzian white-sand forests are white-sand specialists.
  • Researchers argue that white-sand ecosystems require special protections.

Published this month, a special issue of Biotropica, the journal of the Association for Tropical Biology and Conservation (ATBC), focuses on issues in South America’s under-studied neotropical white-sand forests.

Paul Fine, a tropical biologist at the University of California, Berkeley, approached Biotropica with the idea for the special issue after helping organize a symposium at a 2013 meeting ATBC held in Costa Rica on white-sand forest ecology and evolution and coming away impressed by the breadth of the talks.

“I thought a special issue would be a good way to get many of these studies published and it would be fun to include some reviews and syntheses with these articles to spur on future research,” Fine told Mongabay, “and also get some attention paid to the conservation status of these fragile environments.”

Neotropical white-sand forests are unique rainforest ecosystems found throughout tropical South America, often occurring as “habitat islands” surrounded by more typical rainforests that grow in terra firme, where the soil has higher clay content.

The quartz-rich, sandy soil that gives them their name provide very low amounts of nutrients and water, “leading to stunted forests with unique physiognomies and endemic species assemblages,” according to an article Fine and Biotropica editor Emilio Bruna wrote to introduce the issue.

Fine and Bruna said that white-sand forests have been a subject of fascination for biologists since the 19th century, but that it wasn’t until the 1980s that efforts really began to examine their unique forest structure, mechanisms for nutrient cycling, plant adaptations, and biological diversity.

Cattleya eldorado, a white-sand species. Photo by Charles Zartman.

In seeking to represent the current state of knowledge of neotropical white-sand forests, Fine and Bruna collected contributions from authors in Brazil, Ecuador, France, Peru, and the United States.

The two editors say they found three consistent themes in the studies they chose to publish: though millions of years old, white-sand forests are highly dynamic today, which has shaped their unusual biological diversity in a number of ways; many species associated with them, however, also occur in other, similar forest types; and because they are especially vulnerable to man-made pressures, they require special conservation measures.

“A classic case study of habitat specialization”

In some areas of tropical South America, the white-sand soils are ancient — more than 100 million years old. But their recent history, relatively speaking, has seen a lot of change.

White-sand areas have expanded and contracted over the centuries, which has helped shape the plant and animal life that can thrive there. River sediment deposits and geological changes in the Pleistocene, which ended about 11,700 years ago, also helped create the unique conditions of life in white-sand forests, according to Fine and Bruna.

Some plant lineages are so adapted to conditions in a white-sand ecology that they can’t expand into other forest types.

“White-sand forests have been referred to as a classic case study of habitat specialization and a source of high endemism in both animal and plant communities of Amazonian forests,” UC Berkeley’s Juan Ernesto Guevara and co-authors wrote in a paper published in the Biotropica special issue.

While Guevara and team found that, on average, white-sand forests have less diversity than terra firme forests, they said that most of the plants found there are rare or even absent in other forest types.

“This specialization to the white-sand habitat has evolved repeatedly, with some species belonging to lineages that are generally restricted to this habitat and other species that appear to have evolved white-sand specialization relatively recently, descending from non-white-sand ancestors,” they wrote.

White-sand forest. Photo by Paul Fine.

As you might expect, the species inhabiting any given white-sand forest vary by region, but Guevara and his co-authors said they did find a “geographically idiosyncratic dominance of a small number of white-sand specialist lineages.”

In some regions, for instance, flowering plants in the genus Protium, trees in Pachira, and plants in Caraipa account for as much as 70 percent of the total tree abundance.

And yet, many species associated with white-sand forests can and do occur in other forest types, especially those with similar structure or limitations on the amounts of available resources, such as seasonally flooded blackwater forests and disturbed open habitats.

“Conservation of this unique and fragile environment should remain a priority”

In some cases, Fine and Bruna wrote in their introduction, these marginal habitats can “serve as corridors between isolated white-sand forest islands, resulting in greater gene flow and metacommunity dynamics that buffer against local extinction,” explaining why it’s advantageous for white-sand species to expand their range into other forest types.

Scientists say they were surprised to discover that a large majority of species in Amazonian white-sand forests have not adapted solely to that ecosystem.

The University of Edinburgh’s Roosevelt García-Villacorta and co-authors wrote in another paper that they found only 23 percent of plant species in western Amaonzian white-sand forests are white-sand specialists.

The other 77 percent of species are found in other nutrient-poor forest types, suggesting that dispersal from these neighboring habitats has been fundamental to shaping Amazonian white-sand biodiversity.

“Although endemism in Amazonian white-sand forests may be lower than previously estimated, conservation of this unique and fragile environment should remain a priority,” García-Villacorta and co-authors write.

“Such conservation will require the maintenance of regional dispersal processes that connect these archipelagos of habitat islands and other ecologically similar oligotrophic [nutrient-poor] habitats across the Amazon and the Guiana Shield.”

White-sand ecosystems are “inadequately protected”

Scientists say that white-sand forests are extremely susceptible to anthropogenic change.

Map via “White-sand Ecosystems in Amazonia,” published in Biotropica, Volume 48, Issue 1, January 2016.

Unfortunately, Duke University’s J. Marion Adeney and the other authors of a contribution to the Biotropica special issue find that white-sand forests “are inadequately protected and, where accessible, are regularly mined for sand, logged, or burned and cleared for agriculture.”

Adeney and team looked at 25 years of field data, performed an extensive review of existing literature, and reviewed numerous maps and spatial analyses to not only summarize the most important characteristics of white-sand areas in the Amazon Basin, but to compile the first ever comprehensive map of Amazonian white-sand ecosystems (seen above).

While many white-sand forests are protected by geographic isolation and the fact that their nutrient-poor, sandy soil isn’t that useful to humans, they are being exploited near major urban centers, like Iquitos in Peru and Manaus and Belém in Brazil.

Adeney and co-authors argue that white-sand areas should be given special protections that take into account their fragmented distribution pattern, the species that have adapted to live in them and other low-nutrient environments, the fact that they’re extremely slow to recover after disturbance, and their importance to biodiversity and ecosystems across the entire Amazon Basin.

“In a fully functioning Amazonian ecoregion,” the authors write, “small and unusual ecosystems such as [white-sand forests] have a special role to play and should be given special consideration in conservation efforts.”

Tachyphonus phoenicius, a white-sand species. Photo by Camila Ribas.
Polytmus theresiae. Photo by Camila Ribas.
Cyanocorax hafferi. Photo by Camila Ribas.