Any idea, how does the dirt/each return over time to sustain plant life? Or is it just a gradual grow inwards with plant matter slowly forming topsoil?
The Amazon rainforest's soil is extremely poor in nutrients and very shallow due to all the rain. In addition, at least 50% of the rain it gets comes from the trees themselves. The division between abiotic and biotic factors is a lot more fuzzy than people realize
The Sahara's sand is actually full of a lot of nutrients. In fact the massive Saharan sandstorms that traverse the Atlantic Ocean are a significant source of fertilizer for the Amazon rainforest
All the elements are there. You just need the right weather and plant life to get things going.
Of course we can't ignore the human element. Archeological evidence suggests humans played a massive role in transitioning the Amazon Rainforest from the grassland it once was to the jungle it now is. Reforestation projects, Great Green Wall, half-moon berm projects, etc are all way way more effective than they were just a few years ago. We're learning a lot about how much humans can do to revitalize soils
Pioneer species that can grow on basically nothing are slowly replaced by other species as the ground builds up organic matter. Fungi play a very large part in building soil as well as big and small animals.
