Trivially, one could synchronize around the entire tree.
However, if modified nodes are always copied, then the only node that matters is the root node (since there can never be a partially modified tree accessible from the root), which boils down to atomic update of a root node pointer. Should be very efficient.
There is, of course, a performance implication of node copying, but it only affects add/delete/replace performance - read access runs at full speed - and it is cache-friendly, as well.
However, if modified nodes are always copied, then the only node that matters is the root node (since there can never be a partially modified tree accessible from the root), which boils down to atomic update of a root node pointer. Should be very efficient.
There is, of course, a performance implication of node copying, but it only affects add/delete/replace performance - read access runs at full speed - and it is cache-friendly, as well.