Well, obviously it's going to have an effect. Deforestation is in my opinion one of the more dangerous issues we are having to tackle right now. Why? Besides the obvious oxygen thing, which is probably a non-issue, deforestaton causes other things like erosion and desertification. Cut down the trees in some areas, and say bye bye to organisms living there. Good soil? It will be leeched like a torrent.
Luckily, I think we are starting to see more industries that cut down trees, than wait a couple of years while planting new ones, then continue on. It's the fact that some people constantly do it without giving anything a chance to grow back is the problem, I think.