In this brief talk trees impacted adjacent wheat field yield all the way out to about 20m from the tree line. Approximately 15% yield reduction alone was likely from trees shading the wheat. The rest up to 85% close to the trees likely being as a result of soil moisture and root competition below 20cm.
This picture is an example of what can be achieved by managing existing trees that have been over harvested by man that still have large roots hidden below the surface. These trees are managed for regrowth. And the picture is from desert to regrowth in four years. The tree then created microclimates under the trees that increased yields.