Some interesting experiments were run by the guys of Myth Busters on this and it was determined by them that even with huge amounts of high powered lights the tree would never burst into flames as was shown and as is reported by about 10,000 fires nationwide between Dec 24th and Dec 25 each year. But what they also showed is that if there is one spark, the tree would burst into flame as was shown.
So the obvious question is what causes a spark and why is this small spark so lethal on a real tree vs. an artificial tree.
While the experiments on MythBusters test only one source of spark source, a melted wire from the heat of the bulbs, there are usually other possibilities that are far more probable than the melting of the wires. The most common source of a spark in tree lighting is due to a loose bulb that sparks as it makes and breaks contact in the socket. If your lights are non- blinking but flicker at times, they are loose and will generate a small spark in the socket when flickering.
Next- why does a pine tree ignite with a spark? I know the answer to that one well! A pine tree is loaded with pine sap and that pine sap with heat gives off pine oil which is volatile and flammable! The heat from the bulbs raises the temperature of the pine oil vapors and lowers the ignition point of the oil enough that the speed of the fire once started from the spark is much like a tree doused with gasoline!
Many years ago when I was still a kid, our neighbor's house burned to the ground on Christmas Eve due to a tree fire. Very sad. from that time on, we never had a real tree in the house and I forbid it today.
My daughter always complained about it and now, on her own, she has the real tree. It makes me nervous.
So if you want a real tree and want to be safe, here is my suggestion- DO NOT PUT any ELECTRICAL ON YOUR TREE!
Decorate the real tree with reflective tinsel and then from a nice safe distance spot a series of lights with rotating color wheel on the tree and light it up that way. Keep the tree wet with water spray and in a base pan and cool and the pine oil that will still evaporate will stay way below ignition temperature.