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Forest productivity is determined by the amount of carbon fixed through photosynthesis, the carbon lost through respiration, and the allocation of carbon to above-ground and below-ground tree components. Forest growth is directly related to the amount of photosynthetically active radiation (PAR) intercepted by the tree canopy. Leaf area index(LAI), measured as the m2 of leaf surface area per m2 of ground surface areas, is a commonly used measure related to intercepted PAR in forests and is thus also directly related to forest productivity. Light interception and associated tree growth initially increases linearly as leaf area increases. The slope of the line relating leaf area and growth is growth efficiency, which varies with climate, species, genotype, soil properties, and silvicultural treatments. Growth efficiency tends to decline as leaf area increases due to self-shading of leaves in the lower canopy. Leaf area index determines the amount of PAR intercepted and growth efficiency determines the rate that intercepted PAR is converted into biomass.