Fragmentation Can Make Seedlings Wimpy

"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 did this research with three co-authors in the February issue of Conservation Biology.

Tropical dry forest is one of the most endangered ecosystems in the world: more than 99% of the dry forest has been logged or converted to agriculture in tropical Mexico and Central America. Tropical trees are particularly vulnerable to fragmentation in part because they grow at low densities and depend on animals for pollination.

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 S. saman trees with those in continuous populations (the isolated trees were about a third a mile apart and were surrounded by agricultural fields, pastures or remnant forest patches; the trees in continuous populations were at 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% less likely to germinate (58% vs. 75%) and the leaves of those that did sprout were 10% smaller (about 9 vs. 10 square inches).

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 may keep the moths from visiting enough paternal trees to maintain genetic diversity levels.

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.