  We evaluate the effects of environment and stellar mass on galaxy properties
at 0.85 < z < 1.20 using a 3.6um-selected spectroscopic sample of 797 cluster
and field galaxies drawn from the GCLASS survey. We confirm that for galaxies
with LogM* > 9.3 the well-known correlations between environment and properties
such as star-forming fraction (f_SF), SFR, SSFR, D(4000), and color are already
in place at z ~ 1. We separate the effects of environment and stellar mass on
galaxies by comparing the properties of star-forming and quiescent galaxies at
fixed environment, and fixed stellar mass. The SSFR of star-forming galaxies at
fixed environment is correlated with stellar mass; however, at fixed stellar
mass it is independent of environment. The same trend exists for the D(4000)
measures of both the star-forming and quiescent galaxies and shows that their
properties are determined primarily by their stellar mass, not by their
environment. Instead, it appears that environment's primary role is to control
the fraction of star-forming galaxies. Using the spectra we identify candidate
poststarburst galaxies and find that those with 9.3 < LogM* < 10.7 are 3.1 +/-
1.1 times more common in high-density regions compared to low-density regions.
The clear association of poststarbursts with high-density regions as well as
the lack of a correlation between the SSFRs and D(4000)s of star-forming
galaxies with their environment suggests that at z ~ 1 the
environmental-quenching timescale must be rapid. Lastly, we construct a simple
quenching model which demonstrates that the lack of a correlation between the
D(4000) of quiescent galaxies and their environment results naturally if self
quenching dominates over environmental quenching at z > 1, or if the evolution
of the self-quenching rate mirrors the evolution of the environmental-quenching
rate at z > 1, regardless of which dominates.
