Generally, waiting to 'harvest' will increase your PV as long as the forest's value is increasing at a greater rate that your PV discount rate. The 'optimal harvesting' moment is that point in time at which the forest's value increase rate has diminished such that it exactly equals your discount rate.
As a mini-illustration, suppose that by allowing the forest to grow, the expected selling price of the cut timber grows by 20%, 15%, 10%, and 8%, in the first four years, respectively. Suppose further that your discount rate for NPV purposes is 9%. In this case your NPV is optimized by cutting at the end of the third year.