The treble clef defines the pitch range usually played by violins or sung by female voices as the tonal scope of a stave. This also corresponds to the pitch range of the ukulele (with the exception of the baritone). Therefore, regardless of the tuning of the instrument, pieces for the ukulele are written in the treble clef.
Notes in this system below middle C pertain to the small octave. They can be played only on the 4th string of an ukulele in linear tuning or on a baritone ukulele.