  The spectrum of oscillating compact objects can be considerably altered in
alternative theories of gravity. In particular, it may be enriched by modes
with no counterpart in general relativity, tied to the dynamics of additional
degrees of freedom generically present in these theories. Detection of these
modes, e.g. in the gravitational-wave signal from a binary compact object
coalescence, could provide a powerful tool to probe the underlying theory of
gravity. To access the potential of such a detection, it is crucial to
understand the linear and nonlinear spectral features of dynamically formed,
oscillating compact objects in alternative theories of gravity. As a step
towards that goal, in this work we present a suite of 1+1 numerical relativity
simulations of neutron stars in scalar-tensor theories, we carefully analyze
the spectrum of stellar pulsations, and we compare results with expectations
from linear perturbation theory. This allows us to build intuition for the case
of binary neutron star mergers. Additionally, the models investigated in this
work are representatives of two broad classes, in which the scalar field
couples either strongly or weakly with the fluid. The distinct phenomenology of
the nonlinear dynamics that we identify for each class of models, may find
counterparts also in other alternative theories of gravity.
