There was a historically-significant hole on the White House North Lawn that needed to be filled, as it had been a year before him by George W. Bush and by another president more than a century before. W.'s tree, a Scarlet Oak, "did not take," and so, following White House tradition, Obama was called on to replace it.
From the pool report:
"This is a nice looking tree, don't you think?" Obama said as he approached
the scene, accompanied by Retired Rear Admiral Stephen W. Rochon, chief
usher of the White House, about 5:30 p.m.
"We've got to get a couple, do a little work here," Obama said, reaching
for a ceremonial shovel. Nine times he plunged it into the topsoil.
In a minute, he was done, leaving a mostly empty hole but fulfilling an
obligation of planting his first tree on the White House grounds.
"This a little easier than it should be," Obama joked.