Question

Why is irrigation necessary for new plantations of trees?

Answer 1

Irrigation is needed so that new plants grow more strongly ND get enough water

Answer 2

Irrigation is the process of providing plants with nutrients and water.  

If the plants are not provided with water and nutrients in the right amount and at the right point of time, they will not grow properly.

When the plants are new, their roots are not deep and thus they require proper irrigation.