http://pastebin.com/11Bxj4JV

Minor modification to the built-in excavate script, but it goes upwards until it hits an air layer instead of downwards until bedrock. Additionally it goes forward one block first. So if you place it facing the left hand side of the base of a 2x2 fir tree or redwood, then "fell 2" will go into the tree and just cut down the wood. It returns down the column that it started in though so it does cut some leaves. Makes no attempt to re-plant the tree, it's easy enough for me to do that.

You can get it to cut down a large, sprawling tree by placing it at the corner of the canopy and, say, "fell 16" will probably chew up a whole Rubberwood tree. It follows the standard excavate pattern so it spends a lot of time sailing through air blocks on large trees.

Of course this is a terribly inefficient way to cut down a 2x2 tree. It would be much better to go up one side digging above and in front, and then down the other side digging below and in front. But I built it to cut down big sprawling trees, and the fact that I now use it mostly for 2x2 firs is just laziness. I need to stop growing firs though as the wood can't be used in some crafting recipes e.g. Piston.