It's that time of year - the festive jumpers are going on, the lights are going up, and we're ready to decorate our Christmas trees. But when it comes to choosing your fir, what do you go for - real or fake? Perhaps you have a trusty old favourite that comes down from the attic each year? Or maybe going to choose a real tree gets you into the festive spirit? But have you ever wondered which is best environmentally? We chat to forest scientist Henry Hung about which trees are the greenest (not literally!).