Responses of 20 Native Tree Species to Reforestation Strategies for Abandoned Farmland in Panama

Deforestation in the tropics often leads to unproductive agriculture and results in abandoned, degraded grasslands that tree species recolonize poorly. To evaluate why forests do not regenerate naturally and to identify potential species for use in reforestation of degraded areas, we planted 15 000 seeds of 20 native tree species, varying in seed size and shade tolerance, in abandoned Panamanian farmland dominated by the exotic grass, Saccharum spontaneum. To determine the effects of above- and belowground constraints on tree seedling germination, survival, and growth, we used four mowing and shading treatments of the Saccharum. Shading the Saccharum effectively eliminated it, whereas mowing led to increased light aboveground, but did not reduce Saccharum growth rate. Germination, survival, and growth of tree seedlings approximately doubled in shade treatments compared to the unshaded control, but were lowest when the Saccharum was mowed three times. Fire significantly decreased germination and survival. Some species did not follow these general trends, however; we identified four species groups that varied in their response to Saccharum competition. Very small-seeded, light-demanding species performed poorly, and we do not recommend their use in reforestation because they tolerate neither above- nor belowground constraints imposed by the Saccharum. Light-demanding species with large seeds were limited by aboveground constraints, namely, shading. Small-seeded, shade-tolerant species were limited by belowground constraints imposed by the Saccharum. Large-seeded, moderately to highly shade-tolerant species performed well in the Saccharum; we recommend a reforestation strategy that includes planting this last group.