Pecan is a shade intolerant species. If leaves do not receive enough sunlight to remain photosynthetically active, the tree will shed them. Look at the photo and you'll see that the limb with yellow leaves is completely shaded by the limbs above. After the leaves turn yellow and fall from the tree, all water and nutrient flow through that branch stops and the branch begins to wither and die. Later, the entire limb will be shed from the tree, usually during harvest season with the aid of my trunk shaker.
A large amount of this natural self-pruning in a pecan orchard can be a indication that the trees are growing too close together, sunlight is limited by excessive shading, and the orchard needs thinning. I usually survey my orchard at this time of year to look for branches that have bright yellow leaves. That way I can make plans to remove trees in areas that would benefit from a tree thinning operation.
