Cutting down only some trees in a forest and leaving a mix of tree sizes and species standing is called

Selective Cutting

Explanation:

Selective cutting can be defined as cutting down of trees selected in a forest so that growth of trees is affected.

This is done based on some of the criteria regarding minimum tree size for harvesting, spacing and residual trees per area, specifications of the number.

It is done selectively for the trees whose canopies are no more useful and need harvesting to grow further. The selection of trees is done so that they can be made useful.

Hence, the correct answer is Selective cutting.

the cell cycle is the final answer! : )