The spread of pines in the Southern Hemisphere is germane to the assessment of the causes of persistence because nonindigenous pines have rarely become established in the United States. Naturalized pines are largely restricted to Pinus sylvestris in New York State and Pinus nigra (Leege and Murphy 2001) on dune sets around Lake Michigan. The current restriction of pines is curious for at least four reasons. First, the conterminous United States has about 36 native pines that occur in a wide range of physical habitats (Critchfield and Little 1966), from some of the most arid habitats in which trees occur to sites with abundant moisture and from sites with air temperatures that routinely exceed 35°C to the upper timberline in the Appalachian, Cascade-Sierra Nevada, and Rocky Mountain ranges. Second, a long history of deliberate introduction of pines continues. Pinus sylvestris, P. mugo, and P. nigra are common horticultural species. Third, with so many native pines that all maintain associations with one or more native mycorrhizal fungi, it would be surprising if host extensions among the fungi to the introduced pines had not occurred. Finally, as with the inadvertent introduction of the requisite fungi in the Southern Hemisphere with potted pine seedlings from the native range, it would be surprising if similar introductions had not occurred in the long history of pine cultivation in the United States. The further naturalization and even invasion by foreign pines in the United States remain possibilities that should be experimentally evaluated.

LIFE HISTORY TRAITS

Thus far in this chapter, two major categories of factors that influence the process of establishment have been presented: stochasticity and environmental (abiotic and biotic) forces. However, no factor related to predicting the establishment of nonindigenous species has been pursued more assiduously or longer than a link between the life-history traits of species and their ability to become established in a new range. The reason is obvious: life-history traits are related directly to species growth, reproduction, and survival. The value of any such links is appealing and deserving of much further investigation. Having failed, however, to find broad taxonomic agreement between life-history traits and species’ performance in new ranges, we outline here the traits that appear to play a role in influencing establishment for plants, pathogens, and insects.

Plants

Reproductive Systems

Although plants collectively display much diversity in reproductive systems, two fundamental dichotomies are apparent. First, reproduction can be sexual or asexual; and second, sexual reproduction can involve a single parent (uniparental) or two parents (biparental). Biparental sexual reproduction, often termed out-



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