common buckthorn (Invasive Plants of Maryland)

common buckthorn

Rhamnus cathartica

Conservation status ⁵

As of 2010, common buckthorn was listed as a noxious weed in Connecticut, Iowa, Massachusetts, Minnesota, New Hampshire, and Vermont in the United States, and in Ontario and Manitoba in Canada [175]. Information on state-level noxious weed status of plants in the United States is available at Plants Database.

Description ⁶

Common buckthorn is a shrub or small tree that can grow to 22 feet in height and have a trunk up to 10 inches wide. The crown shape of mature plants is spreading and irregular. The bark is gray to brown, rough textured when mature and may be confused with that of plum trees in the genus Prunus. When cut, the inner bark is yellow and the heartwood, pink to orange. Twigs are often tipped with a spine. In spring, dense clusters of 2 to 6, yellow-green, 4-petaled flowers emerge from stems near the bases of leaf stalks. Male and female flowers are borne on separate plants. Small black fruits about 1/4 inch in cross-section and containing 3-4 seeds, form in the fall. Leaves are broadly oval, rounded or pointed at the tip, with 3-4 pairs of upcurved veins, and have jagged, toothed margins. The upper and lower leaf surfaces are without hairs. Leaves appear dark, glossy green on the upper surface and stay green late into fall, after most other deciduous leaves have fallen.

A similar problem exotic species is Rhamnus frangula, glossy buckthorn. Glossy buckthorn does not have a spine at twig tips, leaves are not toothed, and the undersides of the leaves are hairy.

NOTE: Several native American buckthorns that occur in the eastern U.S. that could be confused with the exotic species. If in doubt, consult with a knowledgeable botanist to get an accurate identification. Carolina buckthorn (Rhamnus caroliniana), is a lovely native shrub that has finely toothed leaves somewhat resembling those of black cherry, and are smooth on the underside; it produces attractive fruits from August to October. Alder buckthorn (Rhamnus alnifolia), is a low-growing shrub that may grow to a maximum of 3 feet in height, and has leaves with 6-7 pairs of veins.

Ecological threat in the united states ⁷

Exotic buckthorns tend to form dense, even-aged thickets, crowding and shading out native shrubs and herbs, often completely obliterating them. Dense buckthorn seedlings prevent native tree and shrub regeneration. In fire-adapted ecosystems such as savannas and prairies, the lack of vegetation under buckthorn prohibits fires. Buckthorn control is also of interest to small grain producers; the shrub is an alternate host of the crown rust of oats, which affects oat yield and quality.

Successional status ⁸

More info on this topic.

More info for the terms: alvar, basal area, competition, cover, density, facilitation, facultative wetland species, fen, fire exclusion, forbs, frequency, fuel, hardwood, importance value, interference, invasive species, litter, mesic, natural, nonnative species, peat, prescribed fire, restoration, shrub, shrubs, succession, swamp, top-kill, tree, xeric

As of this writing (2010), no information was available regarding shade tolerance or successional status of Dahurian buckthorn. Information in the following sections applies to common buckthorn.

Shade tolerance
Successional role

Oak savannas and forests

Maple-beech, riparian, and coniferous ecosystems

Old-field succession

Influence on succession and plant community dynamics

Shade tolerance: Common buckthorn tolerates shade [6,71] but may be more abundant and grow more quickly under intermediate levels of sunlight [4,74,89,131] if moisture is not limiting [120,243]. Heidorn [90] suggests that common buckthorn requires more light than Dahurian buckthorn, although the reasons for this assertion are unclear.

Throughout its worldwide distribution, common buckthorn does not typically occur in densely shaded communities [74]; however, qualitative descriptions of invaded communities suggest some shade tolerance in common buckthorn. In its native range, common buckthorn frequently occurs in open areas or forest edges [116]. It occurs in similar environments in its invaded range (see Habitat Types and Plant Communities), but it can also establish and persist in closed-canopy forests and is sometimes a dominant understory species in forests and woodlands [6,74,120,131,140,158,226]. In the Thames River watershed in London, Ontario, common buckthorn was positively correlated with canopy closure (P=0.002) [226]. Common buckthorn seedlings are common in low-light environments such as the shade of mature common buckthorn trees 6,74,243 and at the base of other shrubs and trees where birds perch and deposit seed (see Seed dispersal). Common buckthorn seedlings below a large common buckthorn shrub at Wicken Fen branched and produced leaves in the shade [69,72]. However, most common buckthorn seedlings observed in areas of deep shade in London, Ontario, died [175].

Several studies indicate higher densities and/or greater persistence and growth of common buckthorn at intermediate light levels. At 4 study sites in west-central Minnesota, common buckthorn did not grow well in light, dry microsites and grew in substantially darker microsites than bur oak did; however, common buckthorn seedling abundance peaked at intermediate light levels [243]. In an experiment where seedlings of common buckthorn were grown in a forest understory across a light gradient from 1% to 16% midsummer canopy openness, seedling biomass and survival were greater in the highest-light environments after 3 years (P<0.0001) [118]. In a greenhouse experiment, common buckthorn seedling survival and growth were best in 25% to 50% full light. The number of seedlings was reduced in 100% full light, but those that established grew well. Seedlings in the 12.5% light level grew slowly and were "quite sprawling" [74]. Survival of transplanted common buckthorn seedlings was lower in shadier maple forests than in old fields and plantations in central New York [143], and seedling abundance was significantly greater on sites with greater light at the forest floor (P=0.019) [144].

