For instance if I install the closure_tree gem the ancestry gem will still define acts_as_tree even though it is defined in closure_tree as well. This breaks the closure_tree gem definition of acts_as_tree. My guess is that ancestry is loaded before closure_tree and that is why the code breaks.
Unsure if this is still a problem for you but I just encountered this problem myself.
We were on version 1.2.0 of acts_as_tree and 1.3.0 of ancestry (pretty old project).
We found a combination which worked for us:
gem 'acts_as_tree', '~> 1.6.1'
gem 'ancestry', '~> 2.0.0'
Hope this helps you or anyone else in this predicament.
Similarly here. Ancestry 2.0.0 works with acts_as_tree, but 2.1.0 doesn't.