Variation in growth responses of neotropical pioneers to simulated forest gaps

1. One proposed mechanism by which tree species coexist is through partitioning gradients of light availability. We performed a pot experiment in which seedlings of 12 pioneer species were established in 30% light, then transplanted to six light treatments designed to simulate natural gaps ranging from 25 m² (≈10% full sun) to 800 m² (60% full sun). Plants were harvested after 56-117 days, and comparisons were made of allocation patterns and growth and carbon assimilation rates. 2. Species varied strongly in their maximum relative growth rate (RGR, range 15.4-83.6 mg g^-1 day^-1). However, we found little evidence for gap-size partitioning based on growth rate, as species RGR in large and small simulated gaps was strongly correlated (r = 0.83, P < 0.001). 3. Species differences in growth reflected variation in both physiology and allocation. Net assimilation rate was a strong determinant of RGR across all simulated gap sizes (r² = 0.60-0.71, P < 0.001). Leaf area ratio was a poor predictor of growth rate under all gap sizes (r² = 0.04-0.08, NS). 4. The maximum rate of net C assimilation (A_max) increased significantly with simulated gap size for all but one pioneer species, but only when measured on a per area basis. Among species variation in A_max was only weakly related to RGR. Foliar nitrogen concentration varied widely among species (range 2.2-4.7% dry mass), but was only weakly correlated with RGR (r² = 0.04-0.30). 5. Previous growth analyses of tropical seedlings have identified both specific leaf area (SLA) and seed mass as key traits correlated with growth rate. Although SLA varied twofold and seed mass more than a thousand-fold among the pioneer species in this study, neither trait was significantly correlated with among-species variation in RGR. Although these traits underlie major differences in life history between shade-tolerant and pioneer species, they contribute little to variation in growth performance within the pioneer functional group.