Common buckthorn can survive in deep shade in experimental settlings (e.g., [80]); however, growth and abundance tend to decrease with increased shade (e.g., [80,118,131]). Aboveground growth rates of common buckthorn were greater in an open habitat (0.58 g/g/year) than in a hardwood forest understory (0.23 g/g/year) in southern Wisconsin [89]. See Plant growth for more information on this study. In the greenhouse, Leitner [131] found a general decrease in common buckthorn growth as light decreased; this was especially pronounced under the greatest degree of shading (23% of total exterior solar radiation). In upland, oak woodlots in southeastern Wisconsin, common buckthorn tree/shrub and seedling densities appeared to decrease with shade, and tree/shrub density appeared to increase with canopy openness [131]. See Habitat Types and Plant Communities:Forest and woodland for further details from Leitner's study. In remnant oak savannas in southern Wisconsin [129] and old fields in southeastern New Hampshire [103], common buckthorn was associated with intermediate light levels [129]. In central New York, common buckthorn density was greater on plantation and old-field sites, which had intermediate light levels. Common buckthorn occurred in maple-beech forests, although it was closer to walking trails than expected in those forests (P<0.05), with a strong tendency to occur within 33 feet (10 m) of the trail [144]. Mean attributes of 4 plots in each of 3 site types in central New York. All woody plants >1 m in height and all common buckthorn individuals were censused at each plot. Values within columns followed by different letters are significantly different (P<0.05) [144]. Site type Tree density/ha
(all spp) Shrub density/ha
(all spp) Herbaceous cover (%) ISF* DSF* Common buckthorn density/ha Maple-beech forest 2,802a 10a 18a 0.11a 0.11a 112a Old field 365b 397ab 86b 0.79b 0.78b 5,912ab Plantation 1,719a 794b 57b 0.20a 0.23a 7,717b *Indirect site factors (ISF) is an estimate of the relative amount of diffuse radiation.
**Direct site factors (DSF) is an estimate of the relative amount of direct sunlight given location-specific solar tracks. Both indices range from 0.0, which indicates no radiation, to 1.0, which indicates full and unobstructed sunlight.

Common buckthorn is present in forests dominated by maple (Acer spp.), pine (Pinus spp.), or spruce (Picea spp.) and fir (Abies spp.), although it occurs at low densities and is typically confined to trail edges or gaps in the canopy. Reduced growth and abundance of common buckthorn at very low light levels suggests the very deep shade of these ecosystems may limit common buckthorn invasion as compared with other deciduous forests, shrublands, and grasslands where common buckthorn is common [74].

Common buckthorn reproduction may be best at intermediate light levels, and its shade tolerance may decrease with age. At the University of Wisconsin Arboretum, common buckthorn grew and reproduced in both open wetlands and oak woodlands, and similar numbers of seedlings occurred in each site type. In the open wetlands, common buckthorn trees were younger, larger, and produced more fruit, and common buckthorn seedlings produced fruit at an earlier age than in the oak woods. Shade in the oak woods ranged from 9% to 23% of total sunlight. Many common buckthorn seedlings were observed growing under parent trees in this study; however, common buckthorn saplings seemed not to survive there, but grew around the canopy perimeters [74].

Lower common buckthorn seedling establishment in areas with surface litter may be a function of light availability. Regression analyses showed that common buckthorn seedling density and biomass decreased with lower light transmittance at the soil surface in greenhouse and field experiments [18]. However, many common buckthorn seedlings survive in deep shade (1-2% of full light in midsummer) in the field [118].

Success of common buckthorn seedlings under parent shrubs may be related to factors other than light availability, such as reduced competition with herbaceous species for water and nutrients or facilitation from mature common buckthorn trees. In an oak-dominated forest in east-central Minnesota, common buckthorn seedlings growing near mature conspecifics exhibited greater growth and survival than seedlings far from mature conspecifics in similar light environments [118].

Competition for soil resources can affect common buckthorn seedling survival and growth in shade (e.g., [80,115]). A significant interaction between the effects of irradiance and nutrient supply was observed for common buckthorn (P<0.01) [80]: Seedling mortality and total dry mass of seedlings of common buckthorn after 110 days in 4 shade treatments on soils with differing nutrient supply [80] Numbers of plants dying Mean (SE) dry mass (mg) % daylight 0.3% 1.6% 11% 63% 0.3% 1.6% 11% 63% Grassland (low nutrient supply) 1 0 0 0 20 (3) 45(8) 366(40) 956(98) Scrubland (high nutrient supply) 3 0 0 0 16 (1) 40(3) 458(52) 2145(252)

The number of common buckthorn leaves per plant, total leaf area, and shoot height increased significantly with higher irradiance (P<0.001) and was higher on nutrient-rich soil (P<0.05). Plants responded to increased shading with etoliated stems and decreased root length (P<0.001) [80]. Preliminary results from a field experiment at the Warner Nature Center in east-central Minnesota found that common buckthorn seedlings with no interference from understory plants showed a strong positive response to light with increased survival and growth. However, in subplots planted with native herbaceous plants (to create belowground competition), common buckthorn seedlings had only a small response to increased light [115]. Five- and 17-month-old, experimentally planted common buckthorn seedlings in old fields in New York had significantly greater dry mass in open microsites than under herbs. Growth was also greater for seedlings competing for soil resources than for those competing for light (P<0.025) [65]. These results are consistent with data that show decreased common buckthorn seedling abundance in areas with greater cover of understory herbaceous plants [117] and native shrubs [116].

Desiccation and competition from other plants in open areas may restrict common buckthorn to shaded areas in some habitats [120,243]. Within the grassland biome of North America, common buckthorn is associated with shrublands, hedgerows, lowland river vegetation, and open woods [74]. A detailed survey at the University of Wisconsin Arboretum indicates that common buckthorn is very rare in prairies and savannas, but it is most common and grows best on sites that are either open or moist or both [74,75]. At 4 study sites in west-central Minnesota, common buckthorn saplings were found in significantly darker microsites with higher soil moisture than those where bur oak occurred. Light and soil moisture explained approximately half of the variation in growth seen among common buckthorn saplings [243]. Common buckthorn was not common at Wolf Road Prairie Nature Preserve in Illinois, a rare savanna remnant with an open overstory of mature bur oak. However, it occurred most frequently on sites with lowest mean photosynthetically active radiation (PAR) (about 143.1 mol/m²/sec) and less frequently on sites with a mean PAR ranging from about 164.7 to 826.6 mol/m²/sec [23].

