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I study on the responses of three birch species to elevated CO2 at the Free Air CO2 Enrichment (FACE) system. Previous studies revealed that photosynthetic capacity and number of leaves was increased by elevated CO2. With an increase of leaf numbers at high CO2, incident light to forest floor is reduced. As leaf life span is influenced by light, nutrient condition, I hypothesized two ideas with regards to leaf life-span: 1) leaf fall starts earlier because the leaf under canopy cannot get light sufficiently and may show the compensation of limited light utilization, 2) leaves increase their life-span longer because leaves may increase photosynthetic capacity under elevated CO2. For clarifying these ideas, I have been observing leaf phenology, leaf nitrogen concentration and light condition of the three birches
