Although even the most optimistic predictions would not suggest that tropical forests will regain the coverage they enjoyed a century ago, research to augment restoration efforts are an important step toward sustainable forests. Several Earthwatch-supported projects have made gains in understanding the tropical forest structure and function necessary to expedite reforestation efforts.
In the early 1980's, Dr. Steven Hubbell (University of Iowa) approached Earthwatch with a mammoth task. He wanted to find out how an entire Panamanian rainforest community and its component members work. Intrigued by the scope of the project and excited about its applications, Earthwatch sent 80 volunteers, often up to 18 teams per year, to help. Censusing and recensusing a stunning 250,000 trees, Hubbell's teams identified how a forest ecosystem lives, dies, and regenerates in order to make recommendations about successful reforestation techniques. Hubbell went on to publish The Unified Neutral Theory of Biodiversity and Biogeography, in 2001, a widely acclaimed but controversial book in which ecological patterns are explained by treating competing species as essentially identical.
In the early 1990s, Dr. Rolf Borchert (University of Kansas) explored the seasonal development (phenology) of trees in tropical dry forests in Costa Rica. He found that, contrary to common classifications, these forests are far from uniform. They constitute mosaics of functional tree types whose distribution varies with soil water availability in the dry season. Species range from evergreen trees at moist riparian sites to deciduous and stem succulent trees at dry upland sites. The work of Borchert's teams provided the basis for an ongoing reforestation project in the Guanacaste Conservation Area, Costa Rica.
Dr. Lynn Carpenter (University of California, Irvine) tested the impact of exotics on reforestation in a Costa Rican rainforest in the early 1990s. Earthwatch teams compared pollination, herbivore damage, and seed production of a rainforest native, Columnea raymondii, at the Wilson Botanical Garden. Their results showed that any benefit in terms of attracting pollinators to planted stands of C. raymondii was more than outweighted by the disadvantage of attracting such herbivores as birds, bats, and green hermits.
Earthwatch teams working with Dr. Colleen Kelly (University of Oxford) and colleagues explored the determinants of rarity in a Mexican tropical dry forest, with implications for forest restoration. They found that low water density and sparse canopy prevent an effective seed bank of dry forest trees, and that soil nitrogen levels influence the phylogenic diversity of trees. Teams also found that rarer species have more variable size distributions than more common species, indicating more variable recruitment of new individuals from year to year in less abundant trees.
Earthwatch teams working with Dr. Colleen Kelly (University of Oxford) and colleagues explored the determinants of rarity in a Mexican tropical dry forest, revealing a previously unsuspected reason for biodiversity in trees. Rather than competitive exclusion by similar species, where one species tends to ‘win out,' the teams working with Kelly found that the tree seedlings can ‘take turns' over time.
"The tree seedlings of one species will be successful under one sort of conditions and the seedlings of the other species under another sort of conditions, at a later time," said Kelly. "In this way, there can be two or three or possibly more times as many species at a site as expected from other explanations of biodiversity."
The Earthwatch teams working with Kelly also helped to show that there are few tree seeds in the soil of this dry tropical forest, so that the forest would not be able to recover easily from disasters such as fire or large-scale tree-cutting. Lastly, tree censuses completed by Earthwatch volunteers were instrumental in indicating that soil nitrogen can be an important determinant of the taxonomic range of the tree species that can be found in a tropical forest, with implications for forest restoration.
Dr. Karen Holl (University of California, Santa Cruz) brought Earthwatch teams to the Costa Rican rainforest in 1997 to monitor the recovery of a tropical montane forest in abandoned agricultural lands. Their findings showed that lack of seed dispersal and competition with exotic pasture grasses are major factors limiting forest recovery. Although they found that bird perching structures do not serve to accelerate recovery, remnant trees and shrubs do make a significant difference. These kinds of findings will have important applications for rainforest restoration efforts around the world.
"I regularly receive requests for copies of my articles in Spanish from people working on forest restoration throughout Latin America," said Holl, who also provided training for several Costa Rican high school students on the project (see Legacy of Learning). "I feel good that my research may be helping forest restoration in countries I have never even visited."