Establishment and persistence of common buckthorn in shade may depend on propagule pressure. Common buckthorn was artificially seeded at varied rates into 1-m² treatment plots with varied light and nutrient availability in a 60-year-old red pine plantation in Ontario. Percent cover of common buckthorn was dependent on both resource availability and propagule pressure (seeding rate). Common buckthorn had significantly greater average cover in shaded subplots (2.3%) than in unshaded subplots (1.3%) (P=0.003), and high propagule pressure subplots had significantly greater common buckthorn cover, density, and aboveground biomass than low or no pressure subplots (P<0.001) [212]. A roost area in a 16-acre (6.5-ha) forested wetland in New Jersey characterized by a dense, closed canopy with a sparse shrub layer of southern arrowwood and highbush blueberry (Vaccinium corymbosum) was first noted by Moskowitz [158] in August 1992. In August 1993, changes in the vegetative community beneath the roost were apparent. There was more than 50% mortality of highbush blueberry and a pronounced change in the species composition of the understory, including seedlings of 4 new species beneath the roost: common buckthorn, American black elderberry (Sambucus nigra subsp. canadensis), pokeweed (Phytolacca americana), and bittersweet nightshade (Solanum dulcamara). Common buckthorn increased from 0% cover on 6 August 1992 to 15% cover on 14 August 1993 [158], likely a result of high propagule pressure from seed in bird feces. No changes were observed in the vegetation of the wetland beyond the limits of the roost. Vegetation changes under the roost suggest a change toward more upland and facultative upland species and a loss of obligate and facultative wetland species [158].

Successional role: Little information was available regarding common buckthorn's successional role where it is native, with the exception of a detailed study at Wicken Fen in southern England. On alkaline peat at Wicken Fen, common buckthorn is characteristic of seral communities and codominates with glossy buckthorn, large gray willow, and other shrubs if left uncut. In the early stages of succession from mixed sedge communities to shrub carr after cessation of haying, glossy buckthorn is frequent. As carr ages, undergrowth is killed out and common buckthorn tends to replace both glossy buckthorn and large gray willow. Succession proceeds toward dense and widespread stands of common buckthorn about 50 years after cessation of haying [69,70,71,72]. The character of the shrub stages depends on initial densities of colonizing shrubs [69,70]. The rapid overtaking of glossy buckthorn by common buckthorn at Wicken Fen was apparently the result of severe and widespread fungal attack on glossy buckthorn [69] and may not represent the "typical" successional pattern. Godwin and others [72] provide further details of plant community changes over 50 years at Wicken Fen.

In its nonnative range, common buckthorn's successional role varies among plant communities and depends on the disturbance history of a given site. Its ability to establish and persist in low-light environments such as a deciduous forest understory suggests that it may occur in these communities during mid- to late-successional stages. However, it may be limited to forests where the native species are less adapted to such conditions or to canopy gaps. For example, invasive populations of common buckthorn often occur in the understory of oak forests that were maintained as oak savannas before fire exclusion began (e.g., [83,131,164,243]), but it rarely establishes in the understories of naturally occurring sugar maple forests, except in canopy gaps such as those occurring along walking trails (e.g., [144]) or following other types of disturbance (e.g., [44,140,246]). Common buckthorn also commonly establishes and persists in abandoned agricultural fields (e.g., [37,103,144,208]), especially former pastures (e.g., [17,193,208]).

The ability of common buckthorn to both tolerate shady conditions and grow quickly in open conditions may give it a successional advantage when canopy gaps are created in areas where it occurs in the understory [116]. Common buckthorn was the most responsive to light among 4 shade-tolerant, woody species tested in Europe [80]. In open woods in eastern Ontario, thousands of small common buckthorn saplings and seedlings covered the ground in shaded locations, and common buckthorn trees and large shrubs occurred in openings. When large common buckthorns were removed, small common buckthorn plants grew rapidly, forming a solid stand [160,175]. Three years after thinning treatments in pine plantations in the Oak Openings Preserve in northwestern Ohio, common buckthorn occurred in 10% of control plots and 36% of thinned plots. Common buckthorn cover and frequency increased after thinning, and common buckthorn and sheep sorrel (Rumex acetosella) were the most important nonnatives on thinned plots [1].

Oak savannas and forests: Much of the following information on succession and fire regimes in oak savannas was compiled from literature reviews in several papers: [4,22,23,24,113,125,131]. See Habitat Types and Plant Communities: Savanna for examples of savanna community composition. Historical descriptions [23] and fire scars on old savanna oaks [83] suggest that savannas in the central United States were maintained by frequent fires, often set by humans [4,22,23,24,125,131]. See FIRE REGIMES for more details. Frequent fires and moisture gradients resulted in communities ranging from xeric prairies, to savannas with an open understory, to relatively open oak woods, to shady, mesic stands dominated by maples [131]. The boundaries among these communities were neither discrete nor stationary but changed in response to climate fluctuations and the frequency and intensity of fires [113].

With postsettlement fire exclusion, some savannas succeeded to oak forests, sometimes with dense understories, as oak sprouts grew into oak trees (e.g., [23,59,113]), and native and nonnative woody species established, spread, and interfered with native savanna plants [4,5,83,125,131]. Common buckthorn was introduced to this landscape around the same time that fire exclusion began, as birds dispersed its seeds into a niche that was not suitable for native savanna species that were adapted to the frequent-fire regime [131]. Common buckthorn is now a common component in the understory of some oak forests and savannas [83,113,131]. It is often concentrated along edges of eastern oak forests that were forested at the time it was introduced, whereas some oak forests that originated as woodlands or savannas have common buckthorn throughout [59]. This suggests that common buckthorn established when the canopies were more open and has persisted as the canopies became more dense. While it has been suggested that fire exclusion along the prairie-forest border may increase common buckthorn invasion of prairies (e.g., Leitner 1984 personal communication cited by [36]), the limited success of common buckthorn in light, dry microsites within bur oak communities suggests that common buckthorn is not a pioneer species in fire-excluded prairies. However, it might establish after other pioneer woody species establish and provide some shade [243] and perches for bird dispersers [144].

