Patterns of light distribution, leaf area index (LAI), and canopy photosynthesis were studied in Metrosideros polymorpha forests at both ends of a nutrient and substrate age gradient in the Hawaiian Islands. The two sites differ in the age of the underlying volcanic substrate, and in being limited by nitrogen (N) in the young site (Thurston, Hawaii) and phosphorus (P) in the oldest site (Kokee, Kauai). Long-term fertilization with nitrogen (N), phosphorous (P) and N+P at both sites has resulted in leaf and canopy level changes. Light transmission through the canopy was lower in the N-addition plot than in the control plot. Increased LAI was observed at the N-amended plot, also suggesting greater light attenuation with increased Í availability. While light intensity was not measured in the old site, LAI was much lower in the subcanopy and near ground levels than in the young site. Furthermore, the effects of nutrient amendments on LAI at the old site were not as large as those observed in the young site. Responses of net CO₂ assimilation (A) to fertilization were different at the young, and old sites. Instantaneous values of Ë remained constant across all plots at the young site, but significantly increased with nutrient amendments at the old site. In contrast, calculated values of canopy carbon gain (annual carbon gain * LAI) at the young site more than doubled with fertilization. The increase in canopy carbon gain at the young site was the result of larger leaves rather than higher photosynthetic rates. Variation on leaf area index and in light distribution was correlated to physiological and morphological traits of M. polymorpha dominated rain forests.
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