Genetic implications of silvicultural regimes

Human impacts on genetic structures of forest tree populations are manifold. Changes of environmental conditions and forest stand structures brought along by silvicultural practices concerning artificial and natural regeneration, regulation of species mixtures, thinning, and harvesting operations have a strong influence on evolutionary forces like drift, mating systems, gene flow and selection. The observation of long-term effects of silvicultural regimes on genetic structures and adaptive potentials is hampered by Iona rotation cycles of most forest trees. Artificial regeneration is the most obvious silvicultural practice resulting in possibly drastic changes of genetic structures not only in planted forests, but, in later generations, due to gene flow also in neighbouring forests. However, natural regeneration is no safeguard against unintentional changes of genetic structures by forest managers. Loss of genetic variation and adaptive potentials may be expected if reproduction-effective population sizes are seriously reduced or, mainly in the case of rare and scattered species, if the gene flow through pollen is severely restricted by low population densities. Harvesting and thinning operations may have serious impacts on adaptive and economically important traits due to selection in favor of particular phenotypes. Target diameter felling serves as an example to describe possible consequences for genetic structures on subsequent generations in more detail. Consequences of silvicultural management practices on genetic structures of tree populations are most severe and detrimental in tropical forestry. (C) 2004 Elsevier B.V. All rights reserved.