4) less stable and less diverse compared to the original forest.
This is because the original forest was a native forest, which would have been adapted to the local climate and soil conditions. The newly planted Christmas tree farm, on the other hand, will be a monoculture of Frasier fir trees, which are not native to the area.
A monoculture is less stable than a diverse ecosystem, because if a pest or disease affects one tree, it can easily spread to the entire population. Additionally, a monoculture is less diverse, because it does not support the same range of plant and animal species that a native forest would.