Growth Responses of Tropical Shrubs to Treefall Gap Environments

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Abstract
To investigate the effects of differences in light and nutrient availability on growth, we planted seven species of shrubs in two genera, Miconia (Melastomataceae) and Piper (Piperaceae), into the centers, edges, and adjacent forest understory of four natural treefall gaps (275—3355 m2) in the tropical premontane rain forest of Costa Rica. We used rooted cutting of species typical of forest understory environments on the one hand and large clearings or disturbed areas on the other. We also compared growth rates of three Miconia species grown in shade houses under 2, 20, and 40% full sunlight. 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 (.04—2%), but varied among similar microhabitats from different sites. Relative stem growth rates (RGRs) 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 faster at 20% full sunlight and light—demanding species grew faster at 40% full sunlight. We found no evident of a trade—off between growth and foliar phenolic concentration in these species.