According to Leitner [131], the most prevalent forms of anthropogenic disturbance in southern Wisconsin oak woodlots after the cessation of frequent fires were cattle grazing and timber cutting. Selective feeding by cows was thought to eliminate seedlings of desirable species such as maples, ashes, and basswoods while leaving the undesirable species such as hawthorns, barberries, and prickly-ashes. When cows were removed, the browse-resistant species had an initial advantage. While common buckthorn bears thorns, these seem to confer little protection from browsing, and common buckthorn was practically absent from the most heavily grazed oak woodlands studied in southeastern Wisconsin. However, common buckthorn can establish and spread in the openings created by browsing when cows are removed. A thick, sometimes impenetrable shrub layer developed in many of these woodlands following the cessation of grazing and fire. Over 50% of the stands in this study had shrub densities exceeding 10,000 shrubs/ha, with a high of 19,400 shrubs/ha, composed of native species such as gray dogwood and viburnum, nonnative species such as common buckthorn and honeysuckles, and survivors from the grazing period [131].

Several studies note dense and abundant common buckthorn populations in bur oak communities (e.g., [83,131,243]). Leitner's study of common buckthorn in 28 upland, oak-dominated woodlots in southeastern Wisconsin suggests that the bur oak savanna presents the ideal set of conditions for common buckthorn invasion and growth. However, the abundance of mesic species in the understory of all but the most xeric sites suggests that with continued exclusion of fire these communities may succeed to more mesic communities. These woodlots were never clearcut, so common buckthorn could not have been part of the pioneering vegetation on these sites, but instead invaded established forest or savanna [131]. Oak savanna remnants with canopy intercept of 40% to 70% may have been the vegetation types most vulnerable to common buckthorn invasion [4]. Field observations of 24 oak woods in northern Indiana and Illinois, and southern Wisconsin found common buckthorn was most common in tallgrass savanna dominated by bur oak, but also occurred in mesic savanna dominated by white oak and northern red oak. Fire exclusion from mesic savanna leads to increased abundace of sugar maple, red maple, and other hardwood species [83]. In several of the study areas surveyed in 1986, canopy oaks were nearly gone, oak seedling regeneration was poor to absent, and a dense understory of 16- to 26-foot (5-8 m) buckthorn shaded virtually bare ground [4]. In xeric, bur oak woodlots, common buckthorn produced denser shade than that of the surrounding forest matrix. Increased shade under common buckthorn may further reduce oak regeneration, reduce cover of native spring ephemerals, and favor more shade-tolerant tree species as succession proceeds [131]. See Influence on succession and plant community dynamics.

If oak forest and savanna sites are subjected to additional disturbances that open the canopy, such as fragmentation resulting from urbanization or logging operations, increased edge habitat and light penetration and simplified stand structure may result in a more xeric microclimate and favor continued dominance of common buckthorn [131].

Maple-beech, riparian, and coniferous ecosystems: Common buckthorn is less likely to establish in maple-beech and coniferous ecosystems than in oak-dominated ecosystems, and it seems to require some type of canopy-opening disturbance to establish. Once it has established, additional canopy disturbance may lead to increased growth and abundance of common buckthorn.

In Syracuse, New York, common buckthorn occurred in 78% of regenerated urban forest patches (had no canopy cover in 1939, but had canopy cover in 1978) and 100% of remnant urban forest patches (had canopy cover in both 1939 and 1978 aerial photos). In regenerated forest patches, it was sometimes dominant in communities characterized by sugar maple, Norway maple, or boxelder. In remnant forest patches, it had significantly greater average stem density and basal area in black oak communities (330 stems/ha and 0.58 m²/ha, respectively) than in sugar maple communities (70.7 stems/ha and 0.11 m²/ha) (P<0.05). The authors suggest that as gaps are formed in black oak canopies, common buckthorn can colonize available sites due to lack of oak regeneration in the absence of fire. Sugar maple, on the other hand, can establish beneath its own canopy, thus limiting recruitment sites for other species such as common buckthorn [246].

Results of studies in central New York suggest canopy openings or edges are important for common buckthorn establishment. Common buckthorn seed dispersal was lowest, seed predation highest, and seedling survival lowest in closed-canopy maple forests compared to plantations and old fields. A combination of low dispersal by frugivores, low seed survival due to predation, and low seedling survival due to dim light apparently prevents common buckthorn from invading intact maple forests [143]. These maple forests were free of anthropogenic disturbances for a century or more and had relatively intact canopies. The small populations of common buckthorn observed in these habitats were associated with walking trails and the canopy openings created by these trails. Disturbances that cause the opening of intact, closed-canopy forests in the Northeast will likely promote invasion by common buckthorn, as long as seed sources are nearby. Once common buckthorn is established, mature plants can serve as points of recruitment for new seedlings [144]. Common buckthorn occurred at the edge of a riparian forest along Little Otter Creek, a slow-moving stream in Vermont that floods each spring into the riparian forest to distances of about 328 feet (100 m) from the stream edge. Common buckthorn occurred in shrub-dominated plots 16 to 82 feet (5-25 m) from the stream edge; it did not occur in the forest understory plots 164 to 328 feet (50-100 m) from the stream edge [105].

Several studies provide examples of common buckthorn establishment following canopy-opening disturbances. Common buckthorn did not occur in an eastern white pine forest in Cedar Creek Natural History Area, east-central Minnesota, before or 2 weeks after a July 1983 storm that resulted in widespread windthrow with substantial tree mortality. However, 10 years later (1993) common buckthorn stems >1 inch (2.5 cm) DBH had a basal area of 0.01 m²/ha and a density of 6.6 individuals/ha; in 1997, basal area was still 0.01 m²/ha but density had increased to 9.9 individuals/ha [7]. In Ottawa, Ontario, common buckthorn was rare in maple-beech forest interior plots prior to an ice storm in January 1998, but it was abundant in the 4 years following the canopy-opening disturbance [44]. Common buckthorn seedling density was lower in undisturbed control plots than in treefall gaps created by severe windstorms in a mixed oak-Virginia pine (Pinus virginiana) forest in northern Virginia in July 1990. Although differences were not significant, common buckthorn averaged 347 seedlings/ha in gap plots and 55 seedlings/ha in control plots [163]. One common buckthorn stem, <6 inches (15 cm) DBH, was recorded in an American elm swamp where the elms had been killed by Dutch elm disease. It was not recorded in 2 similar swamps in the area [9]. Common buckthorn was recorded after flooding ceased in a green ash-red maple forest that had been flooded for waterfowl Habitat management at Montezuma National Wildlife Refuge, central New York. It occurred in the ground layer 2 years after cessation of flooding and in the shrub layer 18 years after flood cessation. Densities of some native shrub species, green ash saplings, and overstory American elm were reduced during the same period, likely providing the openings for common buckthorn establishment. Common buckthorn was not mentioned as occurring in the control site (a swamp) [46].

