  The surface [C/N] ratios of evolved giants are strongly affected by the first
dredge-up (FDU) of nuclear-processed material from stellar cores. C and N also
have distinct nucleosynthetic origins and serve as diagnostics of mixing and
mass loss. We use subgiants to find strong trends in the birth [C/N] with
[Fe/H], which differ between the low-$\alpha$ and high-$\alpha$ populations. We
demonstrate that these birth trends have a strong impact on the surface
abundances after the FDU. This effect is neglected in current stellar models,
which use solar-scaled C and N. We map out the FDU as a function of
evolutionary state, mass, and composition using a large and precisely measured
asteroseismic dataset in first-ascent red giant branch (RGB) and core
He-burning, or red clump (RC), stars. We describe the domains where [C/N] is a
useful mass diagnostic and find that the RC complements the RGB and extends the
range of validity to higher mass. We find evidence for extra mixing on the RGB
below [Fe/H]= -0.4, matching literature results, for high-$\alpha$ giants, but
there is no clear evidence of mixing in the low-$\alpha$ giants. The predicted
signal of mass loss is weak and difficult to detect in our sample. We discuss
implications for stellar physics and stellar population applications.
