Pre-commercial thinning is performed when trees are __________.

The correct choice indicates that pre-commercial thinning occurs when trees are at a size that is not yet commercially viable for sale. This practice focuses on selectively removing some trees in a stand that have not reached the point where they can be marketed, ensuring that the remaining trees can grow stronger and healthier.

Pre-commercial thinning typically targets trees that are still relatively young and smaller than those suitable for timber harvest. This technique is essential for managing forest density, as overcrowding can hinder growth, light penetration, and overall forest health. The main goal is to improve the growth potential of the remaining trees rather than to prepare them for market sale at that stage.

Considering the other alternatives, trees that are still seedlings would not usually undergo pre-commercial thinning, as this level of intervention is aimed at more developed trees. Similarly, the practice is intended for stands that are dense; however, the concept of reaching a size where the trees can be sold is more pertinent to the timing of when pre-commercial thinning is necessary. Lastly, while trees not growing properly can be a factor in deciding which ones to thin, it is not the defining characteristic of when pre-commercial thinning is done. The focus remains on managing density and optimizing growth potential.