This description provides characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [18,28,83]).
Glossy buckthorn grows as a multistemmed shrub or single-stemmed small tree up to 23 feet (7 m) tall [21,28,60,83]. Trees produce stout, erect branches [9]. Leaves are deciduous, simple, and generally arranged alternately. Blades are dark-green and shiny above, measure 2 to 3 inches (5-8 cm) long, and are about half as wide. Margins are entire to slightly wavy [9,28,83]. Studies in European habitats suggest that plant age and site conditions can affect glossy buckthorn's appearance. While common as a multistemmed shrub when young, glossy buckthorn develops into a small tree with age. In the shade, shrub forms are sparse and leaves are larger, thinner, and less rigid than those in the sun [31]. Glossy buckthorn flowers are small (diameter <6 mm), inconspicuous, and occur singly or in clusters of 2 to 8. The 2- to 3-seeded, berrylike drupe fruits occur in small clusters [9,28,83]. As of 2008, descriptions of the glossy buckthorn root systems were generally lacking, although its shallow root system has been described as "extensive" (Fagan, personal observation cited in [20]).
Native plants that resemble glossy buckthorn include alderleaf buckthorn (Rhamnus alnifolia) and lanceleaf buckthorn (R. lanceolata) [15].
Glossy buckthorn is a nonnative species common throughout southeastern Canada and the northeastern United States. Populations are less common in western and central states and provinces including Idaho, Wyoming, Colorado, Nebraska, Saskatchewan, and Manitoba. In the United States, glossy buckthorn's southern limits are West Virginia, Tennessee, Illinois, and Iowa [28,89]. Plants Database provides a map of glossy buckthorn's North American distribution.
Introduction(s): Glossy buckthorn is native to Europe, northern Africa, and central Asia [64]. The first known North American collection occurred in 1898 in London, Ontario [27]. Spread of glossy buckthorn in North America was likely facilitated through ornamental and rehabilitation plantings. In New England and the Midwest, glossy buckthorn was planted as an ornamental and used for wildlife habitat improvement [95]. In an article published in 1963, glossy buckthorn was suggested for ornamental use in the southern Great Plains [44]. In a Nevada roadside vegetation guide published in 1963, glossy buckthorn was noted as a species that would "prove useful in time" [84]. Glossy buckthorn was also recommended for use in field break plantings in a 1993 publication from Minnesota [78]. However, the sale of nonnative buckthorns (glossy buckthorn and Rhamnus spp.) is now banned in Minnesota and Illinois [15].
Local North American distributions: In North America, glossy buckthorn has been described as "widely naturalized", "rather local", and "locally well established" from Nova Scotia to southern Manitoba and from Minnesota to New Jersey [3,65,75,83]. In the 1950 Gray's Manual of Botany, glossy buckthorn was described as "recently and rapidly spreading" and "likely to become obnoxious" [22]. In the 1982 New England Flora, glossy buckthorn was considered uncommon but described as "rapidly spreading" [80]. In Tennessee, glossy buckthorn was first reported in 1974 from Marion County [51]. Along the Middle Loup River in Sherman County, Nebraska, glossy buckthorn was "thoroughly established" as of 1999 [76]. In 1996 Colorado flora, glossy buckthorn populations were noted on slopes above the Boulder Creek floodplain [94].
Local distribution changes: The spread of glossy buckthorn has been documented in parts of southeastern Canada and the midwestern and northeastern United States. Often initial establishment occurred in urban and/or disturbed areas. Spread rates and abundance generally increased over time.
Noncultivated glossy buckthorn plants were first collected from the southern Ontario cities of London, Ottawa, and Guelph in 1898, 1899, and 1906, respectively. By 1930, glossy buckthorn occurred up to 25 miles (40 km) from these urban areas, but by 1950, occurred up to 93 miles (150 km) from the first collection sites. In the early 1990s, glossy buckthorn primarily occurred in native communities near urban areas, but its rate of spread into other native and agricultural areas has increased. In some areas, glossy buckthorn comprises more than 90% of the vegetative biomass over several acres. Glossy buckthorn dominates the Leitrim fen near Ottawa and the Sifton bog near London [5].
