Potential invasiveness of Eucalyptus species in the U.S.: Implications for bioenergy crop development

Background/Question/Methods

Many of the life history, physiological, and competitive traits selected for in bioenergy crop species are consistent with those that increase the probability that a plant species will become invasive. Several eucalypts are under exploration as bioenergy crops because of such characteristics as rapid growth rates, high productivity, wood properties, and low susceptibility to pests and pathogens. The Australian Weed Risk Assessment (WRA) includes many of these traits, allowing identification of species at high risk for invasion. When tested on vascular terrestrial plant species in both temperate and tropical regions, the WRA has been shown to correctly identify 90% of the major plant invaders as of high invasion risk on average, and 70% of the non-invaders as having low risk. We used the WRA to evaluate the potential invasiveness of 37 Eucalyptus species in the U.S. and identify the traits distinguishing predicted invaders from non-invaders.

Results/Conclusions

We found that 22 taxa have a low probability of invasion, 8 have high probability of invasion, and 7 require further information. Of the four Eucalyptus species most commonly cultivated world-wide: E. grandis, E. urophylla, E. camaldulensis, and E. globulus, all but E. urophylla are predicted by the WRA to have a high risk of becoming invasive. This latter species requires further evaluation, as does the commonly cultivated hybrid E. grandis X urophylla. Consistent with results for other species and locations, history of purposeful introduction and invasiveness in other areas outside the U.S. significantly contributed to the risk of becoming invasive. Traits that also distinguish predicted invasive species from non-invasive species include broad habitat breadth, growth in dense thickets, allelopathy, and viable and prolific seed production. These results suggest both a wealth of Eucalyptus species at low risk for invasion that might be explored for bioenergy crop potential and traits that might be selected against to reduce the invasion risk of currently used species. The pollen sterility of a GMO E. grandis X urophylla is one example of selection to reduce the probability that eucalypts used for bioenergy become invasive threats to natural areas and agricultural systems.