2007 NORTHEASTERN NATURALIST 14(3):387–402 Rhamnus cathartica L. (Common Buckthorn) as an Ecosystem Dominant in Southern Wisconsin Forests Joseph Mascaro1,* and Stefan A. Schnitzer1 Abstract - Recent work on exotic species in island ecosystems has revealed that many exotic woody plants are capable of dominating forests in which they occur, substantially altering forest structure and nutrient cycling. In mainland forests, however, few empirical examples of exotic dominance exist. The invasive shrub Rhamnus cathartica L. (common buckthorn) is reported to infest temperate forest understories in North America and displace native species, but its degree of dominance has been described only anecdotally. We investigated the extent to which common buckthorn can dominate forest ecosystems, and found strong evidence for monotypic dominance in several mesic and wet sites in southern Wisconsin. Among eight forest sites where common buckthorn was dominant, its mean relative density and basal area was 81% and 45%, respectively. Compared to eight native-dominated sites on similar soils, common buckthorn dominance fundamentally altered forest structure: total woody stem density at Rhamnus-dominated sites was more than twice that of native-dominated sites (two-way ANOVA; P < 0.05, n = 16), but total basal area did not differ significantly (P > 0.3). When considering dominance by size class within only the eight Rhamnus-dominated sites, common buckthorn genets were more abundant than native genets at 5-cm size classes up to and including 20–25 cm diameter at breast height, evidence that common buckthorn dominance can extend well beyond understory size classes. Within Rhamnus-dominated sites, mean relative density and basal area for common buckthorn exceed that reported for four other woody invaders found in the northeastern US, and thus we suggest that common buckthorn is a particularly successful invasive species in eastern temperate deciduous forests of North America and is capable of acting as an ecosystem dominant. Introduction Invasion by exotic plants may cause a myriad of community- and ecosystem- level changes, notably alterations in local diversity, trophic structure, and nutrient cycling (Elton 1958, Mack et al. 2000, Vitousek et al. 1997). Dominant species play a central role in community and ecosystem dynamics (e.g., Crooks 2002, Ellison et al. 2005), and thus alterations caused by exotic plants are likely to be most substantial when successful invaders become dominant (i.e., in terms of relative abundance, basal area, or both; Denslow and Hughes 2004). Forest ecosystems in which the dominance of exotic species exceeds 50% of importance value (mean of density and basal area) have been recently described on islands (e.g., Puerto Rico and Hawai’i; Hughes and Denslow 2005, Lugo and Helmer 2004) or geographically 1Department of Biological Sciences, University of Wisconsin - Milwaukee, 3209 North Maryland Avenue, Milwaukee, WI 53211. *Corresponding author - jmascaro@uwm.edu. 388 Northeastern Naturalist Vol. 14, No. 3 isolated areas (e.g., the fynbos biome in South Africa; Macdonald et al. 1986); however, only anecdotal accounts of such high levels of exotic dominance occur in large mainland ecoregions, such as eastern temperate forests in the US (e.g., Knight 2005). If they are abundant, exotic-dominated temperate forests may already be causing substantial changes to the productivity and nutrient cycling of the region. Rhamnus cathartica L. (“common” or “European” buckthorn) is a small tree or shrub native to Europe and Asia that was introduced to North America in the 1800s as an ornamental hedge and windbreak (Barnes and Wagner 2004, Knight 2005). The species is naturalized throughout temperate forests of the Midwest, northeastern US, and southeastern and central Canada, commonly invading old fields, roadsides, powerline corridors, and fencelines. Although it may reach heights of several meters, common buckthorn is typically considered a shrubby invader of forest understories (Archibold et al. 1997, Gill and Marks 1991, Harrington et al. 1989, Heneghan et al., 2006, Leitner 1985, Stover and Marks 1998, Zipperer 2002). For example, Leitner (1985) found that common buckthorn was widespread in southern Wisconsin, but largely confined to a shrubby growth form on open, upland sites. Similarly, Stover and Marks (1998) found that buckthorn composed 􀂕 20% of basal