As of Patch 0.4.6 trees are only harvestable in creative mode.
As of Patch v0.5a, trees are harvested with a chainsaw weapon.
As of Patch V0.7.5, trees can be harvested in Survival mode.
In creative mode, the player spawns with infinite quantities of literally everything. This includes weapons and ammunition, as well as crafted, smelted, and raw resources, and tiles and blocks. "Everything" is an all-inclusive term. The player is also free to use those resources as the player wishes.