Tree line is used to divide the area where trees grow or no longer grow. It is often called forest boundary, which refers to the boundary between whether trees can grow or not. In the area within the line, trees can grow normally. Beyond this line, most plants will be unable to grow for various reasons.

There are many reasons why trees cannot grow in these places, such as water, soil, wind or other climatic reasons. In the desert, the driest place where trees can grow is the desert forest line. Where it continues to extend into the desert, there is too little water for the growth of trees.

Source: @andrewtatcarlson