THE Urban Council's Green Hong Kong campaign is being highlighted in a stunning new advertisement featuring the bizarre imagery of ''morphing''. Made by Lintas Hong Kong, the 30-second ad drums home the message that trees are an integral, but too often overlooked, part of the environment. Devised to increase the awareness of the importance of trees among the city's concrete-jungle dwellers, the commercial features a tree which ''morphs'', or changes, into a variety of shapes. First it becomes a painting, then a parasol, a bird-house and oxygen tanks, to symbolise the life-sustaining function of trees. Finally it takes the form of a piggy bank to emphasise the Urban Council's investment in the environment. As the tree's form evolves, the actor standing close by reacts to the changes and gives a running commentary. ''Like most people, you probably don't give much thought to trees, except, perhaps, that they're good to look at. Or that they provide soothing shade from the searing sun,'' the actor says. ''But you'll be surprised to know that trees provide a home for most of our wildlife. And most importantly, they provide us with fresh air . . . which is why we're investing so heavily in them.''