The oak tree by his grave has a sidewalk wrapping around it, didn't find any significant details on it. JFK has a dedicated oak tree along with others who requested trees at there headstones.
Under the oak trees is where the bugler stands according to the creed below.
THE BUGLER OF ARLINGTON
I am the bugler of Arlington
But this honor belongs not to one man alone.
Today, I stand the watch.
Yesterday in our history, it was stood by the many who have gone before me.
Tomorrow in our future, others will carry on when I am gone,
Because this great nation of ours will always have a need
for paying tribute to its honored dead.
So today, I am the bugler of Arlington.
I stand alone with my thoughts
On the brow of a windswept hill
Beneath the swaying bows of a protective oak
Watching o'er our nation's dead.
Around me stand the living,
Beneath me lie the dead.
The open grave not far away
Leaves little to be said.
So, I stand alone with my thoughts today,
And find so much for which to be thankful.
Yes, I'm profoundly thankful
For these stones upon this hillside
Over which I daily play,
They tell a story of our country; For them I duly pray