  I present a simple phenomenological model for the observed linear scaling of
the stellar mass in old globular clusters (GCs) with $z=0$ halo mass in which
the stellar mass in GCs scales linearly with progenitor halo mass at $z=6$
above a minimum halo mass for GC formation. This model reproduces the observed
$M_{\rm GCs}-M_{\rm halo}$ relation at $z=0$ and results in a prediction for
the minimum halo mass at $z=6$ required for hosting one GC: $M_{\rm
min}(z=6)=1.07 \times 10^9\,M_{\odot}$. Translated to $z=0$, the mean threshold
mass is $M_{\rm halo}(z=0) \approx 2\times 10^{10}\,M_{\odot}$. I explore the
observability of GCs in the reionization era and their contribution to cosmic
reionization, both of which depend sensitively on the (unknown) ratio of GC
birth mass to present-day stellar mass, $\xi$. Based on current detections of
$z \gtrsim 6$ objects with $M_{1500} < -17$, values of $\xi > 10$ are strongly
disfavored; this, in turn, has potentially important implications for GC
formation scenarios. Even for low values of $\xi$, some observed high-$z$
galaxies may actually be GCs, complicating estimates of reionization-era galaxy
ultraviolet luminosity functions and constraints on dark matter models. GCs are
likely important reionization sources if $5 \lesssim \xi \lesssim 10$. I also
explore predictions for the fraction of accreted versus in situ GCs in the
local Universe and for descendants of systems at the halo mass threshold of GC
formation (dwarf galaxies). An appealing feature of the model presented here is
the ability to make predictions for GC properties based solely on dark matter
halo merger trees.
