When reading through "The Dream of the Rood," no metaphors explain the tree's determination to hold Jesus as he dies. Instead, the rood (as the narrator of the text) uses personification. When charged with being the cross which holds Jesus' body, the tree (initially) refuses. As the tree comes to understand that this is what must happen, the tree "durst not against God's word / Bend down or break" (39-40).
In this sense, the tree is personified as being able to bend down. While one could interpret this as bending under the weight of Jesus, it can also be interpreted as the tree's refusal to bow down to his enemies. The tree is only standing tall because it is God's will.
This said, a metaphor does exist within the poem. When the rood speaks of itself as a symbol of healing (as the cross). This type of metaphor is called an absolute metaphor. An absolute metaphor is one which one term cannot readily be distinguished from the other. Essentially, the rood and the cross are one.
Also, the rood and Jesus suffer together. The rood states that "I underwent / Full many a dire experience on that hill" (59-60). IN this sense, metaphorically, the rood and Jesus are one. This acts as an extended metaphor (given the rood continues to suffer as Jesus did).