Full Idea
I think that we should consider Berkeley as believing in trees; we should simply claim that he has false beliefs about what trees are.
Gist of Idea
Berkeley does believe in trees, but is confused about what trees are
Source
report of George Berkeley (The Principles of Human Knowledge [1710]) by Ross P. Cameron - Truthmakers, Realism and Ontology 'Realism'
Book Reference
'Being: Developments in Contemporary Metaphysics', ed/tr. Le Poidevin,R [CUP 2008], p.118
A Reaction
I can be realist about spots before my eyes, or a ringing in my ears, but be (quite sensibly) unsure about what they are, so Cameron's suggestion sounds plausible.