Ordinarily, I'd place these images in the image sharing section; but, will present them here to illustrate the problem. Here are two renders; the first using 'ambient occlusion'. The second using 'global illumination'.
The AO version seems to eliminate the banding issue while the GI version displays the banding issue at the lower left. The overall best image quality still lies with the GI version except for the banding.
Some borrowed elements are:
• FrankB's 'Simple Camoflage Surface' added to my surfaces.
• dandelO's 'PGS Evolution' grass surface on distant mountains.
• dandelO's 'Altocumulus' clouds.
• Walli's 'Dry Shrubs' and XFrog cactii.
Since I cannot find the reason for the banding issue, I'll use the 'sweep it under the rug' fix by just placing a single model of the gray/green bush directly in front of the cactus to hide the banding. Additionally, I'll tone down the color of the small straw colored bushes.
Here is the AO version.