Rainforest trees experience understory to canopy light conditions, representing a larger range than typically received by shrubs. Higher irradiances at maturity might result in a broader range of physiological plasticity and higher photosynthetic rates in high light for trees over comparable shrub species. Comparative studies tested hypotheses that physiological plasticity and photosynthetic capacity are related to the range of conditions encountered by particular growth forms. The trees Miconia affinis DeCandolle and M. multispicata Naudin and the shrubs M. gracilis Triana and M. nervosa (Smith) Triana (Melastomataceae) have overlapping habitat distributions in the study area of lowland tropical rainforest in Costa Rica.;Experiments and a survey of naturally occurring individuals examined physiological responses across a range of light conditions, utilizing gas exchange measurements of photosynthesis. Shadehouse studies revealed higher whole-shoot photosynthetic and respiration rates for tree seedlings. Measurements of seedling daily carbon gain were positively correlated with maximum photosynthetic rates, but poorly correlated with daily irradiance.;Photosynthetic rates were positively correlated with light availability, but photosynthetic plasticity of these species appears limited to the range below 20% of full sun. Field light conditions were estimated from hemispherical canopy photograph analysis, and the strongest correlations of physiological characteristics were found for estimates of indirect light availability. The biochemical capacity for CO;Strong growth form differences were not observed for leaf-level responses to experimental and field light conditions. The species showed a high degree of plasticity in photosynthetic capacity and nitrogen partitioning characteristics, and similar patterns in the biochemical components of CO
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