In order to unlock certain items you will need to gain XP. You can gain woodcutting XP by chopping down wood. Levels need to completed before you can unlock items like tools and saplings. Take advantage of the bonus XP when it is offered. You can check your XP Levels by clicking on the second icon down to the left of your screen, then click on Woodcutting. There you can view where you are currently and what items you can obtain on future levels.
LEVELS-
3- Stone Axe Crafting Ability
5- Pine Sapling
7- Iron Axe Crafting Ability
10- Birch Sapling
12- Avocado Tree Sapling
14- Maple Tree Sapling
20- Gilded Steel Axe Crafting Ability
24- Hickory Sapling