area at just one of 21 secondary forests on abandoned agriculture and pastureland in New York. The potential effects of common buckthorn on temperate-forest ecosystem dynamics are substantial, and are likely to be exacerbated if the species is capable of dominating these ecosystems. For instance, several authors have noted the tendency for common buckthorn to exclude woody and herbaceous plants, presumably due to its ability to cast dense shade at levels only conspecifics can tolerate (e.g., Barnes and Wagner 2004, Harrington et al. 1989, citations in Skinner 2005). Furthermore, common buckthorn may increase decomposition and nitrogen turnover where it invades (Heneghan et al.,2006; K.S. Knight, US Forest Service Northern Research Station, Delaware, OH , pers. comm.). Despite the demonstrated importance of common buckthorn for the understory dynamics of temperate forests, little is documented about the extent to which the species is able to dominate temperate forest ecosystems. Thus, we surveyed 16 forest stands in southern Wisconsin to determine the general extent to which common buckthorn is a dominant tree in the area, and the structural characteristics of buckthorndominated stands. Methods Site descriptions and land-cover assessment As part of a larger study to explore the functional characteristics of exotic-dominated ecosystems, we surveyed the woody vegetation of 16 wet to mesic-forest sites in southern and southeastern Wisconsin (Fig. 1, Table 1). We chose eight sites that were dominated by native vegetation and eight dominated at various levels by common buckthorn (hereafter 2007 J. Mascaro and S.A. Schnitzer 389 “Rhamnus-dominated”), and we allowed a site to be included only if it was located on a separate landform. Based on our observational assessment of physical site factors, we differentiated among three landform types: swamps (n = 4; defined by histosol soils), floodplains (n = 6; defined by a level grade and the presence of coarse debris that had been moved by floodwaters), and mesic forest sites (n = 6; defined as upland sites lacking both histosols and evidence of flooding). Half of the sites in each landform type were dominated by native tree species, and half were dominated by common buckthorn. We qualified historical land cover for all sites using aerial photography available at the American Geographical Society Library at the University of Wisconsin, Milwaukee (Table 1). Current (i.e., 2004) land cover was determined in the field to be closed-canopy forest for all sites. Sampling procedure At each forest site, we established four permanent circular subplots, each 12 m in diameter (0.011 ha) and separated by 18 m. Because the subplots were particularly close together (30 m from center to center), we considered each array of four subplots to be a single sample for data analysis (0.045 ha). Within each subplot, we identified and measured the diameter at breast height (dbh; 1.3 m from roots) of all woody stems 􀂕1 cm dbh, and marked each genet (see below) with a uniquely numbered aluminum tag. For our calculations of basal area and density, we differentiated between living and Figure 1. Map showing the relative position of all 16 sites. Symbols are not drawn to scale. Circles denote Rhamnus cathartica (common buckthorn)-dominated sites (1–8), while triangles denote native-dominated sites (9–16). Numbers correspond to site information in Table 1. From left to right, Wisconsin counties depicted in detail are: Columbia, Dodge, Washington, Ozaukee (top row), Dane, Jefferson, Waukesha, Milwaukee (middle row), Green, Rock, Walsworth, and Racine on top of Kenosha (bottom row). Note: while some sites appear to be in close proximity to one another, all are located on unique landforms, and no two are contiguous. 