If we cut all the trees in place, will it affect land? How?

Yes, it will definitely affect land by the following ways:

Soil erosion: Trees hold the soil and prevent their erosion from various sources like wind, water etc. But due to cutting of trees soil erosion takes place and top layer of soil is removed. Due to this the lower, rocky and hard surface is exposed which due to less humus content is less fertile in nature.

Therefore, cutting of trees by causing soil erosion affects the fertility of land.

Water holding capacity: Due to cutting of trees water holding capacity of soil is also decreased. The movement of water from the soil surface to the ground is decreased and due to this flood are there as water is not absorbed or moved to the inner ground and level of ground water is also decreased.