The younger oaks to the right hint at a 'family tree' and the children they might expect to have.
In Room 35, there are several portraits of landowners, and just about all of them are shown against oak trees.
Looking at the painting, you might assume that the Milbankes had this oak on their land - therefore they must have been there for many generations. But Stubbs used the same tree in a portrait of the Pocklington family! So did he have a drawing of an actual tree that he had seen, and use it as a kind of studio prop?
This tree could be several hundred years old. It fits beautifully into the composition, sheltering and embracing the family and its newest member.