Isn't this a great and versatile little tool? Love it!
This farm is 74.75% efficient; that is, 74 of the 99 squares are trees with the potential to actively grow and no downtime at night.
The addition of 4 recessed wall torches adds 4 additional trees to the design, although it technically lowers space efficiency to 72.9%, since two open spaces must be left next to the torches at the north and south (top and bottom) of the design to allow light to flow diagonally. Another alternative uses 6 recessed torches and no open space, resulting in an efficiency of 74.29%.
Although the extra recessed torches decrease efficiency, they will, of course, increase your wood harvest for small amount of effort. Then again, so would any expansion of the farm's size. I mean, duh.
This small fish-shaped tree farm is designed to provide the farmer with a regular stack of 64 wood blocks (which I estimate requires 13-15 trees; this design includes 18 for good measure). It is small, space efficient, and aesthetically appealing. Note that, as usual, the ceiling should be 8 squares above ground level to keep all of your wood within easy reach. I recommend constructing the exterior using colored wool in a colorful aquatic theme.
Nothing fancy or clever here. Note that a single torch should be set on a floating dirt block above the water. From ground level, the dirt block should be 2 squares up with the torch on top.