Common buckthorn occurs in a variety of anthropogenically impacted and disturbed forests. It occurs in urban forests throughout its distribution in Canada [45,154,175,226], in the Great Lakes area (e.g., [74,97,133]) including Minnesota (e.g., [100,157,243]), and further east (e.g., [246]). Populations in urban forests can act as propagule sources for spread into wildlands and nature preserves. In central and western Massachusetts, common buckthorn was among several species associated with high-intensity tree harvest and not with low-intensity tree harvest in 4 forest types (mixed-hardwood, oak, eastern hemlock, and white pine forests). No comparison was made to unharvested forests [147]. Common buckthorn occurred in 8.1% of 148 randomly selected forest sites that had been harvested between 1984 and 2003. Though it was typically rare (1 individual) or uncommon (2-10 individuals) in these sites, it was common (>10 individuals) in 17% of invaded plots. Common buckthorn occurred most often on sites that were formerly plowed, and less often on former pastures or continuously forested woodlots [146]. Study results suggest that regional patterns of invasive species distribution are affected not only by disturbance but also by soil characteristics and "the current and historical landscape context" [146,147].

A comparison of historical data from a 1950 survey of floodplain forest vegetation on the Lower Wisconsin River [41] to data collected there in summer of 2001 [82] suggests that these forests are shifting to a later successional stage that is outside the historical range of variation. In the 1950s, floodplain forests in southern Wisconsin were typically colonized by black willow and eastern cottonwood on recently scoured or deposited substrates. River birch and swamp white oak (Quercus bicolor) were early-successional species in forest gaps. These communities typically succeeded to dominance by American elm, green ash, and silver maple, with a sparse woody understory. In the past, it was thought that regular flooding would typically prevent succession beyond this stage. Since the 1950s, however, the interaction of river regulation, elm mortality, increased browsing pressure from white-tailed deer, timber harvest, and invasive species establishment has led to changes in floodplain forest vegetation. The understories of 3 of the 5 study sites surveyed in 2001 were dominated by 2 invasive species: common prickly-ash and common buckthorn. Both species are thought to have established and spread in canopy gaps resulting from timber harvest and Dutch elm disease. Neither of these species is considered particularly flood tolerant, nor are they particularly palatable to white-tailed deer due to their thorns. Common buckthorn had the highest importance value of all understory species at Ferry Bluff, which is 1 of 2 sites that apparently experienced greater vegetation removal since the 1950 sampling. Common buckthorn was not recorded on this site in 1950 [82].

Mascaro and Schnitzer [140] found evidence that common buckthorn may attain its highest dominance in forests when it establishes prior to afforestation, then persists as forests develop. No examples were found of common buckthorn becoming a canopy dominant when colonizing intact, closed-canopy forest [140]. Eight forested sites where common buckthorn was a dominant species were studied in southern Wisconsin. The 3 mesic sites in this study—where common buckthorn ranks 1st or 2nd by basal area—were largely open areas in 1963. The 3 floodplain sites—where common buckthorn dominates the subcanopy—were completely forested in 1963, probably by native trees. The 2 swamp sites—which have the largest common buckthorn individuals encountered (>12 inches (30 cm) DBH)—were closed-canopy forest in 1963, probably with some common buckthorn present [140]. Site type, land cover history (as determined by inspection of aerial photography, or in the field for 2004), basal area of living stems, and generalized composition of dominant species on 8 forested study sites where common buckthorn was a dominant species in southern Wisconsin [140] Site type and # Land cover history by year* Basal area
(m²/ha) Most dominant species*
(% relative basal area, % relative density) 1963 1967 1970 1975 1985 2000 2004 1st 2nd 3rd FP 1 F F 49 SiM (44,4) AM (29,10) EC (14,2) FP 2 F F 68 EC
(41,1) SiM (40,3) AM (14,7) FP 3 F F 42 EC
(45,1) GA (35,6) CB (10,76) Ms 4 A A A F F 25 CB (95,99) AE (5,1) -- Ms 5 S O F F 10 CB (54,75) WO (19,2) WA (16,5) Ms 6 S S O O F 36 WA (61,13) CB (23,67) BC (23,2) Sw 7 F F F F F 48 CB (100,100) -- -- Sw 8 F F F F F 29 CB (66,93) BC (18,4) GA (8,1) *FP= floodplain, Ms= mesic, Sw= swamp.
**A= active agriculture, F= closed-canopy forest (ground not visible), O= open forest (ground visible, but trees touch each other), S= sparse trees (do not touch each other); blank cells indicate no data available for that site in that year.
**AB=American beech, AE= American elm, AM=boxelder, BA=basswood, BC=black cherry, BW=black walnut, CB=common buckthorn, EC=eastern cottonwood, GA=green ash, HA=hawthorn, RM=red maple, SH=shagbark hickory, SM=sugar maple, SiM=silver maple, WA=white ash, WO=white oak.

Old-field succession: Former agricultural fields throughout the eastern United States are undergoing succession to forest. No generalized pattern of old-field succession exists because of variability in agricultural use (e.g., cultivation, pasture), potential seed sources, and high variability in site conditions (review by [37]). Common buckthorn occurs on many former agricultural sites at various stages of succession (see below). As old fields in northeastern temperate forests undergo afforestation, there is potential for common buckthorn to become dominant in the plant community, alter successional trajectories, and have other adverse impacts on the invaded community [140].

