  We study algebraic synchronization trees, i.e., initial solutions of
algebraic recursion schemes over the continuous categorical algebra of
synchronization trees. In particular, we investigate the relative expressive
power of algebraic recursion schemes over two signatures, which are based on
those for Basic CCS and Basic Process Algebra, as a means for defining
synchronization trees up to isomorphism as well as modulo bisimilarity and
language equivalence. The expressiveness of algebraic recursion schemes is also
compared to that of the low levels in Caucal's pushdown hierarchy.
