If you want to help reforestation, it's important to know about a process called ecological succession. This is how nature does it.
The plants most people think of as weeds are very important in this. Botanically, they're known as pioneer species. They're tough, tenacious, and will grow even in unforgiving places.
Pioneers lay the groundwork for larger plants to gradually take over, eventually making a comfortable environment for climax species, like forest trees.