Melania Trump made no such decision....it’s the GROUNDS KEEPER...or the Park Service.
You are right that she didn’t just make the decision without it being recommended to her, but she would have to sign off on it as the upkeep of the grounds and the WH fall under the responsibilities of the FLOTUS. The grounds keepers can’t just uproot and change the grounds of the WH without authorization.
“First Lady Melania Trump gave the final order to fell the so-called Jackson Magnolia, which the Democratic seventh president planted in memory of his late wife, Rachel, following the divisive 1828 election that Jackson blamed for her death, the station reported.
The FLOTUS made the decision following a recommendation from the US National Arboretum, which found “the overall architecture and structure of the tree is greatly compromised and the tree is completely dependent on the artificial support.”
https://nypost.com/2017/12/26/white-house-to-cut-down-200-year-old-tree/