Trees can serve as decoration for your prison or as a source of income (or both of course). The buying, planting, dismantling and selling of trees can be automated through Forestry.
As of Alpha 16 there are multiple tree types, randomly chosen when you plant a tree. Trees can be dismantled and the wood sold; if you have a Workshop the wood can be cut in to planks which sell for more money. Trees which have been cut down (dismantled) leave behind a stump which must be removed before another tree can be planted. Tree Stumps can be dumped by workers or left to decay after a short period of about a day (in-game time).
A forestry "room" will automate the process of tree planting and cutting but you will need gardeners to plant the trees and workers to cut them down.
Any tree planted manually outside of forestry will be considered decoration and will not be cut down unless you manually order them to be cut.
Tip: You can mass order the cutting of trees (such as large areas of tree clumps on a starting map) by using the dismantle object tool on the objects list and dragging it across all the trees that need to be cut. (this is much faster than individually selecting each tree)
Strategy: As of alpha 23 contraband can be thrown over walls (the prisoners request this by phone or when speaking with family). The contraband throwing range is 10 squares. One strategy to prevent prisoners obtaining thrown contraband is to create a double fenced prison. This potentially large path of land between the two fences makes for a useful forestry growing area (a few trees wide and as long as the fences) all the way around the prison. The prisoners have no need to enter forestry rooms so it can be marked staff only between the fences and they will not have the chance to retrieve thrown contraband.
Selling Wood and Planks
Wood is obtained by dismantling any tree, placed by the player or spawned in by worldgen. Once dismantled, trees drop about 3 wood logs. These logs can be sold for $50/per or taken to a workshop and, using the saw, sliced into 4 wooden planks. These planks also sell for $50/per piece, however they can be stacked until 20 which can be sold for $1,000. Logs are rarely stacked above 7. That means, when running a tree farm, 1 tree ($100), can be turned into about 12 planks ($600). If worked efficiently, a Forestry yields around $500 profit per tree, making it a viable, but lengthy source of income.
Planks can be further refined into furniture using the carpentry table in the workshop and sold for even more money. However, as of alpha 23 finding a prisoner capable of being trained to carpentry level is exceedingly rare.
Workshop Saw