390 Northeastern Naturalist Vol. 14, No. 3 Table 1. Site location (section, township, and range), land-cover history (as determined by inspection of aerial photography at the American Geographical Society Library at the University of Wisconsin, Milwaukee, or in the field for 2004), basal area of living stems, and generalized composition. Site numbers correspond to those appearing in Figure 1. Most dominant species Location Land-cover historyA Basal area (% relative basal area, % relative density) Site type and # Sec T R 63 67 70 75 85 00 04 (m2 ha-1)1st 2nd 3rd Rhamnus sites Floodplain 1 1 7N 8E F F 49 SiMB (44,4) AM (29,10) EC (14,2) 2 27 7N 9E F F 68 EC (41,1) SiM (40,3) AM (14,7) 3 5 5N 21E F F 42 EC (45,1) GA (35,6) CB (10,76) M e s i c 4 17 7N 17E A A A F F 25 CB (95,99) AE (5,1) — 5 13 8N 21E S O F F 10 CB (54,75) WO (19,2) WA (16,5) 6 28 6N 21E S S O O F 36 WA (61,13) CB (23,67) BC (23,2) Swamp 7 17 7N 17E F F F F F 48 CB (100,100) — — 8 17 7N 17E F F F F F 29 CB (66,93) BC (18,4) GA (8,1) AA = active agriculture, F = closed canopy forest wherein the ground is not visible, O = open forest wherein the ground is visible but trees touch each other, S = sparse trees that do not touch one another. BAB = American beech (Fagus grandifolia Ehrh.), AE = American elm (Ulmus americana L.), AM = ash-leaf maple (Acer negundoL.), BA = basswood (Tilia americana L.), BC = black cherry (Prunus serotina Ehrh.), BW = black walnut (Juglans nigra L.), CB = common buckthorn (Rhamnus cathartica L.), EC = eastern cottonwood (Populus deltoides Bartr. ex Marsh.), GA = green ash (Fraxinus pennsylvanica Marsh.), HA = hawthorn (Crataegus spp. L.), RM = red maple (Acer rubrum L.), SH = shagbark hickory (Carya ovata P. Mill.), SM = sugar maple (Acer saccharum Marsh.), SiM = silver maple (Acer saccharinum L.), WA = white ash (Fraxinus americana L.). 2007 J. Mascaro and S.A. Schnitzer 391 Table 1, continued. Most dominant species Location Land-cover historyA Basal area (% relative basal area, % relative density) Site type and # Sec T R 63 67 70 75 85 00 04 (m2 ha-1)1st 2nd 3rd Native sites Floodplain 9 28 1N 10E FC F 24 SiM (45,16) SH (27,6) BW (14,3) 10 6 7N 21E S S O F F 25 EC (57,7) AE (24,28) GA (16,41) 11 6 7N 21E S S O O F 25 GA (56,25) AE (32,28) HA (7,34) Mesic 12 13 8N 21E F F F F 38 RO (35,2) AB (23,17) WA (22,6) 13 35 5N 21E F F F F 47 BA (52,45) BC (15,9) SM (12,23) 14 34 5N 21E F F F F 29 BA (44,35) WA (32,6) SH (6,3) Swamp 15 8 7N 20E F F 31 RM (76,67) GA (12,4) AE (6,24) 16 31 11N 21E F F 36 RM (91,53) GA (6,22) AE (4,16) AA = active agriculture, F = closed canopy forest wherein the ground is not visible, O = open forest wherein the ground is visible but trees touch each other, S = sparse trees that do not touch one another. CPhotography was not available for this site for any year, but it was designated a state natural area in 1958 and field reports confirm it was in closed canopy forest in 1967 (Londré and Schnitzer 2006) . 392 Northeastern Naturalist Vol. 14, No. 3 dead stems, and for density, we also differentiated between independently rooted genets and their attached ramets (methods follow Mascaro et al. 2004, Schnitzer and Carson 2001). In the field, we defined a genet as a woody stem or clump of stems not connected aboveground to neighboring stems. We considered a stem to be a ramet if it was clearly connected between 1.3 m in height and ground level to a confirmed genet. Because we did not excavate the root structures of any individuals, our method likely overestimated the number of true genets in the case of any hidden belowground connections. However, common buckthorn is not known to be clonal (Barnes and Wagner 2004), and cases of misidentified ramets are probably few. Time constraints during the 2005 growing season limited our available sample area at site 4 to two subplots (0.022 ha); however, the total number of trees sampled at this site exceeded the number at any other, and we feel that these data are representative of the particular forest. Statistical analyses We compared differences in structural response variables using a twoway analysis of variance (ANOVA) with both treatment group (i.e., native- v. Rhamnus-dominated) and landform type (i.e., swamp, mesic, and floodplain) as factors (n = 16; SAS 2002). Our response variables were density, basal area, importance value (mean of density and basal area), and woody diversity (species/site), and we differentiated between living and dead stems for all analyses and between genets and ramets for density comparisons. Samples for basal area, diversity, and two density measurements (total living stems and living genets) were normally distributed (Shapiro-Wilk W test; SAS 2002), meeting the assumptions of ANOVA; however, the remaining density samples were not normally distributed. Therefore, we ln-transformed the entire dataset and repeated the analysis. Among the eight Rhamnus-dominated sites, we examined the relative contribution of common buckthorn to density and basal area with respect to landform type. Because the