It is thought that early Pagans in the north brought evergreen boughs into their homes and decorated them at the Winter Solstice. Some tales suggest this was a superstitious thing... providing a home for the wood nymphs through the cold winter so they wouldn’t play tricks on them come summer and mess up hunting, growing, etc. Others say it was just part of the celebration acknowledging the seasons. The Yule log in the north and laurel wreaths in the south also came from this seasonal celebration. Also in the south (Rome), where Pagans celebrated Saturnalia, houses might be decorated with evergreen clippings.
Honestly, Pagans could be credited with inventing the concept of the wreath more than the Christmas tree. But some Pagans in the south of Europe did decorate living trees with bits of metal and replicas of their Pagan Gods. It was this practice that may have frightened some early Christians, so there was intense opposition to the Christmas tree, strangely, mostly in northern Europe, in centuries past. They were outlawed in England for some time. But Martin Luther, German founder of the Lutheran Church, found a way to make the Christmas tree a symbol of Christian celebration, and so Germans began to once again enjoy the smell of evergreen in their homes at the Yuletide... or Christ Mass, or Christmas.
Christian holidays tend to fall at the same times of the seasons as Pagan celebrations. Christmas fell at the same time as Saturnalia (Roman holiday) and is close to the Winter Solstice, for instance. Easter is named for the Pagan Goddess of Fertility, Eostre. She was the symbol of spring time, when the ewes were lambing, chickens and other birds were laying, rabbits multiplied with extra zeal, etc. It is these symbols that were carried over into Christian celebrations, which fall near the Spring Equinox. It is thought this was done to lure Pagans into Christian churches to hear the stories of Christ in an environment that wasn’t totally strange to them. Either that, or Christians were just copy-cats, eh?
Anyway, in Victorian times, Queen Vic married Prince Albert, a German, and Albert brought the Christmas tree to the English palace. Well, you can’t very well outlaw a custom in all the country when the head family is enjoying it, can you? So opposition to this Christmas custom slowly waned and Christmas trees enjoyed a resurgence in England as well. It was probably brought to North American by Hessiens (German soldiers) but there are many legends as to how the custom crossed the ocean and spread.
Segue to modern day: Christmas trees have seen many incarnations, from real trees, to plastic, to silvery foil and more. But the original custom of bringing an evergreen into the house for the holidays seems to remain a favourite. Occasionally some well-meaning environmental group, organization, publication... will object on environmental grounds. This is because, though they mean well, they haven’t informed themselves. There is no need to “save” real Christmas trees as if someone has schlepped into the boreal forest to cut them down. They haven’t. Christmas trees are grown on tree farms. They are an economically and environmentally viable, renewable crop resource that is actually good for the environment. I mean, fields upon fields of little evergreens can only be helping the Earth breathe.
Some people object to disposal methods when the holidays are over, but this is something that people need to address in their communities by being proactive in the political decisions being made in their community for waste management. There is NEVER any excuse to throw used Christmas trees in landfill. That’s just wrong. They are compostible, at the very least. If your community does not have a waste management strategy that deals responsibly with used Christmas trees... well, for one thing, you need to get more involved in your community and change that, but for another, you can ask a local farmer if they'll take trees to compost or you can compost your tree yourself, either by cutting or grinding it up and throwing it into your composter, or by putting it with your autumn leaves to compost. If you’re in a municipality that allows outdoor fires, just throw it into a corner of the yard till spring and there you go... fire wood. (Outdoors only... soft, sappy evergreen wood is not good for an indoor wood stove or chimney.) Or of course, if you're in a snowy region, you can always stand it up in a snow bank and hang orange peel in it for the birds until it falls apart of its own accord as Spring approaches. Then compost it. There are a number of responsible ways to deal with used Christmas trees.
