Sasu is a modern diminutive of
Sakari, a Finnish form of
Zachary. Pihla is derived from "pihlaja", which means rowan tree in Finnish. Nuutti is a Finnish form of
Knut.
Yes, nature names are very popular in Finland :) Myrsky and Metsämansikka, however, are very rare among those names, but I think Myrsky has become a bit more common in the recent years.