Elvish Branchbender

Creature — Elf Druid

: 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.

2/2

2007-10-01:
The value of X is determined when the ability resolves. It won’t change later, even if the number of Elves you control changes.
2007-10-01:
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.
2008-08-01:
A noncreature permanent that turns into a creature can attack, and its {T} 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.