Plants in the understory of tropical forest tend to become filthy with
algae and lichens, dead insects, and poop. Only the newest leaves look
as shiny and clean as a well kept houseplant. Rain helps only a little
because while it may wash off some debris, it brings more debris down
from the canopy above, and the algae etc flourish.

As for desertification in the cradle of humanity, people who make their
livings by studying this say the single most important factor has been
centuries of intensive and large scale resource extraction. That means
clear cut logging followed by commercial agriculture in which harvests
are not followed by return of organic matter to the soil. The great
cities of history and prehistory alike have been on waterways, aka giant
sewers. All food and wood for building and cooking and heating for
millions of people living in relative luxury is imported and consumed,
and what is not burned is flushed into the water and goes to the bottom
of the sea. None is returned to the agricultural soil. The result is
steady depletion of organic matter and reduced ability to retain water
in the soil. This contributes to erosion and further loss of that
ability.

A similar trend has occurred in history, in the southwestern United
States. In Spanish Colonial times the land supported vast quantities
of grazing animals. Not anymore.

Una