  Aims. We study how IMF sampling affects the ionizing flux and emission line
spectra of low mass stellar clusters. Methods. We performed 2 x 10^6 Monte
Carlo simulations of zero-age solar-metallicity stellar clusters covering the
20 - 10^6 Mo mass range. We study the distribution of cluster stellar masses,
Mclus, ionizing fluxes, Q(H0), and effective temperatures, Tclus. We compute
photoionization models that broadly describe the results of the simulations and
compare them with photoionization grids. Results. Our main results are: (a) A
large number of low mass clusters (80% for Mclus = 100 Mo) are unable to form
an H ii region. (b) There are a few overluminous stellar clusters that form H
ii regions. These overluminous clusters preserve statistically the mean value
of <Q(H0)> obtained by synthesis models, but the mean value cannot be used as a
description of particular clusters. (c) The ionizing continuum of clusters with
Mclus < 10^4 Mo is more accurately described by an individual star with
self-consistent effective temperature(T*) and Q(H0) than by the ensemble of
stars (or a cluster Tclus) produced by synthesis models. (d)Photoionization
grids of stellar clusters can not be used to derive the global properties of
low mass clusters. Conclusions. Although variations in the upper mass limit,
mup, of the IMF would reproduce the effects of IMF sampling, we find that an ad
hoc law that relates mup to Mclus in the modelling of stellar clusters is
useless, since: (a) it does not cover the whole range of possible cases, and
(b) the modelling of stellar clusters with an IMF is motivated by the need to
derive the global properties of the cluster: however, in clusters affected by
sampling effects we have no access to global information of the cluster but
only particular information about a few individual stars.
