The saplings that sprout up in denuded rainforest might seem to herald a recovery. But a report in the current Science  says that a few trees dominate the regrowth, reducing plant diversity in the logged forests and altering pollination patterns for nearby animals. Such inbred populations are usually much more susceptible to disease, drought, or other threats, the researchers warn.
James Hamrick of the University of Georgia, Athens, and his graduate student Preston Aldrich were concerned that trees left standing in the midst of cleared areas--so-called forest fragments--would not be able to exchange pollen and seeds with trees from a different fragment. To explore this possibility, the duo spent 3 years surveying the genetic diversity of Symphonia globulifera trees in a circular 38-hectare section of Southern Costa Rican rainforest that had been logged roughly 20 years ago.
They extracted DNA sequences from leaf and bark samples and identified which adults had borne the seeds which gave rise to the younger trees. The researchers found that 52% of the new seedlings in the logged area came from just two prolific adults in pasture area outside the fragmented forest. These "supertrees" produced bigger flowers and more fruit, attracting more pollinators such as bats and hummingbirds. "If the patterns continue, we will see some loss of genetic diversity," Hamrick says. The loss of diversity could extend to the pollinators as well, whose normal patterns were disrupted by the scattered locations of the superproducers.
Hamrick fears that such a pattern of regrowth may be forcing both the trees and the pollinators into a genetic bottleneck. But other researchers say the evidence isn't so clear. Chris Dick, a tropical ecologist at Harvard University, says that the research points to "plenty of gene flow" within the fragmented forest. "These fragments are not isolated." Dick also cautions against drawing too broad a conclusion from a snapshot of a plot that may continue to evolve.