  In this paper we prove a space lower bound of $n^{\Omega(k)}$ for
non-deterministic (syntactic) read-once branching programs ({\sc nrobp}s) on
functions expressible as {\sc cnf}s with treewidth at most $k$ of their primal
graphs. This lower bound rules out the possibility of fixed-parameter space
complexity of {\sc nrobp}s parameterized by $k$.
  We use lower bound for {\sc nrobp}s to obtain a quasi-polynomial separation
between Free Binary Decision Diagrams and Decision Decomposable Negation Normal
Forms, essentially matching the existing upper bound introduced by Beame et al.
and thus proving the tightness of the latter.
