  The mechanism of hadronic interactions at very high energies is still
unclear. Available accelerator data constrain weakly the forward rapidity
region which determines the development of atmospheric showers. This ignorance
is one of the main sources of uncertainty in the determination of the energy
and composition of the primary in hadron-induced atmospheric showers. In this
paper we examine the effect on the shower development of two kinds of
collective effects in high-energy hadronic interactions which modify the
production of secondary particles. The first mechanism, modeled as string
fusion, affects strongly the central rapidity region but only slightly the
forward region and is shown to have very little effect on the shower
development. The second mechanism implies a very strong stopping; it affects
modestly the profile of shower maximum but broadens considerably the number
distribution of muons at ground. For the latter mechanism, the development of
air showers is faster mimicking a heavier projectile. On the other hand, the
number of muons at ground is lowered, resembling a shower generated by a
lighter primary.
