There's a little more to this story than just Lillian giving it to Disneyland.
Walt found the tree for sale on a trip to Colorado. He really liked it so he purchased it, and later in what can only be called a "gag gift"...presented it to Lillian on their 31st Wedding Anniversary on July 13th, 1956.
Well, Lillian...not wanting anything to do with this beast...came up with her own way to pawn it off...so she in turn gifted it to Disneyland later the next year.
Here's a shot of Lillian and Walt at the tree in what can only be speculated as them smiling at each other acknowledging their own "inside joke" about the tree.
Don't know which of the two ended up the winner in this exchanging gag. Something tells me Walt wanted the thing for Disneyland all along.