Does anyone know or has experiences of how to discriminate soil, Oak tree, and residues of harvested rainfed cereals-wheat (cultivated in the open areas between Oak trees) in a mixed dryland forest ecosystem based on the MODIS products such as GPP, NDVI, LAI, and fPAR? Is there a specific index to do this discrimination? Is there a proved "False Color Composition" to distinguish these three elements?

Our problem is that both soil surface and cereal residues are dry in June (North hemisphere) and not easy to distinguish them. Moreover, when the Oak trees and cereals are green and healthy, we need to define a threshold NDVI (or another index) to distinguish the NDVI and GPP of cereal and Oak trees. The goal is to define a threshold for NDVI above which shows the Oak tree GPP and below that it shows the soil section. We need to make a correlation between GPP and NDVI (excluding the soil-induced NDVI). In June, we know that we have only Oak tree (green leaf) and soil (+residue) and obviously those open areas with no tree should have GPP=0 but it is not so in the MODIS products and we need to set a threshold value of NDVI (or a similar index) below which the GPP is set to 0. In addition, in May we have still some high NDVI in the open area between trees that show cereals might be still green and active in GPP. So, how to distinguish green cereals with the green Oak trees to find their associated GPP?

So, any hint and clue is appreciated.

Regards

Seyed