The spell now checks if there are trees within the area once upon targeting. It doesn't check for trees during the cast time. So if you cast Sprout and want to follow up with Nature's Call, if the sprout trees disappear during the cast time, Call still gets cast and wasted.

The spell should periodically check for trees within the area and cancel if none are detected, just like how e.g. single-target spells which don't pierce spell immunity get canceled when the target turns spell immune during the cast time.