, 2009, Huang et al., 2011 and Alfaro et al., 2014). Introduced tree species can sometimes become invasive of agricultural and natural ecosystems, and there has been much debate in the literature about this danger (e.g., Richardson et al., 2011). Many introduced tree species have been recognized as invasive only fairly recently, Bcl-2 inhibitor despite the long history of the transfer of tree germplasm. A global survey conducted by Richardson and Rejmánek (2011) found a total of 357 introduced tree species known to

be invasive in some part of the world. The majority of species were introduced for horticulture, but some were introduced for forestry and agroforestry (Richardson and Rejmánek, 2011). Better-studied taxa, such as Pinus spp. and Australian Acacia spp., are considered as model groups in plant invasion ecology

( Richardson, Cilengitide 2006 and Richardson et al., 2011), but in many other cases little is known about invasiveness. The case of Australian acacias illustrates the benefits and risks: an introduced species can be simultaneously a commercially important crop and, if it escapes from plantations, an invasive. Not all introduced tree species of invasive genera, however, turn out to be weedy in new environments. Of the 386 acacia species that have been transferred outside of Australia, only 23 are currently invasive (Richardson et al., 2011). Although they are relatively few, these invasive acacias have caused significant damage to natural ecosystems, especially in Mediterranean-type climatic regions (Gaertner Branched chain aminotransferase et al., 2009). In South Africa, for example, nine Australian acacias are classified as ‘major invaders’ and another three are considered

as ‘emerging invaders’ (Nel et al., 2004). In a review of tree invasions, Lamarque et al. (2011) noted that large propagule pressure is often an important factor for an introduced species to become invasive. A similar conclusion was made by Procheş et al. (2012), who reported that the number of experimental plantings strongly correlated with the invasive range size of certain pines in southern Africa. In northern Europe, Kjaer et al. (2014) observed that the few introduced tree species planted on a large scale were the ones that created invasiveness problems later. The benefits and risks of introduced tree species change over time and include social aspects. This is illustrated by the introduction of several Prosopis species from Latin America to Africa, Australia, India and other tropical regions of the world at the end of the 19th century. These introductions were first considered very valuable sources of shade, fodder, fuel wood and other products (e.g., gums, honey and resins), as they were able to grow in extreme conditions ( Felker, 2009).