analysis did not include treatment as a factor (i.e., Rhamnus- v. native-dominated), we used a one-way ANOVA (n = 8; SAS 2002). Here all samples were normally distributed with the exception of relative importance value, for which we used ln-transformed values. Results In a general structural comparison between the native- and Rhamnusdominated sites, we found that that those dominated by common buckthorn had significantly higher living stem densities (i.e., all woody stems, ramets + genets; F5,10 = 9.92, P < 0.05; Table 2). In fact, Rhamnus-dominated sites averaged more than twice the stem density of the native-dominated sites (4200 ± 650 [SE] v. 1900 ± 370 stems ha-1). The disparity between living genet density between groups was also large, but highly variable (􀂧2500 v. 1600 genets ha-1 for Rhamnus- v. native-dominated sites, respectively), and thus not significant (F5,10 = 2.82, P = 0.12). Living ramets were far more 2007 J. Mascaro and S.A. Schnitzer 393 abundant in Rhamnus-dominated (􀂧1800 ramets ha-1) than in native-dominated sites (􀂧300 ramets ha-1; F5,10 = 31.86, P < 0.001), and the disparity in ramet density was much greater when dead ramets were included (􀂧3800 v. 􀂧400 ramets ha-1; F5,10 = 44.20, P < 0.01). Despite the density differences, living basal-area values were remarkably similar between Rhamnus- and native-dominated sites (38 ± 6 v. 35 ± 4 m2 ha-1), as were the number of woody species >1.0 cm dbh (10 ± 1 v. 8 ± 2 species site-1). The basal area of all living stems ranged from 10 to 68 m2 ha-1 for Rhamnusdominated sites, and from 24 to 58 m2 ha-1 for those dominated by natives. Taken together, these values are evidence that the Rhamnus-dominated forests we sampled are characterized by relatively high levels of bifurcation, and thus increased stem densities, but by marginal differences in genet densities and nominal differences in basal area. Woody-species richness ranged from one (common buckthorn at site 7) to 17 (at a native site 13) among all 16 sites, although mean richness values between Rhamnus- and native-dominated sites did not differ significantly (Table 2). Woody-species richness differed significantly among swamps, floodplains, and mesic forests (F5,10 = 4.77, P < 0.05), but there was no significant interaction between treatment (i.e., Rhamnus- v. native-dominated) and landform type (F5,10 = 0.61, P = 0.56). Table 2. Mean density, basal area, and woody-species richness in native- versus Rhamnusdominated sites in southern Wisconsin. A genet was defined as a single woody stem or clump of stems not connected aboveground to neighboring stems. Ramets were defined as stems connected (<1.3 m from the ground) to an individual already included in the census. P values (normal and Lntransformed) refer to the treatment effect in a two-way ANOVA with treatment (native v. exotic) and landform type (floodplain, mesic, and swamp) as factors, and an interaction term (SAS 2002). Native Rhamnus P valueA Characteristic dominated (n = 8) dominated (n = 8) Normal Ln trfmd Density (stems ha-1) All stems 2310 (461) 7065 (1523) *** Living stems 1879 (366) 4244 (650) * ** Dead stems 431 (113) 2820 (1050) ** All genets 1901 (351) 3459 (953) ms Living genets 1592 (281) 2465 (477) ns ms Dead genets 309 (89) 995 (587) ns All ramets 409 (126) 3796 (1000) *** Living ramets 287 (104) 1790 (499) *** Dead ramets 122 (36) 2006 (669) ** Basal area (m2 ha -1) All stems 38 (5) 41 (6) ns ns Living stems 35 (4) 38 (6) ns ns Dead stems 3 (1) 3 (1) ns ns Number of woody 10 (1) 8 (2) ns ns species (>1 cm dbh) (species site-1) ANot significant (ns) = P > 0.1; marginally significant (ms) = 0.05 < P 􀂔 0.1; significant (*) = 0.01 < P 􀂔 0.05, (**) = 0.001 < P 􀂔 0.01, (***) = P < 0.001. 394 Northeastern Naturalist Vol. 14, No. 3 Within the eight Rhamnus-dominated sites, the relative density of common buckthorn ranged from 75–100% of all living stems (mean 84 ± 4%). When considering only living genets, common buckthorn density was essentially the same (mean 81 ± 4%, range 66–100%). Thus, while Rhamnus-dominated sites were characterized by higher ramet densities, but only marginally higher genet densities compared to native sites (Table 2), the high relative density of common buckthorn within the eight Rhamnus-dominated sites was consistent for both stem types (Fig. 2). To determine the sizes classes to which common buckthorn is typically dominant, we compared the mean