Common buckthorn may establish in early succession after field abandonment. A study of seedling establishment suggests that common buckthorn establishes and persists better in open microsites than in those with herbaceous cover [65]. However, observations by Whitford and Whitford [236] suggest that dense, ungrazed bluegrass (Poa spp.) sod does not prevent establishment of common buckthorn. Common buckthorn and other woody species occurred as small and suppressed individuals beneath the herb canopy in a field in London, Ontario, that was mown yearly for at least 35 years until 2001. After annual mowing ceased, common buckthorn dominated the shrub layer within 5 years, and female trees produced large seed crops in 2006 (Cavers personal observations cited by [175]).

Some authors have suggested that common buckthorn does not establish in early old-field succession because these fields lack perches to aid in common buckthorn seed dispersal by birds [57]. In southwestern Michigan old fields, common buckthorn established 6 years after field abandonment. Common buckthorn stem density was 0.17 stem/200 m² in year 6 and 3.17 stems/200 m² the following year. The authors noted that at this time other woody species, especially staghorn sumac, had established and provided bird perches [57]. However, other authors observed common buckthorn establishment in fields without perches [236].

Many old fields in central and western New York have a similar land use history: a century of dairy farming followed by abandonment beginning during the 1930s. Similarities in old-field succession may be expected on these sites (Stanton and Bills 1996 as cited by [37]). In a chronosequence study of old fields in western New York, common buckthorn was dominant in early to middle succession but decreased in importance 70 years after abandonment. Common buckthorn averaged 20.3% cover and was among the dominant species on old fields 30 years after abandonment. Fifty years after abandonment, common buckthorn averaged 39.4% cover. On old fields in later succession (70 years after abandonment), common buckthorn averaged only 0.6% cover [37]. Common buckthorn population dynamics in old fields and abandoned plantations in central New York were studied by McCay and others [143,144,145]. Old fields were abandoned 10 to 30 years prior to the study; they had a dense cover of grasses and forbs and sparse cover of shrubs and small trees, of which common buckthorn was among the most abundant. The conifer plantations were planted 35 to 45 years prior and had not been thinned; before planting, they were in agricultural land use. Plantations had many canopy gaps and an "overwhelming number of buckthorn" [143]. Common buckthorn populations in plantations were larger, more evenly distributed, and more mature than populations in old fields or maple-beech forests in this landscape [144]. Common buckthorn is not a universal dominant in abandoned agricultural land, however. Of 483 mesic, abandoned pasture plots surveyed in the Bristol Hills in central New York, common buckthorn dominated 3 plots where quaking aspen and black cherry had high importance values. The author speculated that common buckthorn would eventually be replaced by quaking aspen and black cherry on these sites. Common buckthorn also occurred with hawthorn in shrub-dominated areas on mesic and dry-mesic fields. Most common buckthorn and hawthorn on these sites were in poor condition and shaded by eastern white pine, suggesting eventual replacement of these shrubs by eastern white pine [193].

A study of old-field succession comparing sites that were historically in pasture to those that were historically in cropland in Tompkins County, central New York, found that common buckthorn was more common in abandoned pastures than in abandoned cropland. Common buckthorn was among the most common species in the tree and shrub layer on former pasture sites, occurring in 4 of the 12 sites and contributing ≥20% of the basal area on 1 of those sites; it occurred on only 1 of the former crop sites. Common buckthorn occurred in the ground layer on 10 of the 12 pasture sites and on 5 of the 9 crop sites. The authors suggest that common buckthorn persisted on pasture sites because it is not commonly browsed by livestock (but see next paragraph). Crop sites ranged in age from 12 to about 40 years since abandonment; pasture sites were similar in age with the exception of 2 younger sites (age not given) [208]. A study of 36 farmlands abandoned in the 1970s in southwestern Quebec reveals 2 ecological groups: one comprised of former pasturelands and dominated by spiny shrub vegetation, including common buckthorn, and the other comprised of formerly cultivated fields dominated by either hydric herbs or shrubs. It is not clear whether common buckthorn occurred in the latter ecological group, but it was not among the dominant species. Differences in common buckthorn occurrence and plant community composition between sites was associated not only with land-use history and time since abandonment, but also with abiotic variables (e.g., slope, surface stoniness, drainage), which in turn influence how a parcel of land was most likely used [17].

Despite its common occurrence on abandoned pastureland, some authors indicate that common buckthorn is subject to browsing pressure by livestock and wildlife (e.g., [131,236]), and removal of browsing pressure may increase its importance. Observations in a small permanent pasture in Wisconsin with a history of use by both cattle and white-tailed deer suggest that common buckthorn establishment and persistence may have been facilitated by eastern redcedar acting as a nurse plant and protecting common buckthorn from browsing. Apparently common buckthorn was more palatable to browsers than eastern redcedar. Only the youngest tips of the eastern redcedars were browsed, and the common buckthorn seedlings were able to establish within the dense older growth of these eastern redcedars. Near the middle of each clump, the topmost buds of common buckthorn eventually grew above the reach of browsers and spread into normal crowns, overtopping the eastern redcedars. All common buckthorn individuals had been repeatedly browsed on all branches <4 feet (1.3 m) above the ground, such that remaining branches at that level were very short and densely branched. No other common buckthorn were found in this pasture except 2 individuals that were protected against rocks but were severely browsed and greatly suppressed [236].

Common buckthorn appears to be most abundant in midsuccessional old fields but may persist in later stages of succession on some sites. In a chronosequence study of old field succession in southwestern Ohio, common buckthorn did not occur in 2-year-old, 10-year-old, 90-year-old or 200+-year-old stands. It was present but not common (importance percentage or relative cover ≥5%) in 50-year-old stands dominated by goldenrod and with canopy tree cover of about 30% [222]. A 22-site chronosequence was used to study upland old-field succession in southeastern New Hampshire. Five woody community types were identified ranging from recently abandoned fields to eastern hemlock forests greater than 200 years old. Common buckthorn occurred at low densities in the tall shrub stratum in all types, with the highest relative importance values (<5%) in early- and midsuccessional types. Common buckthorn also occurred in the herb stratum in early-successional and midsuccessional community types [103].

