The current readme indicates that :adopt should be a valid orphan strategy. I get AncestryExceptions when attempting to specify this.
ruby 2.0.0p195 (2013-05-14 revision 40734) / Rails 3.2.13
Don't know whether this is a documentation or feature issue, but my expectation would have been that this would work.
2.0.0-p195 :001 > TreeNode.find(:first)
Ancestry::AncestryException: Invalid orphan strategy, valid ones are :rootify, :restrict and :destroy.
class TreeNode < ActiveRecord::Base
has_ancestry :orphan_strategy => :adopt
Solved my immediate issue by upgrading to ancestry 2.0.0