Restoration of tropical forests gets under way

But scientists still have a lot to learn

provided by University of California, Santa Cruz


n an unparalleled ecological disaster, more than half of the world's tropical forests have been lost since 1950, due in large part to the developed world's appetite for agricultural and wood products. While environmentalists lobby for conservation, some scientists are trying to find ways to restore tropical forests that have been cleared and abandoned. So far, however, they are finding that they have much yet to learn.

    Karen Holl, assistant professor of environmental studies at the University of California, Santa Cruz, is one of the few ecologists who has completed multifaceted studies of tropical forest restoration. In the December 2000 issue of Restoration Ecology, Holl reviews the results of her own work on abandoned pasture in Costa Rica and warns that the challenges of habitat restoration demand far greater site-specific knowledge than is currently available.

    “Tropical forests have been impacted by humans for a long time, going back to before the Mayans, but we're seeing a scope of destruction now that is unprecedented,” said Holl. “By contrast, recovery efforts are very new, and we don't know very much, especially when we try to put all the pieces of the ecosystem puzzle together. In this field, a five-year study is considered long-term.”

    The pressure is on, although even the most successful restoration projects couldn't keep pace with the grand scale of the ongoing destruction, noted Holl. Tropical forests are disappearing throughout Asia, Africa, and Central and South America, and the future of the Brazilian Amazon the greatest remaining tropical rainforest in the world is bleak: up to 40 percent of the Amazon may be cleared in the next 20 years if proposed infrastructure projects are approved, according to a report by other scientists in the January 19 issue of Science.

The big picture


    Unlike many researchers, whose work focuses on a piece of the puzzle, such as soil nutrients or seed dispersal, Holl takes an ecosystem approach to forest restoration. Her research and that of others suggests that the chances for recovery depend a lot on the intensity of the disturbance whether heavy equipment was used to clear the forest and whether a nearby source of seeds remains. “Some problems, like seed dispersal, are common to all tropical forest restoration projects, but others are site-specific,” said Holl, who has studied tropical forest restoration for six years.

    Ecologists lack data on natural forest recovery processes, which hampers their ability to compare the effectiveness of human-managed intervention strategies with nature's own power. But Holl's work in Costa Rica has illuminated some success stories as her team tries to accelerate the natural recovery process. “It would be impossible to reintroduce all the species that have been lost, so we're focusing on identifying the factors that limit recovery and designing strategies to maximize our effectiveness,” she said.

Gone to seed

    By removing trees, logging eliminates the primary source of seeds that would facilitate the recovery of abandoned pastures. Tree loss triggers the disappearance of birds, which are a critical agent of seed dispersal between forest and pasture. Dispersal efforts alone may not yield desired results, however, as seed predation rates can exceed 65 percent, depending on the species, according to Holl. Germination rates also vary greatly by species, but competition with pasture grasses is the primary factor impeding seedling survival. At Holl's research site, pasture grasses commonly grow to a height of more than five feet, and they produce a dense layer of matted litter about four inches thick.

    The most successful strategies Holl has identified include planting native tree seedlings and shrubs to enhance seed dispersal and shade out pasture grasses.

    Other highlights of Holl's Costa Rica project include:

  • Seedlings are much more likely to survive if they are planted in areas that have been cleared of pasture grass or if they are transplanted under shade-generating pasture shrubs.
  • Tropical light levels can stress seedlings once they overtop pasture grasses.
  • Soil compaction and low levels of nutrients slow seedling growth.
  • Rabbit predation greatly reduces the survival and growth of seedlings.

    Rabbit herbivory is an example of the site-specific nature of reforestation challenges. Previous research has reported the damage done by leaf cutter ants, but Holl's team is the first to document the importance of rabbit predation. A factor for further evaluation is the role forest “remnants” play in reforestation. If patches of forest are left undisturbed, they may “fill in” the forest faster, while also providing habitat for animals that can disperse seeds, said Holl.

    Holl's work underscores the need for increased basic knowledge about tropical ecology, as well as comparative studies across a range of sites to identify the most useful restoration strategies. That information should shape allocation decisions about where to initiate large-scale restoration efforts, she said. “Unfortunately, what we've found is that many factors operate simultaneously to inhibit reforestation, which means recovery efforts will require site-specific preliminary research and then aggressive oversight,” said Holl. “It's not impossible, but it's going to take a strong commitment from the international community.”

    Holl hastened to note that tropical forest restoration is not a solution to deforestation. “Current deforestation is having devastating effects on numerous species, including humans,” she said. “It is unlikely that we will be able to 'restore' exactly what was there before. Therefore, we must redouble efforts to develop conservation strategies that meet human needs while protecting forests.”

    Karen Holl may be reached directly at (831) 459-3668 or via e-mail at