relative density of native versus common buckthorn genets by 5-cm (dbh) size-class within the eight Rhamnus-dominated sites (Fig. 3). We found that common buckthorn genets were more abundant than co-occurring native genets at all size classes below and including 20–25 cm dbh, and that common buckthorn thinned much faster than native plants with increasing diameter size class. Thus, the degree of dominance by common buckthorn appears to extend beyond the smaller understory size classes (i.e., <10 cm dbh), but does not reach the largest size classes encountered overall (>25 cm dbh). The absolute contribution to basal area of living common buckthorn stems at the eight Rhamnus-dominated sites was widely variable, ranging from 3–48 m2 ha-1, with a mean of 15 ± 5 m2 ha-1. However, the relative Figure 2. Relative density, basal area, and importance value (mean of density and basal area) of living Rhamnus cathartica (common buckthorn) individuals (i.e., genets) by ecosystem type in eight sites in southern Wisconsin where it was dominant. See Methods for a description of ecosystem types. Not significant (ns) = P > 0.1, marginally significant (ms) = 0.05 < P 􀂔 0.1, and significant (*) = P < 0.05. Significance refers to the results of a one-way ANOVA among the exotic sites (n = 8), with landform type as the factor (SAS 2002). Importance value data were lntransformed to ensure normality. 2007 J. Mascaro and S.A. Schnitzer 395 contribution of common buckthorn to basal area varied significantly with landform type (F2,5 = 5.96, P < 0.05; Fig. 2). Relative and absolute common buckthorn basal area was lowest in floodplains (7 ± 2%, and 4 ± 0.4 m2 ha-1), intermediate at mesic sites (58 ± 21%, and 14 ± 5 m2 ha-1), and highest at swamps (83 ± 17%, and 34 ± 14 m2 ha-1). Importance values followed the same trend as basal area, but differences among landform types were only marginally significant (F2,5 = 5.35, P = 0.06; Fig. 2). At site 7, near Oconomowoc, WI, common buckthorn composed 100% of all woody stems, reaching 48 m2 ha-1 in living basal area. This high basal area greatly exceeded the mean living basal area for native- or Rhamnus-dominated sites (35 and 38 m2 ha-1, respectively). Relative densities of common buckthorn ramets and genets were poorly correlated to landform type, but had trends similar to those of basal area and importance value (Fig. 2). Discussion Common buckthorn as an ecosystem dominant Despite the general perception of common buckthorn as an invader of forest understories (Skinner 2005 and citations therein), we found evidence that common buckthorn can become the dominant woody plant in temperate forests in southern Wisconsin. In particular, common buckthorn can be the dominant species up to 20–25 cm dbh, reaching a mean basal area of 45%, and essentially forming an exotic-dominated ecosystem type that Figure 3. Ln of relative abundance of Rhamnus cathartica (common buckthorn) and native genets versus diameter by 5-cm size class within the 8 Rhamnus-dominated sites in southern Wisconsin. Regression lines depict the best linear fit to the lntransformed data. 396 Northeastern Naturalist Vol. 14, No. 3 is structurally distinct from native-dominated forests (Table 2). At sites 4 and 7 near Oconomowoc, WI, the species exceeded 90% of the total basal area. At site 7, we found six individuals with diameters larger than 25 cm, two of which were larger than 30 cm (Fig. 4). The magnitude of dominance by common buckthorn reported here far exceeds the levels found for other woody invaders of northeastern Figure 4. A large Rhamnus cathartica (common buckthorn) individual adjacent to site 7 near Oconomowoc, WI. The notebook measures approximately 18 by 24 cm. Photo by J. Mascaro. 2007 J. Mascaro and S.A. Schnitzer 397 Table 3. Comparison of structural characteristics among some woody exotic species commonly found throughout northeastern temperate forests of the US. Relative Relative Importance Total No. density basal area value area of Species Growth form (%) (%) (%) (ha) sites Location Reference Acer platanoides L. Large tree 17.2 8.3 12.8 0.25 1 Madison, NJ Webb and Kaunzinger 1993 (Norway maple) 26.0 7.3 16.7 0.50 1 Madison, NJA Wyckoff and Webb 1996 40.0 36.0 38.0 0.15 1 Ithica, NY Martin 1999 Ailanthus altissima (P. Mill.) Large tree < 20.0B - - 0.06 1 WV Kowarik 1995 (tree of heaven) 51.8C - - 0.00D 1 Duchess Co., NY Knapp and Canham 2000 Berberis