Age of Empires II: Definitive Edition doesn't have Steam workshop so you cannot use Steam to subscribe to any mod. Now I'll show you how to activate Small Trees mod in AOE2:DE.
How to Install Small Trees Mod in AOE2 Definitive Edition
- Launch Age of Empires II: Definitive Edition
- Click the Mods button in the main menu.
- On top, click on the Browse Mods
- Type "small trees" in Search box
- Select Small Trees mod in the list.
- Click on the Subscribe button.
- After the download is completed, you can return to main menu and you can play the game.
In this wise, you can find all the mods what you want in Age of Empires II: Definitive Edition and install them easily.