  A tree tensor network variational method is proposed to simulate quantum
many-body systems with global symmetries where the optimization is reduced to
individual charge configurations. A computational scheme is presented, how to
extract the entanglement spectra in a bipartite splitting of a loopless tensor
network across multiple links of the network, by constructing a matrix product
operator for the reduced density operator and simulating its eigenstates. The
entanglement spectra of 2 x L, 3 x L and 4 x L with either open or periodic
boundary conditions on the rungs are studied using the presented methods, where
it is found that the entanglement spectrum depends not only on the subsystem
but also on the boundaries between the subsystems.
