Relationship between woody biomass and PAR conversion efficiency for estimating net primary production from NDVI

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Abstract
Terrestrial net primary production (NPP) may be determined from remotely-sensed vegetation indices by estimating the amount of photosynthetically active radiation (PAR) absorbed by vegetation. Studies from the literature were used to determine the upper limit of the PAR conversion efficiency (ε) as a function of woody biomass for forest vegetation. Without climatic or other limitations, the upper limit was about 3·5gMJ-1 of absorbed PAR at very low values of stem biomass. Because of increased maintenance respiration with increasing woody biomass, the upper limit decreased to 2·0g MJ-1. Therefore, global estimates of NPP based on vegetation indices should include a classification among established forest, young forest and non-forest ecosystems to account for differences in ε.