  We study the complexity and expressive power of conjunctive queries over
unranked labeled trees represented using a variety of structure relations such
as ``child'', ``descendant'', and ``following'' as well as unary relations for
node labels. We establish a framework for characterizing structures
representing trees for which conjunctive queries can be evaluated efficiently.
Then we completely chart the tractability frontier of the problem and establish
a dichotomy theorem for our axis relations, i.e., we find all subset-maximal
sets of axes for which query evaluation is in polynomial time and show that for
all other cases, query evaluation is NP-complete. All polynomial-time results
are obtained immediately using the proof techniques from our framework.
Finally, we study the expressiveness of conjunctive queries over trees and show
that for each conjunctive query, there is an equivalent acyclic positive query
(i.e., a set of acyclic conjunctive queries), but that in general this query is
not of polynomial size.
