EVAC: Zombie Apocalypse

What would you do if every tree on the planet grew over a kilometre tall overnight?
-Wait for the authorities to sort it out?
How about if the crown branches formed an interlocking canopy blocking out 95% of sunlight, plunging the Earth into permanent darkness?
-Purchase a chainsaw?
-Did I mention the ravenous zombie hordes? - Still buy a chainsaw, right?
I dare you to accompany seven year old Susan on her terrifying exodus for survival.