  We study the evolution of a wave packet in a nonlinear Schr\"odinger lattice
equation subject to a dc bias. In the absence of nonlinearity all normal modes
are spatially localized giving rise to a Stark ladder with an equidistant
eigenvalue spectrum and Bloch oscillations. Nonlinearity induces frequency
shifts and mode-mode interactions and destroys localization. With increasing
strength of nonlinearity we observe: (I) localization as a transient, with
subsequent subdiffusion (weak mode-mode interactions); (II) immediate
subdiffusion (strong mode-mode interactions); (III) single site trapping as a
transient, with subsequent explosive spreading, followed by subdiffusion. For
single mode excitations and weak nonlinearities stability intervals are
predicted and observed upon variation of the dc bias strength, which affect the
short and long time dynamics.
