Elvish Branchbender

Oracle Text

T: Until end of turn, target Forest becomes an X/X Treefolk creature in addition to its other types, where X is the number of Elves you control.

Card Rulings

10/1/2007 The value of X is determined when the ability resolves. It won’t change later, even if the number of Elves you control changes.
10/1/2007 If you control no Elves when the ability resolves, the Forest will become a 0/0 creature and be put into the graveyard. However, since Elvish Branchbender is itself an Elf, the Forest will usually be at least a 1/1.
8/1/2008 A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.