He is Groot.
Groot has been killed and regrown before, but in the past, it always happened as a proportionally smaller version of himself. (I believe this is from Guardians of the Galaxy #1 (May 2008), but I could be wrong. Definitely that run, anyway.)
In Guardians of the Galaxy (2014), and especially Guardians of the Galaxy vol. 2 (2017), the new Groot, aka "Baby Groot", has child-like proportions and is considered a new character.
This is also the case in the new series, I Am Groot, out this month.