  We investigate the sensitivity to temperature and gravity of the strong
absorption features in the J- and K-band spectra of substellar objects. We
compare the spectra of giants and young M dwarfs (of low gravity) to field M
and L dwarfs (of high gravity) and to model spectra from the Lyon group. We
find that low-resolution spectra of M4 - M9 stars and young brown dwarfs at R ~
350 and S/N > 70 can determine the spectral type to a precision of pm 1
subtype, using the H2O and CO bands, and can measure the surface gravity to pm
0.5 dex, using the atomic lines of KI and NaI. This result points toward the
development of photometric spectral indices to separate low-mass members from
foreground and background objects in young clusters and associations. We also
emphasize the complexity of the interpretation of the empirical quantities
(e.g., spectral types) in terms of the physical variables (e.g., temperature,
opacities) in the cool atmospheres of young brown dwarfs.
