Aged spruce, cut down the center, but not yet split apart. Looks like a piece of firewood. And if things don't go right, it still could be!
Here's the two pieces of the top, now split. The process is to make perfectly square edges at the joint, using a long, sharp plane.
The top glued together. Here I used a rub joint -- heat the wood with a hair-dryer, apply the warm hide-glue, then rub the joint together until it 'catches'. Clamping on one end to avoid creating any tension. My geraniums, in the background, really want to be outside. I hope it warms up soon.
Joining the back is basically the same process. My jointer plane is partly hidden behind it here.
Here's the back, glued together. A rub-joint, as with the top, then held in clamps. I tried to line up the flame by planing away a bit of the joint prior to gluing, then doing an eyeball match. It's tough, because the flame is not consistently vertical, so it may not match up at the height of the arching anyway. I don't worry about it too much.