Elvish Branchbender

• WWizards of the Coast2007-10-01
  Official Ruling
  The value of X is determined when the ability resolves. It won’t change later, even if the number of Elves you control changes.
• WWizards of the Coast2007-10-01
  Official Ruling
  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.
• WWizards of the Coast2008-08-01
  Official Ruling
  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.

Rulings via Scryfall