Impact of three-dimensional vegetation structure on the canopy radiation regime.

摘要

The three-dimensional (3D) canopy structure determines the spatial distribution of intercepted solar radiation which drives various physiological and physical processes integral to the functioning of plants. Monitoring the 3D canopy structure has long been one of the main goals of vegetation remote sensing from space. The theory of radiative transfer in stochastic media provides the most logical linkage between satellite observations and the 3D canopy. Its potential for satellite remote sensing of vegetated surfaces has not been fully realized because of the lack of models of a canopy pair-correlation function that the stochastic radiative transfer equations (SRTE) require. This function defined as the probability of finding simultaneously phytoelements at two points provides a measure of the canopy structure over a wide range of scales. The objectives of this research are to (1) develop models of the pair-correlation function; (2) investigate the impact of 3D canopy structure and mixture of vegetation species on canopy radiation regime; and (3) separate the structural and radiometric components from canopy hyperspectral reflectances. A stationary Poisson point process was used to generate stochastic models of the 3D canopy and to derive associated pair-correlation functions. The theoretical and numerical analyses suggested that the spatial correlation between phytoelements was primarily responsible for effects of the canopy structure on canopy reflective and absorptive properties. Canopy reflectances predicted by the solutions of the SRTE compared well with field data. The SRTE was extended to account for mixture of spectrally different vegetation species within a satellite pixel. It was found that the linear mixture model underestimated while the turbid medium approach overestimated multiple interactions between species compared to the SRTE. A solid theoretical basis was developed for spectral invariant relationships reported in literature with an emphasis on their accuracies in describing the canopy shortwave radiative properties. The analysis of field data collected during a field campaign in Sweden supported the theoretical conclusions. The theory is essential to the remote sensing community as it allows us to extract structurally variant and spectrally invariant components from the measured hyperspectral canopy reflectances.