A Christmas tree is no different from having a huge bunch of flowers in your house. The moment it is in a warm house it will start transpiring and draw water.
Anything up to a litre or more a day depending on the size of the tree.
If you have not been present when your tree is cut then, we advise you cut a slither off the bottom of the trunk as this quickly seals with resin and restricts water flow.
Lemonade or cut flower sachets can give the tree a sugar rush which will help in needle retention.
With underfloor heating, to avoid a disastreeous situation, do not put a tree on a floor which is under-heated. Warm air rises up and dries the underside of the needles which have no waxy protection.
If there is no alternative, then laying a blanket on the floor to deflect heat and lots of extra water will help.