Question

how do trees save the soil from being washed off

Answer 1

the roots of the trees hold the soil particles around them very tightly which does not allow the soil particles to be washed off

Answer 2

This phenomenon where soil gets washed off is called soil erosion.

When trees are cut, they are removed with the roots which creates an environment around the soil which is susceptible to erosion. Also, if trees were planted on a sloping ground and removed; the process of erosion accelerates.

Therefore when trees are planted, they hold together the soil with their roots and this is how its prevents soil from getting washed off (soil erosion)