Weed-Pruner Poplar rulings If you're the only player who controls any other creatures

[Zippy]

in magic gatherer it says about Weed-Pruner Poplar

Weed-Pruner Poplar (Morningtide) - Gatherer - Magic: The Gathering

4/1/2008 The ability is mandatory. If you're the only player who controls any other creatures, you must target one of your own creatures with the ability.

but what if my only other creatures have shroud?

[riolu]

if there are no creatures to target and like all ur creatures have shroud at beginning of turn then you cant target them and the ability gets countered

[Arix]

Slight correction - the ability doesn't get countered. It gets removed from the stack immediately after triggering. Same basic result most of the time, but being pedantic is kinda my thing.