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Control of the Invasive Exotic Rhamnus cathartica in Temperate North American Prairies.. Page 3 of 11 • Life History Once the production of fruit has begun, the inevitable profusion of progeny is nearly impossible to control as the seeds are readily dispersed in a number of ways and remain viable for several years. Many of the fruit drop below the parent plant and germinate within the immediate vicinity. Some fruit are washed away with heavy rains and germinate where they settle (Gill and Howell 1984). R. cathartica fruit have been known to float for 3 -6 days before sinking (TNC 1995). Insects, birds, and small mammals, such as, voles, mice and other rodents, are also potential vectors (USGS 2000). • • In particular, avian dispersion of seed greatly enhances the invasiveness of R. cathartica. Studies show that the majority of seed dispersed from a parent tree may be attributed to birds (Haber 1998). Several bird species, such as American robins, European starlings, blue jays, and cedar waxwings eat the drupes off mature trees when alternative food sources are depleted (Gourley and Howell 1984). According to several studies, most birds do not prefer R. cathartica drupes as they contain harsh unpalatable chemicals (Maw 1981). However, by late fall avian consumption of the fruit becomes increasingly common (Haber 1998, Rebuffoni 1997). Relatively few of the ingested seeds are destroyed in the digestive tract of the animals while the rest are carried away and redistributed intact (Howland 1996). The chemical compound found in the drupe mesocarp that is responsible for inducing the laxative effect is anthraquinone (Archibold et al. 1997). Quick passage of R. cathartica seed through the digestive tracts of animals is the result. Research also suggests that the seeds germinate more quickly after the fleshy fruit has experienced a degree of digestion (Gourley and Howell 1984). Interestingly, a study examining the effect of exotic shrub species on songbird nest predation found that American robins preferably nested in R. cathartica and also experienced a significantly higher daily mortality rate than those nesting in Crataegus sp., a native shrub species (Schmidt and Whelan 1999). This study suggests that, ironically, a native bird species appears to be enhancing the invasive capabilities of R. cathartica, which in turn serves as an ecological trap for the birds. An extended growing season likely gives R. cathartica a competitive advantage over native plant species as well. Harrington et al. (1989) showed that R. cathartica's leaves remain on the tree an average of 58 days longer than its native counterparts, Cornus racemosa and Prunus serotina. Its leaves emerge earlier and senesce later. In both cases, upper canopy foliage is largely absent. Consequently, photosynthesis under high light availability conditions is significantly greater for R. cathartica than for native shrub species. Indeed, 27 to 35 percent of R. cathartica's annual carbon gain occurs before C. racemosa leaf emergence (Harrington et al. 1989). Allelopathy may also contribute to R. cathartica invasiveness. Some observations indicate that R. cathartica possesses allelopathic chemicals within the fruit and leaf structures (Boudreau and Wilson 1992). Allelopathic chemicals act as a germination or growth inhibitor to other organisms that could potentially occupy the affected area. The presence of these chemicals improves R. cathartica's chance of survival by eliminating some or all of its competition. Preliminary studies by Krebach and Wilson (1996) suggest that the flesh of the drupes contain terpenoids or alkaloids which are capable of retarding the growth of competing plants. The extent to which R. cathartica's release of allelopathic chemicals to the soil accounts for native plant displacement has yet to be determined. A greenhouse experiment measuring the fitness of native species grown in soils varying in allelopathic chemical content would help to illuminate the degree to which allelopathy contributes to R. cathartica's competitive superiority. Ecosystem Affected R. cathartica seems to do best in well -lit, well - drained areas, but is tolerant of a broad range of soil types (neutral to alkaline, sandy or clayey soils) and light conditions (TNC 1995). Across its range, R. cathartica is a particularly problematic invasive of deciduous forest communities. For example, the eastern deciduous forest community, found throughout much of the American portion of its range, has experienced extensive invasion. Some eastern deciduous forest tree species include Quercus alba, Q. rubra, Q. macrocarpa, Acer saccharum, Tilia americana, Ulmus americana, Fraxinus pennsylvanica, Celtis occidentalis, Juglans nigra, Carya cordiformis, and many others (Gleason and Cronquist 1963). By reducing recruitment of the above http: / /www.hort. agri. umn.edu /h5015 /00papers /gale.htm 2/15/01