This is a follow up on our earlier queries . We have notice this bulge on the tree and wonder if we should be concerned.
The growth appears to be a burl. Burls on trees are thought be caused by stress of some sort, i.e. insect damage, broken branches, severe weather, or other environmental distress, on the tree. The response of the tree to the stress is hyper growth of the cambrium layer, resulting in a burl containing the affected area of the tree.
Trees with burls are generally healthy, and can live for many years after the growth develops. However it is suggested that you contact an arborist to determine whether you should be concerned about the burl on your tree.