Learn English – the root morpheme of a word when ‘y’ turns into ‘ies’ or ‘ier’

Suppose I have to write out the root morpheme and the bound morpheme. Consider the word 'happier'.

I've seen some examples, for example here on the second slide: http://www.slideshare.net/mobile/fatmasima/morphology-son where the root is 'happy' and the bound morpheme is 'er'.

However, on another website (I forgot the link but I will post it if I find it), the example was 'prettier' and it mentioned that the root was 'pretti' and the bound morpheme was 'er'.

My question is, when y turns into ier or ies (for example supplies or prettier), what is the root and bound morpheme?