Influence on succession and plant community dynamics: Persistence and spread of common buckthorn in native plant communities may alter plant community composition, stand structure, and successional trajectories in invaded communities. Hobbs [100] suggests that if invasive populations of common buckthorn suppress native tree regeneration, succession may tend toward a more open canopy; whereas, if tree regeneration is not suppressed, succession may lead to a more closed canopy and an eventual loss of common buckthorn. However, it is unclear how long common buckthorn can persist under a closed canopy, and observations of other authors (e.g., [6,74,120,131,140,158,226,243]) suggest that it tolerates substantial shading. Research in Chicago-area oak woodlands [95,96,97] and the University of Wisconsin Arboretum [137] showed altered soil properties under common buckthorn such as changes in soil chemistry, nutrient composition and cycling, and soil fauna and microbial communities, which are likely to impact plant community succession on invaded sites. These changes may have a "legacy effect" that impedes native plant establishment even after common buckthorn is removed [93].

Patches of common buckthorn likely interfere with the establishment and persistence of native shrubs, herbs, and tree seedlings. Alsum [3] found differences in woody species composition between invaded and uninvaded sites along the Lower Wisconsin River. Sites where common buckthorn was present also had higher density of woody seedlings (including common buckthorn), lower herbaceous cover, more weedy and nonnative species, fewer sensitive native plant species, lower density of several native woody species, and greater density of invasive honeysuckles than sites without common buckthorn [3]. While largely anecdotal, several other authors note impacts on native species in communities invaded by common buckthorn [20,31,59,74,90,131,144,232]. Observations by Gourley [74] in Wisconsin indicate that thickets of native shrubs generally had an herbaceous ground layer, whereas thickets of common buckthorn did not. She suggests that because common buckthorn leafs out early and retains leaves late in the season (see Phenological niche separation), there are no seasonal light fluctuations that would allow the germination and survival of tree seedlings, spring ephemerals, and understory shrubs [74]. Where common buckthorn occurs in wetlands, the understory tends to be completely devoid of any other vegetation. It is possible that dense common buckthorn shades out the understory or that white-tailed deer, which tend to congregate in these areas, impede vegetation growth. Areas within the wetland that contain the greatest common buckthorn seedling growth are usually the areas where the typical grass and sedge matrix is absent [74]. Catling and Brownell [31] examined 63 alvar sites in the Great Lakes region that had relatively high native plant diversity. Common buckthorn was especially common among invasive shrub species on the Napanee and Smith Falls plains, where it "has reduced native herbaceous flora" [31].

Dense shade under patches of common buckthorn may interfere with native plants [131,164]. Removal of common buckthorn from Chicago-area oak woodlands resulted in an increase of more than 10% in the openness of the canopy, measured at 5 feet (1.5 m) above the floor (Heneghan and Umek in prep cited by [93]). Packard [164] describes areas in Illinois that were dominated by tallgrass oak savanna in presettlement times and dominated by "an unbroken sea of buckthorn" in the early 1970s. Native plants were lacking and bare dirt was common under the buckthorn [164]. Measurements in oak woodlands at the University of Wisconsin Arboretum showed that light penetration was consistently lower under common buckthorn than in the surrounding forest matrix. However, decreases in native plant cover were not consistently associated with common buckthorn invasion. While herb cover was consistently lower in common buckthorn plots, differences were not significant. Similarly, numbers of woody seedlings were highly variable and showed no significant differences between plots with and without common buckthorn (P>0.05) [131]. Experimental studies of individual common buckthorn shrubs showed no negative effects on understory plants [118,119].

Areas where common buckthorn has invaded may be impacted by a variety of other disturbances that may also affect native plant populations. Study sites in common buckthorn-invaded oak woodlands in the Chicago area, for example, had "a depauperate herbaceous flora". The authors suggest that this was likely due to a combination of fire exclusion, dense shading from invasive woody species, a history of domestic livestock grazing, and an overabundant white-tailed deer population [97]. Along the Lower Wisconsin River, changes in plant community composition in the latter half of the 20th century may be attributed to changes in the flood regime, as well as death of overstory elms from Dutch elm disease, invasion of common buckthorn, changes in land use, and large populations of white-tailed deer [3]. Differential insect herbivory between native and nonnative species may also contribute to increases in common buckthorn associated with declines in native species abundance. Herbivory on 8 native species averaged 4.3% of leaf area lost, significantly more than the 0.8% loss to herbivory on common buckthorn [93].

Evidence of a synergy between common buckthorn and nonnative invasive earthworms, in which each invasive species positively reinforces the population of the other, has been observed in oak woodlands in the Chicago area [92,97] and at the University of Wisconsin Arboretum [137]. Nonnative invasive earthworms were most abundant and had the greatest biomass in subcommunities dominated by common buckthorn compared to those dominated by white oak, northern red oak, or sugar maple [97]. Earthworm populations are responsible for a very rapid incorporation of forest floor material into the soil [58], and earthworms showed a preference for common buckthorn litter in a decomposition experiment [92,97]. As keystone detritivores, invasive earthworms can change seedbed conditions, soil characteristics, plant-herbivore interactions, and flow of water, nutrients, and carbon ([58], review by [137]). Some earthworm species have been linked to declines in native plant diversity and declines in native soil micro- and mesofauna in northern hardwood forests (review by [137]).

