  We study the transition in dimensionality of a three-dimensional
magnetohydrodynamic flow forced only mechanically, when the strength of a
magnetic guiding field is gradually increased. We use numerical simulations to
consider cases in which the mechanical forcing injects (or not) helicity in the
flow. As the guiding field is increased, the strength of the magnetic field
fluctuations decrease as a power law of the guiding field intensity. We show
that for strong enough guiding fields, the helical magnetohydrodynamic flow can
become almost two-dimensional. In this case, the mechanical energy can undergo
a process compatible with an inverse cascade, being transferred preferentially
towards scales larger than the forcing scale. The presence of helicity changes
the spectral scaling of the small magnetic field fluctuations, and affects the
statistics of the velocity field and of the velocity gradients. Moreover, at
small scales the dynamics of the flow becomes dominated by a direct cascade of
helicity, which can be used to derive scaling laws for the velocity field.
