You people downvote this, but it really is closer to Lamarck's theory than Darwin's. Initially, it might look like the "arms race" type of evolution in which predator and prey adapt to each other's adaptations through natural selection. However, giraffes don't kill trees, and in fact contribute to their reproduction in some cases by transporting seeds from fruits. Thus, neither giraffe nor tree would become taller through natural selection. Lamarck's theory postulates that offspring inherit characteristics of their parents; his example was that giraffes had long necks because their parents stretched their necks a lot:

As the tree is growing taller despite no evolutionary need to do so, this more closely resembles Lamarck's theory.