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Control of the Invasive Exotic Rhamnus cathartica in Temperate North American Prairies.. Page 6 of 11 Brush- B -Gon. The effectiveness of prescribed burns in controlling R. cathartica is more difficult to determine at this point because of the greater amount of time it takes to realize results and its rather recent advent. A study conducted in 1998 indicated that fire reduced the number and vitality of R. cathartica in a given area (MNDNR 2000). In another study, small seedlings (less than 25.5 cm tall) with shallow roots were particularly impacted by fire (Boudreau and Wilson 1992). Aho (2000) reports a high R. cathartica seedling mortality rate after three consecutive years of conducting prescribed burns. Similarly, Moriarty (2000) has had success with the several burns he has conducted and especially believes in fire as an essential means of controlling the accelerated growth of seedlings that normally ensues after removal of the more mature individuals. To the contrary, a 1988 study on fire and R. cathartica control showed that R. cathartica populations were virtually unaffected when surveyed the following spring (TNC 1995). The study concluded that R. cathartica's ability to re- sprout after top killing was not diminished even after several burns. Furthermore, newly burned areas may serve as a temporary window providing prime conditions for invasion by nearby R. cathartica and dormant seed germination resulting in a re- infestation worse than the original (USGS 2000). Despite being labor and time intensive, mechanical means of controlling R. cathartica are often of necessity when a large number of seedlings is interspersed by desirable vegetation. Under these conditions herbicide treatment is prohibitive. Larger trees can be removed mechanically too. Bohnen (2000), in her restoration efforts, has had success removing all age classes, up to 7 cm in diameter, by sufficiently loosening their roots with a shovel followed by hand pulling/pushing. The recruitment of volunteer help, such as `sentenced -to- serve' crews (those who must perform community service as a means of satisfying their sentence for a minor crime), has made mechanical control methods a more viable option for larger restoration efforts (Hayman 2000). Control Strategy Current efforts on parklands to control R. cathartica are a good start, but the efficient avian dispersal of R. cathartica propagules necessitates the assumption of a regional removal stategy. To reduce the threat of immediate reinfestation of a given park, residential areas within close proximity to that park should be cleared of mature R. cathartica as well. Education efforts that would succeed in mobilizing the public to remove R. cathartica from their properties, or infested areas within their immediate vicinity, should be highly encouraged. The importance of residential control may be further emphasized by an example of R. cathartica infestation of urban parkland along Minnehaha Creek approximately 0.5km from my south Minneapolis residence. An individual adult R. cathartica tree laden with fruit and a R. cathartica hedgerow, also bearing fruit, exist in neighboring yards (Figures 4a and 4b). Five city blocks away, the woodland buffering the creek is experiencing varying states of R. cathartica infestation. Its density is quite heavy in spots. The understory of a 30 m patch of mature native tree species (Populus deltoides, Fraxinus sp., Ulmus sp., and Acer negundo) completely choked with a near singular age class of two to three m tall R. cathartica dramatically demonstrates one such instance (Figure 5). Several large, fruit bearing trees within a short distance of the patch are the likely propagule sources. Their removal should be conducted immediately, but unless the mature R. cathartica capable of producing fruit in the nearby residential area are removed, a successfully restored portion of the park may once again be faced with R. cathartica encroachment. A greater dissemination of a list of alternative native shrubs that could serve as replacement plantings would likely expedite public involvement in residential control. Possible native shrub replacements for Minnesota are Aronia melanocarpa (Michx.) Ell., Alnus incana (L.) subsp. rugusa (Du Roi) Clausen, Amelanchier sp., Cornus sp., Corylus americana L., Hamamelis virginiana L., Symphoricarpos albus (L.) Blake, Viburnum lentago L., V. rafinesquianum Schultes, and V. trilobum (Marsh.) (Moriarty 2000). Figures 4a and 4b. Clusters of fruit are apparent on this mature, female R. cathartica tree (a) and hedgerow (b). http: / /www.hort. agri. umn.edu /h5015 /00papers /gale.htm • 2/15/01