  We report the phenomenon of vibrational resonance in a single species and a
two species models of groundwater-dependent plant ecosystems with a biharmonic
oscillation (with two widely different frequencies \omega and \Omega, \Omega >>
\omega) of the water table depth. In these two systems, the response amplitude
of the species biomass shows multiple resonances with different mechanisms. The
resonance occurs at both low- and high-frequencies of the biharmonic force. In
the single species bistable system, the resonance occurs at discrete values of
the amplitude g of the high-frequency component of the water table.
Furthermore, the best synchronization of biomass and its carrying capacity with
the biharmonic force occurs at the resonance. In the two species excitable and
time-delay model, the response amplitude (Q) profile shows several plateau
regions of resonance, where the period of evolution of the species biomass
remains the same and the value of Q is inversely proportional to it. The
response amplitude is highly sensitive to the time-delay parameter \tau and
shows two distinct sequences of resonance intervals with a decreasing amplitude
with \tau.