thunbergii DC. Low shrub 47.0 - - 0.01E 3 NJ Ehrenfeld et al. 2001 (Japanese barberry) Lonicera X bella Zabel Shrub 2.5F - - 1 Madison, WI Harrington et al. 1989 (showey fly honeysuchle) Rhamnus cathartica L. Small tree 84.0 45.0 64.4 0.36 8 Southern WI This study (common bucktorn) 24.8F - - 1 Madison, WI Harrington et al. 1989 AContiguous with the study area of Webb and Kaunzinger 1993. BEmpirical data not reported; ranked 5th out of eight canopy tree species. CAmong saplings >30 cm in height, in gaps. D(24 m2). ESample points centered around B. thunbergii individuals. FData are for understory only. 398 Northeastern Naturalist Vol. 14, No. 3 temperate forests in the US (Table 3). For example, Acer platanoides L. (Norway maple), arguably the largest exotic tree in the northeastern US, was the most dominant tree at one site in Ithaca, NY, but reached only 36% of basal area (Martin 1999), compared to an average of 45% for common buckthorn across all eight Rhamnus-dominated sites in this study. In a survey of 21 regenerating old fields in New York State, Stover and Marks (1998) found that three woody exotic species exceeded 20% of the basal area at just one site (common buckthorn, Pyrus communis L (common pear)., Pinus sylvestris L. (Sots pine), and a fourth (Malus sp. P. Mill.[apple]) occurred at this level at only two sites. Ashton et al. (2005) described two potentially exotic-dominated forests in Suffolk County, NY, reporting that the percent cover of exotic species was 18 and 51%; however, these values also included herbaceous species (particularly the hyperabundant Alliaria petiolata (Bieb) Cavara & Grande [garlic mustard]). Some degree of underreporting for many species and regions may contribute to our finding that common buckthorn reached higher levels of dominance than other temperate forest invaders. For instance, Kowarik and Körner (2005) noted a tendency for severely invaded forests to be excluded by researchers, and many workers provided only anecdotal accounts of monotypic exotic dominance (e.g., Archibold et al. 1997, Knight 2005). However, empirical data from exotic-dominated forests are certainly lacking, and this omission limits our ability to investigate the importance of these novel ecosystems for temperate-forest dynamics. In contrast, the existence and importance of exotic-dominated ecosystems on tropical islands have been known for some time. For instance, Lugo and Helmer (2004) found that widespread exotic-dominated forests on Puerto Rico may have substantially altered rates of nutrient and carbon cycling, and may encourage the regeneration of native tree species (see also Lugo 2004). Previously described exotic-dominated forests on Hawai’i also exhibit major changes in productivity and nutrient turnover (Hughes and Denslow 2005, Vitousek and Walker 1989, Vitousek et al. 1987). In temperate forests, areas that become dominated by common buckthorn or other exotic species will probably function differently than the native forests they replace. Common buckthorn has been shown to increase carbon and nitrogen mineralization rates where it invades, and these functional effects are likely to be stronger where the species is dominant (Heneghan et al., 2006; K. Knight, pers. comm.). If other invaders function in a similar manner (see Ehrenfeld 2003), the gradual accumulation of exotic-dominated forests could lead to accelerated carbon and nutrient cycling in mainland temperate forests. Natural history and management implications of buckthorn dominance Previous authors that investigated the natural history of common buckthorn in Wisconsin noted that the species prefers upland sites, particularly oak openings or oak savannas (Curtis 1959, Leitner 1985). Our findings are evidence that common buckthorn is also capable of invading and dominating sites with high water tables. In particular, the site with the highest relative 2007 J. Mascaro and S.A. Schnitzer 399 common buckthorn basal area (site 7) also had the highest gravimetric soil moisture content among the eight Rhamnus-dominated sites, and the third highest over all 16 sites (measured in July 2005, data not shown; methods follow Robertson et al. 1999). One study in southern Wisconsin reported that common buckthorn abundance was considerably higher in urban areas (D. Rogers, University of Wisconsin - Madison, Madison, WI, pers. comm.), but our results did not necessarily support