Photosynthesis, Growth, Carbon Allocation, and Fruit Load of Frangula caroliniana and Rhamnus cathartica

Rhamnus cathartica is an invasive shrub capable of high fecundity in North America. The physiology of R. cathartica has received little attention, but more is needed to understand traits that enhance its success in disturbed environments. Such data may help to predict invasiveness of closely related species such as Frangula caroliniana. We hypothesized that R. cathartica would have greater relative growth rate and specific leaf area over time than would F. caroliniana and that fecundity would be higher for R. cathartica than for F. caroliniana. Photosynthesis, growth, and carbon allocation patterns of both species were studied over time from July 3 to October 2, 2003. Carbon allocation to leaves (i.e., leaf mass fraction) of F. caroliniana was greater than that of R. cathartica after 28 d. The partitioning of carbon to stems (i.e., stem mass fraction) of R. cathartica, however, was greater after 28 d than that of F. caroliniana. The allocation of carbon to roots (i.e., root mass fraction) of R. cathartica was greater than that of F. caroliniana after 70 d. Relative growth rate of R. cathartica was greater than that of F. caroliniana after 14 d but not thereafter. Specific leaf area of R. cathartica was greater than that of F. caroliniana after 98 d, but specific leaf area and relative growth rate were poorly correlated. There was a stronger relationship, however, between relative growth rate and net assimilation rate for both species. Photosynthesis of R. cathartica was higher than that of F. caroliniana after 42 d, but there were no differences in plant dry mass after 28 d. Length, leaf surface area, and fruit count of 2-yr-old branches of both species were measured to determine their fecundity. Fruit count of F. caroliniana was 41% of that of R. cathartica. We conclude that under favorable field conditions, both species establish similarly and that growth and photosynthesis of R. cathartica exceed those of F. caroliniana over time. Rhamnus cathartica also has greater fecundity than F. caroliniana.