The final -a is a stem-forming suffix that changes to -e when certain enclitics follow it, e.g., the diminutive =la.  This changing vowel is called 'the ablaut(ing) vowel' by modern Dakotanists, in spite of a general protest among Indo-Euorpeanists that this isn't quite what they had in mind by ablaut.  I suspect the Siouanists are actually thinking of the Proto-Indo-European thematic stem-formant -e/o-, which, of course, does ablaut, like several other PIE stem-formants, though it isn't "the ablaut vowel."