Trees are decoration that can also gain you gold, lets you hide defensive units/buildings or can serve as an indestructible wall segment, while still allowing a unit to fire past it.
Removing trees consumes fuel, but produces gold. The amount of gold you get follows a sequence:
3, 2, 3, 6, 1, 1, 3, 5, 1, 7, 2, X, where X is the level of the current tree + 1. After that the sequence starts over again. This means you would get between 36 and 41 gold each 12 trees (3 - 3.4 gold average).
Each tree has 10% chance to grow into the next level in 12 hours time. They also have each 10% chance to grow a new tree during the same time frame. At the maximum tree level of 6 the tree no longer can spawn new trees. If you remove all trees, your trees might never grow back. You can have maximum 25 trees.