Is the stain strictly from the tree water? Or did the water get a tree skirt or wrapping paper wet, which in turn bled color onto the carpet? If the later, can try a dye treatment, but we’ve never had much luck with those dye bleeds. If it’s just the water, search on the board for plant spills. There have been a thread or two about water from pots that people have commented on. Should be similar situation. Could be some tree sap involved too (for which you could try just hot water, a blood/protein remover, or TGP/Dry solvent). If it’s got a brown/yellowish color, may want to try wood stain remover.