, 2004 and Van Klinken and Campbell, 2001). These examples show that the environmental risks related to the introduction of tree species have been underestimated in the past. However, awareness of these risks has grown in recent years, and the invasive potential of tree species is now considered more carefully before any new introductions. The risks of genetic pollution

and hybridization are related to the transfer of tree germplasm to an area where the same or a related species already occurs. Hybridization and introgression are natural evolutionary processes (Arnold, 1992), but the term ‘genetic pollution’ usually refers to a situation where the mixing of gene pools, between different individuals of the same or related species, has been initiated by, or significantly influenced through, human activity. If the seed source used is not local, then planted trees are likely to have a different genetic composition PR-171 purchase from selleck compound wild

stands, and crossing between them could lead to the dilution and loss of unique diversity in the wild. The subsequent breakdown of co-adapted gene complexes could lead to outbreeding depression (Ledig, 1992). Genetic pollution has been reported for many forest trees. One of them is Juglans hindsii, which is known to have hybridized with many congeners imported for commercial purposes ( Rhymer and Simberloff, 1996). Another well-known example is Populus nigra, which was once widespread but is now extirpated over large parts of Western Europe ( Lefèvre et al., 2001). Its habitats have been considerably reduced by the past transformation of rivers to canals and its gene pool is threatened by the large-scale cultivation of hybrid poplars ( Smulders et al., 2008). Other check examples are Platanus racemosa, which is currently disappearing from its native range through introgression

with Platanus × acerifolia ( Rhymer and Simberloff, 1996), and the genetic pollution of native gene pools of eucalypts resulting from plantation establishments in Australia ( Potts et al., 2004). Concerns have also been expressed that cultivated-wild tree hybridisation could result in traits introduced into cultivars through genetic modification (GM) being transferred into natural stands, with potentially significant evolutionary consequences in the wild (see Delplancke et al. (2012) for concerns regarding cultivated Prunus dulcis and wild Prunus orientalis). The environmental risks associated with genetic pollution were largely ignored in the past and it is important not to overstate them now. Strong barriers to hybridisation exist between some related species, such as differences in flowering time or the poor fitness of hybrids, which reduce the risks. One approach to reduce the potentially negative impacts of cultivated-wild tree hybridisation is to deliberately isolate cultivated material or to plant exotic rather than indigenous trees around natural forests and woodlands (Potts et al., 2001).