No such thing as a tree really (genetically speaking), trees are just a bunch of plants that converged on a similar niche but they don’t all share a common ancestor (some trees are more closely related to brocoli than other trees), lots of different things evolved into what we now call trees seperately from eachother so a bannana tree is as much a tree as any other tree.

Not only that but most of them are played down not up when compared to real life. The discworld is a lot less weird than real life.