New research shows that fragmentation of tropical forests can make trees wimpy. Seeds from isolated trees had less genetic diversity and were less likely to germinate, and the seedlings that did grow had smaller leaves. This is the first study of how forest fragmentation affects seedling quality.
“Fragmentation of tropical dry forests reduces genetic variation and seedling vigor of the tree Samanea saman,” says Mauricio Quesada of the Universidad Nacional Autonoma de Mexico’s Institute of Ecology in Morelia, Mexico, who published this research with three co-authors in the February issue of Conservation Biology.
Quesada and his colleagues studied how fragmentation of a dry forest in northwestern Costa Rica affects a tree in the mimosa family (Samanea saman). The researchers compared isolated trees with those in continuous populations. The isolated trees were about a third of a mile apart and were surrounded by agricultural fields, pastures, or remnant forest patches; the trees in continuous populations numbered about 4/acre and were surrounded by undisturbed forest. The researchers evaluated factors including seed production, relatedness, and germination.
Quesada and his colleagues found that fragmentation decreased the vigor of S. saman seeds and seedlings. For instance, seeds from isolated trees were 15 percent less likely to germinate, and the leaves of those that did sprout were 10 percent smaller.
Fragmentation also decreased genetic diversity: in isolated trees, seeds from different fruits of the same tree were four times more likely to have the same father. S. saman is thought to be pollinated by moths, and researchers speculate that fragmentation keeps moths from visiting enough paternal trees to maintain genetic diversity.
The deleterious effects of fragmentation notwithstanding, Quesada and his colleagues caution that “trees in isolation or small populations should not be considered the ‘living dead.’” Several isolated trees had high levels of genetic diversity and so could serve as important “stepping stones” for the pollinating moths — and thus gene flow — between S. saman populations.
Quesada’s co-authors are: Alfredo Cascante, Jorge Lobo, and Eric Fuchs, all of the Universidad de Costa Rica in San Jose, Costa Rica.
Cascante, A. et al. 2002. Effects of dry tropical forest fragmentation in the reproductive success and genetic structure of the tree Samanea saman. Conservation Biology 16:137-147.