Rapid decomposition of common buckthorn litter by earthworms and soil microbes may alter soil biochemistry in invaded sites. Common buckthorn litter decomposed much faster than litter from the native northern red oak overstory at the University of Wisconsin Arboretum [137] or litter from white oak, northern red oak, or sugar maple in Chicago-area woodlands [92,97]. Common buckthorn litter is high in nitrogen and has a low carbon:nitrogen ratio [95,96]. Soils under common buckthorn patches had higher percentages of nitrogen and carbon, modified nitrogen mineralization rates, elevated pH and gravimetric water content, and modified microbial communities compared to uninvaded parts of the woodland [94,95,96]. It has been suggested that these changes might promote growth of common buckthorn seedlings beneath the canopies of conspecifics [74,116,118], whereas the abundance of herbaceous plants in the genera Aralia, Botrychium, Osmorhiza, Trillium, Uvularia, and Viola can be reduced by invasive earthworm populations [58]. Additionally, loss of forest litter may result in the collapse of invertebrate populations that reside in the litter layer [93]. Because invertebrate populations support a large woodland foodweb that includes mammals and birds, the impacts of common buckthorn and nonnative earthworm invasion on native communities may be extensive [92]. Changes in the litter layer with common buckthorn invasion also impact surface fuel characteristics and may impede the use of prescribed fire for management of invaded communities. A single removal of common buckthorn from invaded areas sometimes allows for native plant establishment (e.g., [20]). However, common buckthorn typically sprouts after top-kill (see Vegetative regeneration), and its seedlings establish from the soil seed bank in disturbed areas (see Germination and Seedling establishment), so it often reestablishes in areas after control efforts (see Control and Use of prescribed fire as a control agent). Other nonnative species may also establish (e.g., [20]). The positive feedback loop between common buckthorn and nonnative earthworms [97] and other changes in soil properties in invaded areas [92] may facilitate reestablishment of common buckthorn following its removal by cutting, chemical treatment, or prescribed fire [97] and retard restoration efforts [92]. After removal of common buckthorn in Chicago-area oak woodlands, populations of earthworms remained high (Heneghan and Umek personal observations cited by [97]), and common buckthorn reestablished and appeared to grow vigorously [97]. However, an experiment at the University of Wisconsin Arboretum in a mixed-hardwood stand dominated by northern red oak in the overstory and common buckthorn in the understory showed a 63% reduction in earthworm abundance the summer following common buckthorn removal (in September 2003 and spring 2004) compared to plots where no common buckthorn was removed. Common buckthorn aboveground biomass was removed again in the late spring of 2005 and 2006. Earthworm abundance in the common buckthorn removal treatments increased over time after the initial reduction, but it remained significantly lower than in control plots throughout the study period (P<0.10). However, no recruitment of native woody species was observed in any of the removal plots in 2004 or 2006 [137].

New york state invasive species information ⁹

Background

Common buckthorn (Rhamnus cathartica) is a small deciduous tree or large shrub that can grow to six meters in height. It has dull green oval or egg shaped leaves and is easily identified by the small thorns at the tip of its branches. It is also known as European buckthorn, European waythorn, and Hart’s thorn. Common buckthorn is considered an invasive species throughout most of the northeastern and central United States and southeastern Canada because of the dense thickets it forms.

Origin

Common buckthorn is native to most of Europe (except Iceland and Turkey) and western Asia. It was brought to North America some time in the 1800s for use as an ornamental shrub and wind break but did not have wide spread distribution until the early 1900s. It is found in hedgerows, along roadsides and on ravine slopes.

Biology and Identification

Common buckthorn is a perennial shrub or small tree. It is found in lightly shaded areas and is tolerant of many soil types from well-drained sand to clay. Branches are tipped with a short thorn; a thorn may also be found in the fork between two branches. The leaves may be opposite or in an alternating pattern (both may be found on the same branch). The leaves are oval or egg shaped with small, serrated teeth. The leaf may be a dull green or a dark green with a lighter green on the under side. Flowers are small with four sepals (a modified leaf that encloses the petals and other parts of the flower) and four petals and they form small clusters from the axils (the space between a leaf or branch and the stem/stalk of the plant) of leaves or on short twigs along the stem. The flowers are a yellowish to green color. Each flower is unisexual with either four stamens or one pistil with a plant being either male or female (dioecious). The fruit or berries are small (5-6 mm in diameter) and are a dark purplish or black color. Each berry will contain four hard seeds. The common buckthorn flowers during late spring (May-June) while leaves are emerging. The berries ripen during August and September and can be found still attached to the plant throughout the winter.

Common buckthorn leaves may be opposite or alternating with both possible on the same branch. Leaves are oval or egg shaped with small, serrated teeth Buckthorn seeds are easily spread by birds and other wildlife. It is fast growing and will reproduce from seeds or by stump sprouting. The seeds may remain viable in the soil for up to five years. Common buckthorn can be distinguished from native and other non-native buckthorns by its sharp, thorn-tipped branches and from native Hawthorns (Crataegus spp.) on which the thorns grow from the sides of branches. It also has noticeable forward-curved side veins on its leaves and clusters of purplish-black berries that have 4 hard seeds.

Impacts

Common buckthorns form thick hedges with long branches that crowd out and shade out native shrub and herbaceous species, preventing regeneration of native plants. In fire prone areas the lack of herbaceous ground cover underneath the buckthorn hedge may prevent fires from spreading. The common buckthorn is a host for the crown rust fungus (Puccinia coronata), an agricultural pest that inhibits the yield and quality of oats. It may also serve as a overwintering host for the Asian soybean aphid (Aphis glycines Matsumura), a pest known to damage soybeans and can spread a variety of horticultural viruses. Buckthorn leaves have a high concentration of nitrogen and the decomposition of leaf litter changes soil nitrogen content and can increase the pH levels in the soil. These changes create better growth conditions for the common buckthorn perpetuating their persistence.

Prevention and Control

There are several methods available for control of common buckthorn. These controls include mowing, excavation, cutting and burning. Repeated mowing and cutting has been shown to reduce the vigor of the plants. The plants may be removed by hand or with heavy equipment depending on the size of the shrubs. Care should be taken to not disturb the roots of other plants. The disturbed area, now devoid of the invasive plant, may become the home for new common buckthorn seedlings or other opportunistic invasive plants. As noted earlier, the seeds may persist in the ground for five years resulting in new growth. Prescribed burns are another way to control buckthorns in fire-adapted ecosystems. Fires will top-kill mature plants; however sprouting can occur from the roots and trunks. There are also several chemical methods (Table 1) available for controlling common buckthorn. These are generally applied to the stumps after cutting to prevent sprouting. There are no currently known biological controls for common buckthorn. Research into biological controls for common buckthorn is in progress.