Remote Sensing Analysis of High-Density Canopy fAPAR Considering the Absorption Ratio of Senescent Leaves

The fraction of absorbed photosynthetically active radiation (fAPAR) is an important biophysical characteristic in models of vegetation productivity. Synoptic estimation of fAPAR has been performed by using Normalized Difference Vegetation Index(NDVI) as a linear proxy of fAPAR. However, in high-density canopies, the sensitivity of NDVI to fAPAR is reduced. The NDVI/fAPAR relationship is analyzed in high-density rice in this paper. In addition, we combine the absorption ratio of senescent leaves and green leaves in the photosynthetically active radiation (PAR) region, re-determines the meaning of green fAPAR, and analyzes the sensitivity of green fAPAR under different nitrogen treatments. Moreover, other more effective alternative vegetation indices such as Wide Dynamic Range Vegetation Index (WDRVI) and absorption coefficient of PAR (αPAR) by remote sensing are also utilized to evaluate changes in green fAPAR.

1. INTRODUCTION

Vegetation is an important component of terrestrial ecosystems. Quantifying the biophysical properties of terrestrial vegetation and their variation through time is important for a rapid and accurate assessment of the status of the vegetation and its responses to changing environmental conditions especially in agriculture. As a key biophysical parameter, the faction of absorbed photosynthetically active radiation (fAPAR) is defined as the fraction of photosynthetically active radiation (PAR) absorbed by vegetation. Which has been widely used for crop growth monitoring and yield prediction, and is also an important parameter to assess primary productivity [1], [2]. Numerous studies [3], [4] found that under specified canopy reflectance properties, fAPAR can be estimated remotely, using the Normalized Difference Vegetation Index (NDVI =(ρ_NIR − ρ_red)/(ρ_NIR + ρ_red), where ρ_NIR and ρ_red are reflectances in the near-infrared(NIR) and the red spectral regions, respectively. Nevertheless, some studies [5], [6] underscored the fact that the linear relationship between fAPAR and NDVI is an approximation, and it is only valid during the growing stage. The main reason may be that the interception of light by the aging leaves of the canopy affects the actual photosynthetic absorption of green vegetation [7]. Since only the green components of the canopy are used for photosynthesis, some studies calculate the fAPAR of the green part(fAPAR_green) through the proportion of green leaf area index (GLAI) [5]. However, this calculation method does not take into account the difference in absorption in the PAR region between aging leaves and green leaves. Which may cause significant errors in high-density canopies. In this paper, we evaluate the NDVI vs. fAPAR_green relationship in high-density canopies (e.g. rice) with different nitrogen treatments. The definition of green fAPAR is improved by considering the absorption ratio of senescent leaves to green leaves in the PAR region, and the sensitivity of green fAPAR in high-density rice canopy is analyzed. Finally, this paper summarizes vegetation indices (VIs) that are more suitable for evaluating green fAPAR changes in high-density canopies.