That yew tree really seems like something out of Poltergeist, arewerite? It's personified in lines 61-64. It "grasps" the gravestones, while its "fibres" snake down under the earth and wrap around skulls and its roots cozy up to the bones of the dead. Freaky, right?
And as if we didn't get the picture already, Tennyson later describes the yew tree as "dark yew" (780).
The yew tree symbolizes a force of nature that is timeless and super-powerful compared to the fragility of humans, whose bones it stands over. Nature will eventually break down all things made by men, suggested by how the roots wrap around the gravestones.