GROWTH-RESPONSES OF TROPICAL SHRUBS TO TREEFALL GAP ENVIRONMENTS

The authors planted 7 species of shrubs in 2 genera, Miconia (Melastomataceae) and Piper (Piperaceae) into the centers, edges and adjacent forest understory of four natural treefall gaps (275-335 m2) in the tropical premontane rain forest of Costa Rica. Both light and nutrient availability in newly formed gaps of these sizes were strongly buffered by the canopy and root systems of the surrounding forest. Total incident radiation was higher in gap centers (9-23% full sunlight) than in gap-forest edges (3-11%) or under intact forest canopy (0.4-2%), but varied among similar microhabitats from different sites. Relative stem growth rates of all field-grown plants were significantly greater in gap centers than at edges or beneath forest understories. Fertilization did not significantly affect growth rate in any light environment. Light appears to be the most critical resource limiting growth at these gap sizes. In general, shade-tolerant species were less plastic than light-demanding species, but at these gap sizes grew as fast or faster in the gap centers. In shade-houses, the shade-tolerant species grew fastest at 20% full sunlight and light-demanding species grew fastest at 40% full sunlight. There was no evidence of a trade-off between growth and foliar phenolic concentration. -from Authors