However, in many arid and tropical areas, forests are often unable to re-establish themselves without assistance due to a variety of environmental factors. One of these factors is that, once forest cover is destroyed in arid zones, the land quickly dries out and becomes inhospitable to new tree growth. Other critical factors include overgrazing by livestock, especially animals such as goats, and over-harvesting of forest resources by native populations or outside businesses. In some tropical areas, the removal of forest cover may result in a duricrust or duripan that effectively seal off the soil to water penetration and root growth.
In some areas, reforestation can easily be accomplished by the mass planting of tree seedlings. In other areas, mechanical breaking up of duripans or duricrusts is necessary, careful and continued watering may be essential, and special protection, such as fencing, may be required.
One great problem with reforestation is that, whereas the original forests often featured significant biodiversity, the replacement trees are often of only one or a few kinds, without the re-establishment of the other plants and animals that is required for a healthy ecosystem. When only one kind of tree (or other plant) is planted, the resulting system is called a monoculture.