The problems of self-pollination among trees are a major concern to foresters. Silvicultural practices have an effect on the frequency of self-pollination.For example, a cut, such as a shelterwood or seed tree, reduces the number of individuals in the breeding population and increases the distance between individual trees. This tends to increase the frequency of self-pollination. These effects can either be helpful or harmful depending on the goals of the forester. Therefore, a basic understanding as to the effects of self-pollination on trees is necessary. The point of focus in this paper is the genus, Pinus. Self-pollination occurs naturally in the forests, but usually not to any great extent. Trees have phenological and morphological barriers to self-pollination that help reduce the frequency. When self-pollination does occur, recessive genes that may be carried in the heterozygous condition can be expressed. These can either cause various degrees of harmful changes or go unnoticed. Because only the lethal or deviant changes are highly observable, those are the ones associated with self-pollination. Therefore, self-pollination is generally thought of as being harmful. Self-pollination can also carry out an important function in the forest. When the environment changes, trees need to cope with that change. The variation carried in the genes of the population allow the species to adapt to environmental changes.A recessive gene that was once considered harmful can be necessary for the survival of the species. Self-pollination does have its place, but in the normal forest situation, outcrossing is the more common method of pollination.