Effect of senescent leaves on NDVI-based estimates of fAPAR: experimental and modelling evidences
Spectral indices from remotely sensed data, such as the Normalized Difference Vegetation Index (NDVI), are often used to estimate biophysical characteristics of vegetation. The objective of this study is to evaluate the effect of senescent leaves on the estimation of the fraction of photosynthetically active radiation absorbed by the green elements of the canopy (fAPARg) from NDVI measurements. An experiment was conducted under controlled conditions over grass canopies. Both NDVI and fAPARg were measured when the cover fraction by senescent leaves was changed. The results demonstrated that the effect of senescent leaves was significant on NDVI values. A similar effect was observed on the fAPARg values. In these conditions, simple models were developed to relate the fAPARg to NDVI values when the cover fractions of the senescent leaves vary. When combining these models, a linear relationship was found between NDVI and fAPARg, which was in good agreement with our experimental observations. Complementary radiative transfer model simulations were then run and confirmed these results. In addition, the vertical and horizontal distribution of the senescent leaves was investigated. They show that the relationship between NDVI and fAPARg was only marginally affected by the distribution of the senescent leaves.