Important aspects of environmental pollution on the genetics of forest trees are reviewed and include: (a) a description of the variation in response to gaseous and metal pollutants among forest tree species and within-species variation on the family and clonal level, (b) heritability of plant response to gaseous pollutants, (c) a genetic assessment of polluted tree populations by means of gene markers, (d) altered gene expression triggered by environmental stress, (e) investigation of whether environmental pollution triggers evolutinary processes in forest tree populations. In most forest tree populations (provenances), differences in resistance to gaseous and non-gaseous pollutants exist. Generally slow-growing and genetically highly variable tree populations seem to be less affected by different gasous pollutants than fast-growing and genetically less variable tree populations. Intraspecific variation in resistance to pollutants increases from the provenance to the family level and is greatest among single trees (clones). Heritability estimates of resistance to gaseous air pollutants in forest trees are mainly high. Inheritance to pollution seems to be polygenic and in some cases in probably pleiotropic with respect to other stress factors. While genetic control of response to environmental stress (e.g. number of genes, gene dose, epistatic effects) is often known in herbaceous plants, knowledge in forest trees is still missing. Results of gene marker studies in polluted forest tree populations indicate that groups of trees (subpopulations, artificial subsets) with high genetic variation tend to be more tolerant to heterogeneous environmental stress than less variable ones. Environmental pollutants repress and/or induce plant genes. Predictions of how forest tree populations will change genetically over time is complicated by the following facts: estimates of genetic variation derived from fumigation experiments cannot be generalised; response functions are unknown in detail; it is unclear how differences in tree vigour and foliar damage is related to fitness in natural tree populations; data on how pollution affects reproductive processes (pollen, ovule production) are scanty; it is still unknown how certain reproductive characteristics (e.g. seotiny) affect the evolutionary change to stress; intrinsically variable pollution over time and in space will cause episodic and space-dependent selection pressures; and data on "net-fitness" regarding evolution of pollution resistance are missing. Nevertheless, there is circumstantial evidence that natural tree populations will change genetically because of pollution.
