Forestry is an activity that was introduced in Farming Simulator 15. It offers a quick, large profit, but can be more labor intensive because you cannot hire a worker to help.
The cost of trees is $7,000 on normal. But the profits per tree vary greatly from about a 50% loss ($3,500 selling price on hard) to a 65% profit ($11,500 selling price on easy). From indications easy can net you over $100,000 for a tree to hard where you can barely get a profit at all buying trees to harvest.
You can use the chainsaw to cut down trees. Press LT and joystick to tilt then press B to cut. A log is worth the most when it is between 6 and 10 meters long (the truck log trailer is 10m long), and is straight. Logs and Wood Chips can be sold at the Saw mill. Wood chips are produced by feeding logs into a Wood Chipper, and after which they come out of the Wood Chipper's pipe. The pipe can be manually controlled to make a pile in a certain spot, or can automatically 'lock' onto a nearby container. If it is not set to make a pile and there is no nearby container, then once the Wood Chipper's internal storage is full, the Wood Chips will be deleted.