Question

how does deforestation lead to desertification

Answer 1

b'coz as deforestation increases slowly the soil will become unfertile and it will be of no use. and when the wind blows the soil Will also blow and the sand from other places come at that place and settled there.
And slowly it becomes the desert area.

Answer 2

The trees hold water at the roots deep inside the EArth.  So the soil remains fertile. Due to the recycling of water from rains, decomposed nutrients from dead plants, insects, animals, birds etc., the soils remains fertile. Due to the inhaling of CO2 by the plants, the environment remains cleaner and cooler. The trees and plants absorb a lot of radiation from the hot Sun and keep the region cool.

If we cut down the trees, then the underground water gets evaporated slowly. The terrain dries up and breaks into pieces. The region and the air becomes hot during the day. It also becomes very cold at night. It becomes a desert where there is no water or greenery.  Soil erosion takes place due to dryness and hot winds.

Obviously people don't prefer to live around there and go further away. Then there will not be much of rains in that region. So gradually the region could become like a desert.