SAPLING SURVIVAL, GROWTH, AND RECRUITMENT - RELATIONSHIP TO CANOPY HEIGHT IN A NEOTROPICAL FOREST

Treefall gaps are thought to contribute to the diversity of plants in tropical forests by providing opportunities for niche differentiation in modes of regeneration. To examine this hypothesis, we studied the survival, diameter growth, and recruitment of saplings in > 100 species of woody plants in a 50-ha permanent plot of moist tropical forest on Barro Colorado Island, Panama, from 1982 to 1985. The performance of saplings in low-canopy sites (< 10 m) was compared to that of saplings in high-canopy sites (greater-than-or-equal-to 10 m), and performance of common species was compared to that of rare species. Of the 108 species for which all three parameters of performance were measured, 104 fell into four response groups, each with characteristic patterns of survival, growth, recruitment, and response to canopy height. Pioneers (six species) survived poorly in both canopy-height categories, and survivors grew rapidly in low-canopy sites. Sapling recruitment was skewed toward low-canopy sites. Understroy specialists (three species) survived well in high-canopy sites and poorly in low-canopy sites. They grew slowly and recruited poorly in both situations. Generalists (79 species) survived well and grew slowly in both canopy-height categories. Per-adult recruitment was usually low, and often skewed toward low-canopy sites. Poorly performing species (16 species) survived poorly, grew slowly, and recruited infrequently in both canopy-height categories. Most of the common (> 10 saplings/ha) species appeared to be generalists. Many rare (< 1 sapling/ha) or occasional (1-10 saplings/ha) species survived significantly (P less-than-or-equal-to .05) less well than the average survivorship of saplings, while many common species survived significantly better than average. Some rare or occasional species grew rapidly, either in low-canopy sites or in both canopy-height categories, while most common species grew slowly in both situations. Rare and occasional species had significantly more recruits per adult than did common species, but often this did not balance their higher mortality. Large differences in survival, growth, and recruitment between canopy-height categories were found only among rare and occasional species.