Population demographics and trade-offs to reproduction of an invasive and noninvasive species of Rubus

Do trade-offs between growth and reproduction differ between an invasive and noninvasive plant species and how do such trade-offs relate to population demographics? To help address these questions, we compared demographics for an invasive plant species, Rubus discolor, with a noninvasive congener, R. ursinus, in several populations of varying density. Removal of floral buds from reproductive canes increased the size of juvenile canes that arose from clonal sprouting in R. ursinus, suggesting a trade-off between current reproduction and growth. Removal of floral buds had no effect on growth of R. discolor. R. ursinus displayed trade-offs between reproduction (sexual and vegetative) and future growth based on negative correlations between leaf area production and both clonal sprouting and seedling production during the previous year. R. discolor did not exhibit these trade-offs. Both species had high population growth rates in low-density populations, but exhibited little or no growth in high-density populations. A life table response experiment was used to determine the underlying cause for the effect of density on population growth. For R. ursinus, lack of population growth in high-density populations was due primarily to increased mortality of clonally sprouting canes, while for R. discolor, it was due to decreased clonal cane production. Elasticity analysis revealed that clonal growth was more important than sexual reproduction for population growth of both species. However, elasticity values for sexual reproduction in R. discolor were greater in high- than low-density populations. This suggests an increased reliance on sexual reproduction in populations that had reached stable sizes, which could increase the capacity of R. discolor to disperse to new sites. Elasticity analyses were also used to simulate the efficacy of various control strategies for R. discolor. Control of this species could be attained by reducing clonal production within existing populations while reducing seed production to limit establishment of new populations.