Relationship between fraction of radiation absorbed by photosynthesizing maize and soybean canopies and NDVI from remotely sensed data taken at close range and from MODIS 250m resolution data

The fraction of incident photosynthetically active radiation absorbed by the photosynthesizing tissue in a canopy (fAPAR) is a key variable in the assessment of vegetation productivity. It also plays tremendous role in accurate retrieval of light use efficiency, which is essential for assessing vegetation health status. The main goal of this work was to study in detail relationships of fAPAR absorbed by photosynthetically active vegetation (fAPAR_green) and Normalized Difference Vegetation Index (NDVI) for two crops with contrasting leaf structures (C3 vs. C4) and canopy architectures, using close range (6m above the canopy) radiometric data and daily MODIS data taken during eight growing seasons over three irrigated and rainfed maize and soybean sites. Our specific goal was to understand differences in fAPAR_green/NDVI relationship when crop canopy was almost vertically homogeneous (with respect to leaf area and leaf chlorophyll content), as in vegetative stage, and vertically heterogeneous as in reproduction stage. Firstly, we established fAPAR_green/NDVI relationships for NDVI, taken at close range, and assessed noise equivalent of fAPAR_green estimation by NDVI, and then we established relationships for NDVI retrieved from daily MODIS 250m data. Daily MODIS data illuminated fine details of this relationship and detected effects of canopy heterogeneity on fAPAR_green/NDVI relationship. In vegetative stage, the fAPAR/NDVI relationships for contrasting in leaf structures and canopy architectures crops were almost linear allowing accurate estimation of fAPAR_green as it is below 0.7. However, very different fAPAR_green/NDVI relationships in reproductive stages for both crops were observed, showing that canopy architecture and leaf structure greatly affect the relationship as leaf chlorophyll content changes and vertical distribution of chlorophyll content and green LAI inside the canopy becomes heterogeneous. We have found fine details of the fAPAR_green/NDVI relationships with two types of hysteresis that prevent the use of a single relationship for fAPAR_green estimation by NDVI over the whole growing season and suggested mechanisms for each type of hysteresis that should be further studied using radiative transfer models.