What determines pine naturalization: species traits, climate suitability or forestry use?

Aim: Species attributes, biogeographic features and human factors have all been shown to discriminate between invasive and non-invasive plant species. However, the relative importance of these factors, their generality in determining invasion outcomes across different regions and their ability to discriminate success and failure at different stages of the invasion process have not been established. Location: New Zealand (NZ) and Great Britain (GB). Methods: For species in the genus Pinus, we used boosted regression trees to identify factors associated with success or failure at the introduction and naturalization stages in each region. Results: Human factors, notably the forestry use index, were the strongest determinants of which species from the global pool were introduced to both NZ and GB. Species with a close climate match were also more likely to be introduced to NZ but not to GB. Human factors and climate match were also the strongest determinants of which introduced species became naturalized in both NZ and GB, although the order of importance differed (human factors followed by climate match for NZ and vice versa for GB). Species attributes (life-history traits and the Z-score) had much less ability to discriminate successful and failed species at both the introduction and naturalization stages in these two regions. Main conclusions: We show for the first time that human factors are more important than either species or biogeographic traits in determining the likelihood of a species being introduced or becoming naturalized. The similarity between two different regions in the factors found to be important in success at both these invasions stages points to potentially general mechanisms underlying these processes. The strong human component to introduction and naturalization highlights a potential conflict between future afforestation using alien species with conservation and management aims in the surrounding landscape, given that the factors desirable for forestry species are also those that may promote invasion.