Does Rhamnus cathartica display flooding tolerance?

Background/Question/Methods

Rhamnus cathartica L. (Rhamnaceae, common buckthorn) is an aggressive invader from Eurasia of North American woodlands, old pastures, and fencerows.  The species is naturalized from Maine to Montana and south to Kentucky and Utah.  In the Midwestern US, the species can be found in wetlands despite not being considered a wetland species.  Because little is known about the flooding tolerance of this species a greenhouse study was conducted to determine how Rhamnus cathartica responds to various flooding regimes.  Also, we investigated the differences in flooding tolerance between Rhamnus cathartica’s seedlings and saplings.  A total of 90 R. cathartica plants, 45 seedlings and 45 saplings were paired and treated to one of four soil moisture regimes (control [C], saturated [S], fluctuating [FX], and flooded [FL]) for 11 weeks.  Stem height and branch length measurements were taken weekly and all plants were destructively harvested for biomass at the end of the study.

Results/Conclusions

Results show R. cathartica seedlings and saplings were able to measurably grow for up to two weeks after flooding treatment was implemented.  Significant differences were found for stem height between plant stages and treatments (two way ANOVA, F = 11.75, P < 0.001).  In the case of seedlings, plants were taller in C > S > FL = FX.  In the case of saplings, plants in the C and S treatments were taller than saplings in FX and FL.  Significant differences were found for plant biomass (i.e., leaves, roots, and woody material) between plant stages and treatments (two way ANOVA, all F values > 7.88, all P values < 0.001).  Seedling and saplings varied in resource allocation to leaves, roots, and woody material within the four treatments.  By the end of 11 weeks, 58% of flooded plants (33% of seedlings and 86% of saplings) appeared dead, while only 43% of plants subjected to fluctuating water levels (50% of seedlings and 28% of saplings) appeared dead.  These results show a difference in how R. cathartica seedling and saplings can respond to various soil moisture levels.  Determination of R. cathartica’s true flooding tolerance is of importance as it has legal implications for wetland determinations and could explain the potential invasiveness of this species into wetlands.