Continental scale variability in vegetation reflectance and its relationship to canopy morphology

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Abstract
Remote sensor studies of terrestrial vegetation have recently begun to address continental-sized geographical areas. If quantitative relationships between satellite-derived spectral indices and vegetation parameters (e.g. biomass, leaf areas index (LAI), absorbed photosynthetically active radiation (APAR)) can be extrapolated to these very large areas, important contributions to models of global primary production and biogeochemical cycles would result. However, little is known of the consequences of extending locally-derived relationships to large areas. We have measured the spectral canopy reflectance, biomass and projected LAI of widely-dispersed plots of a North American coastal plant (salt marsh cord grass) in an attempt to identify potential impacts of continental-scale environmental variability on the assumptions underlying remote vegetation analysis. We found that important systematic changes in the canopy geometry and resultant near-infrared reflectance of this plant occurred within its 17° latitudinal range. Mean infrared canopy reflectances of canopies in the northern half of the range are nearly double those of the southern half despite almost identical total and live standing biomass in the two regions. The transition from one regional canopy type to the other occurs quite abruptly near the centre of the plant's latitudinal range. The difference appears to result from divergent canopy morphologies_northern canopies present greater horizontally project LAIs per unit biomass than southern canopies. Continental assessment of biomass using spectral indices would be severely affected by the observed differences. Use of spectral indices to assess more fundamental radiative transfer properties of the canopy, such as project LAI, is not subject to the observed latitudinal changes, suggesting that large area studies can more reliably address canopy parameters such as APAR. The observed canopy changes also have potential functional consequences for the plant which would be of considerable ecological interest, particularly if similar trends are found for other plants and regions.