The theory and study of invasiveness typically emphasize exceptional early successional life-history traits in invasive exotic plants, enabling the capture of high resources in recently disturbed environments. A key issue in invasion dynamics is whether these pronounced early succesional behaviors come at the expense of conservative, late successional traits which allow plants to increase during succession. We used maximum likelihood analysis to compare the growth and mortality of two exotic trees, Ailanthus altissima and Acer platanoides, with nine dominant native tree species in closed-canopy forests in Connecticut, USA. Growth is predicted as a function of light, and mortality is predicted as a function of recent growth.
Results/Conclusions
The exotic species showed strikingly high growth rates at high light levels, exceeding all native species at light levels ≥ 10% full sun, and growing on average 2.8 times faster at 100% full sun. However, at light levels typical of a deeply shaded understory (<3% full sun), the growth rates of five native species exceed both exotic species. The low-light mortality of the exotic species was strongly dependent on the degree of shading; e.g. at 1% full sun, the annual mortality rate of A. platanoides is 10% and A. altissima is 17%; only two native species had higher mortalities. However, at 5% full sun, A. platanoides’ mortality was < 1%, superior to all but three native species. Mortality of all species dropped to < 1% by 10% full sun, except A. altissima whose mortality remained nearly 10%. Metrics of interspecific life-history trade-offs (based on growth, height allometry and low-light survivorship) show a nearly linear trade-off axis in native species. The life-history of A. altissima trades-off along this same axis—its extraordinary growth rate in high light comes directly at the expense of low-light survivorship. Acer platanoides is the lone exception to this pattern, as it combines very high growth rates with moderately high shade tolerance. This study confirms the predominance of early successional traits (exemplified by rapid growth) in invasive plants, even in exotic tree species noted for invading unfragmented forests. These results indicate that the rate and magnitude of invasions will strongly interact with high light levels created by disturbance. In the long-term, the poor low-light survivorship of Ailanthus altissima will limit its invasiveness, while Acer platanoides’ unusual combination of traits indicates it will be a serious invader of most forests.
