From Brotato Wiki
Trees are neutral units that spawn throughout waves. They can be killed for good Loot.
Trees drop 3 Materials on death, and have a 100% to drop a Consumable. This consumable has a 20% chance to be a Loot Crate.
Note that unlike the Loot Hoarder, trees are not guaranteed to drop consumables. So if you have negative Luck, or are playing Endless Mode, trees might not drop consumables.
The Tree item increases the amount of trees that spawn.
The Lumberjack Shirt allows you to kill trees in one hit, as if the tree had 1 HP.
Explorer Starts with the equivalent 12 tree items.
At the Start of the wave, and each following 10 seconds, trees have a chance to spawn.
A random number between 1 and 2 + the number of Tree items you have is chosen.
Then the game will attempt to spawn a tree with a 33% chance that number of times.
Trees are Neutral. This means that they aren't affected by some effects like Gentle Alien's ability to increase the amount of enemies.
In the same manner, Jack doesn't increase the amount of Materials dropped by Trees.
Bouncing Attacks like the ones from Slingshot do not target neutral units when finding a target for their bounce, but the bounces can still hit trees if it crosses the path of the projectile.
Furthermore, Burn doesn't spread to Neutral Units if you have the Snake item. You can still set the tree on fire by attacking it directly.