Meanwhile, Emmanuel Macron held closed-door negotiations with President Trump over everything from the US involvement in Syria to the fate of the Iran nuclear accord, which he believes Trump will pull out from.
In 1991, Britain's Queen Elizabeth II helped to plant a little-leaf linden tree on the South Lawn after a storm damaged the one her father, King George VI, had planted in 1937.
In their first White House meeting last spring, Trump seemed to ignore photographers' prompts to give Merkel the traditional photo-op handshake.
The White House said more than 9,000 U.S. Marines died in the Battle of Belleau Wood in 1918 - and said the forest is a memorial site and important symbol of the sacrifice made to ensure peace and stability in Europe.
As President George H.W. Bush noted on another tree-planting occasion, in 1990, a number of White House trees have historic connections, including an oak tree planted next to the Oval Office by President Dwight D. Eisenhower.
Now the oak sapling is gone - at least temporarily.
But a few days later, the tree vanished.
In fact, the tree, from Belleau Wood in France where nearly 2,000 American soldiers died in a World War One battle, had been dug up not long after it was planted.
It was put in quarantine, USA and French officials told Reuters.
The official said both sides knew all along that the tree would later have to go into quarantine.
Reuters reported that the tree would be inspected for parasites, which could spread to other trees nearby if left unchecked.