Demographic variation among sites within a continuous population of Rhamnus cathartica

Background/Question/Methods

Introduced species that escape cultivation and become naturalized provide opportunities to study demographic processes involved with range expansions and colonization of new habitats.  Common buckthorn (Rhamnus cathartica) has established itself in many urban and suburban parts of the Upper Midwest since being introduced to the region over 100 years ago.  We mapped buckthorn stands at 30 haphazardly chosen sites within a one-mile stretch along the top of the west-facing bluffs of the Mississippi River in St. Paul, Minnesota, in order to determine the extent to which the local population is demographically uniform or consists of demographically distinguishable subunits.  Each site was roughly 30m from the previous site.  All individuals within a 20mX30m plot (with the 30m side paralleling the river) at each site were mapped to the nearest centimeter and assigned to one of four size classes:  less than 15cm tall, between 15 and 50cm tall, between 50 and 150 cm tall, and greater than 150cm tall. 

Results/Conclusions

Size distributions based upon these four classes were highly heterogeneous:  of the 435 pairwise comparisons only 60 were considered homogeneous by chi-square tests of heterogeneity.  In one group of 10 plots the smallest size class comprised about 50% of the individuals, with about 25% of the individuals coming from the second size class, about 15% from the third, and about 10% from the tallest size class.  The demography of the other 20 plots varied idiosyncratically. This lack of any pattern among the sites suggests that the demographic structure of buckthorn stands may be highly site-specific and that demographic processes such as recruitment, survival, maturation, and reproduction may well reflect site-specific rates, too.