Tree populations suffer from a variety of natural environmental factors and Man’s action. The latter mostly leads to reductions not only in population size but also in population density. Reduction of population density generally means the reduction of possible mating contacts among trees. Consequently, the number of mating partners is reduced. Hence, the number of effective pollen parents of the seed produced by single trees is expected to decrease. This condition is responsible for a tendency towards increased genetic differentiation of pollen contributions. The genetic differentiation of individual effective pollen clouds is an important means of inference on reduced effective number of male mating partners. Reduced population density possibly enforces mating preferences for neighboring trees. If neighboring trees are on average more closely related than more distant trees, preferential mating among neighbors results in inbreeding even for completely self-incompatible or dioecious species. This situation is particularly likely to occur in naturally regenerated stands if gene flow through seed and pollen has been restricted and eventually results in reduced adaptedness and adaptability of progenies under heterogeneous environmental conditions. A reduced number of male mating partners of a cosexual tree may also mean restrictions in the relative amount of neighbor tree pollen and, hence, a relative increase of the tree’s own pollen present in its crown.
Consequently, the population density is likely to influence the adaptive potential of tree populations. However, it is difficult to define critical population densities which must be kept in order to avoid intolerable losses of adaptive capacities. Such critical densities are an outcome of the genetic system of a species but they also depend on environmental factors including pollen vectors. The critical population density may vary greatly depending on the environment, the physical structure of forests, and the mode of pollen transport. Although there exists ample evidence of long-distance transport of effective tree pollen, the reproduction system of trees is also characterized by a small-scale spatial pattern. This has a variety of consequences and is sensitive to anthropogenic influence.