this. While some common buckthorn sites (1, 2, 3, 5, and 6) were all located very close to urban centers, several native sites (10, 11, and 12) were equally urban, and site 12 was very close to site 5 (Fig. 1). Several authors have noted that common buckthorn dominates the understory by creating thickets of sprouting stems (Heneghan et al., 2006; Knight 2005 and citations therein). Our finding that ramet densities differ significantly between native- and Rhamnus-dominated sites supports this observation (Table 2). However, we also found that common buckthorn genets are more abundant than those of native trees up to 20–25 cm dbh, and that common buckthorn was the dominant species in all size-classes at two of eight Rhamnus-dominated sites. Thus, common buckthorn may dominate forest ecosystems by reproducing sexually (i.e., producing genets), manufacturing numerous shoots (i.e., ramets), reaching large size, or some combination thereof. We also found a plethora of dead stems in Rhamnus-dominated sites— nearly seven times the abundance found in native-dominated sites (P < 0.05; Table 2). Although dead basal area did not differ between treatment groups, the high abundance of dead stems, mostly ramets, may make Rhamnusdominated sites more prone to fire. Common buckthorn stems are highly combustible, and the species is not fire resistant (while it is resilient via resprouting, fire suppression has been implicated in its proliferation; Curtis 1959, Leitner 1985). Furthermore, the accumulation of ramet thickets in the understory at some sites may promote crown-fire development (Turner and Romme 1994). Most of the sites described herein are small, fragmented stands in close proximity to roads. Thus, the risk of human-caused fires at these sites may be especially high. Common buckthorn may attain its highest dominance when colonizing unforested sites. All three mesic sites, where common buckthorn ranks 1st or 2nd by basal area, were largely open areas in 1963, while the three floodplain sites were completely forested, probably by native trees that are today far larger than the mostly common buckthorn understory (Table 1). The two swamp sites had the largest common buckthorn individuals found (30+ cm dbh), and were in closed canopy forest by 1963. We have not cored common buckthorn individuals at either site, but smaller (18–20 cm dbh) individuals near site 5 were cut at the base and dated at 42 years old (J. Mascaro and S.A. Schnitzer, unpubl. data). Thus, it is likely that the forest canopy observed at the two swamp sites in 1963 includes common buckthorn. We have no examples of common buckthorn becoming a canopy dominant when colonizing intact, closed-canopy forest. 400 Northeastern Naturalist Vol. 14, No. 3 Ecosystem dominance by common buckthorn, although here observed only in southern Wisconsin, is probably not an isolated phenomenon. Common buckthorn is known to dominate the understory throughout suburban Illinois, mid-Minnesota, and the Saskatoon area of Saskatchewan, Canada (Archibold et al. 1997; Heneghan et al., 2006; Wyckoff et al. 2005), and is also widespread in the northeastern US (Skinner 2005). The species is potentially dispersed over long distances by birds and has shown remarkable resilience to removal campaigns, sprouting vigorously after cutting, fire, and even poisoning (Barnes and Wagner 2004). Forest-floor conditions under common buckthorn are not conducive to native-tree regeneration, and may also exclude woody and herbaceous ground flora (Wyckoff et al. 2005). Furthermore, common buckthorn has been shown to significantly alter ecosystem function, and such effects are likely to be stronger where the species is dominant (Heneghan et al., 2006; K. Knight, pers. comm.). Thus, as old fields throughout northeastern temperate forests undergo natural aforestation, land managers should be aware of the substantial community and ecosystem effects of widespread dominance by common buckthorn. Acknowledgments We thank the cities of Middleton, Madison, Brookfield, and Milwaukee, WI, and the WI Department of Natural Resources, University of Wisconsin - Madison Arboretum, Indian Mound Camp, and County of Milwaukee for site access. We also thank Erica Young, Norm Lasca, and two anonymous reviewers for comments on a previous version of this manuscript. Ron Londré provided assistance in the field. 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