In a period of 20 years, glossy buckthorn has "overrun" about 30% of the 2,500-acre (1,000 ha) Cedarburg bog in Wisconsin (Reinartz University of Wisconsin-Milwaukee Field Station 2001 as cited in [15]). Through a review of the available literature, Taft and Solecki [86] found reports of a well-established glossy buckthorn population in Cook County, Illinois, in 1912. By 1955, glossy buckthorn occurred in 5 northeastern Illinois counties and by 1978, occurred in 18 counties [86]. Glossy buckthorn was first collected from Michigan in 1934, where it has been more recently described as "locally aggressive" and as a "serious pest" in bogs, fens, and disturbed damp areas [92]. The first glossy buckthorn collection from Ohio was made in Lake County in 1927 by a botanist who noted it was "becoming well established". Researchers speculated that European starlings, introduced into northeastern Ohio in 1921, may have been instrumental in the dispersal and spread of glossy buckthorn. As of 1977, glossy buckthorn in Ohio was still distributed primarily in the northeastern portion of the state [42]. Based on the study of 11 early successional sites in southeastern New Hampshire, glossy buckthorn occurrence was positively correlated with the abundance of agricultural fields and other disturbed lands (R²=0.176, P=0.057) [45]. In eastern white pine (Pinus strobus)-dominated stands in southeastern New Hampshire, researchers estimated that glossy buckthorn had established about 36 years earlier and spread as an "advancing front" at a rate of about 20 to 22 feet (6.2-6.7 m)/year. Spread rate was significantly slower in the time soon after establishment (1.1 feet (0.35 m/year)) than in the later stages (P=0.004). In southeastern New Hampshire, canopy openness was positively related to the basal area of glossy buckthorn (P<0.001) [26,27].
Glossy buckthorn occurs on moist to wet sites in mixed woodlands, thickets, sphagnum bogs, and on roadsides, lake shores, ditches, and streambanks across its range [18,28,80,83,92].
Continental climates predominate in glossy buckthorn's southeastern Canadian and northeastern US habitats. Farrar [21] indicates that glossy buckthorn is tolerant to USDA hardiness zone 2, which may reach temperatures of -50 °F (-58 °C) or lower. While glossy buckthorn may be "most aggressive" on wet sites, it occurs on "drier" sites as well [15]. It occurs in permanently moist wetlands, in shallow soils over rocks where moisture collects temporarily, and on sandy sites with high water tables [81]. In glossy buckthorn's native European habitats, moderately dry soils are tolerated, but glossy buckthorn is more characteristic of moister soils [31]. In its native habitats, glossy buckthorn occupies nearly any soil texture and is recommended for revegetation of waterlogged clay soils that are low in nutrients and humus [13]. However, plants likely will not survive permanently waterlogged conditions [31]. In eastern Massachusetts, glossy buckthorn dominated the shrub layer of red maple-silver maple-swamp white oak floodplain woodlands with silty loam soils where the pH ranged from 4.5 to 5.5 [47]. In Ottawa, glossy buckthorn occurs on moist to mesic acid, neutral, or alkaline soils as well as low-nutrient wetlands and fens [5].
Glossy buckthorn grew tallest and most rapidly where depth to the water table was intermediate and water table fluctuations were lowest on a mined peat bog in Delafield, Wisconsin. Site 1, mined in the 1960s, had more vegetation cover than sites 2 and 3, which were mined in the 1980s. Glossy buckthorn growth rates were lowest at site 2, where depth to the water table was intermediate. Growth rates were highest at site 1, which had the lowest water table fluctuations [96].
Glossy buckthorn growth as related to water table characteristics in a mined Wisconsin peat bog [96] Year Average height increase (cm) for the growing season Average growth rate (g/g/yr) Average depth to water table (cm) Average water table fluctuation (cm)Site 1
1986 34.1 1.5 23.1 26 1987 24.1 0.80 37.2 32Site 2
1986 22.3 1.0 17.8 44 1987 7.1 0.36 22.5 67Site 3
1986 30.4 1.3 23.9 37 1987 21.6 0.98 24.5 68Impacts: Many North American studies have shown that glossy buckthorn dominance can negatively affect native species richness, simplify vegetation structure, disrupt food webs, and delay succession [12]. However, in native Scots pine (Pinus sylvestris) forests of northeastern Germany, there was a significant (P<0.001) positive relationship between the occurrence of glossy buckthorn and species richness. Forests with glossy buckthorn had significantly more threatened vascular species (P<0.05), threatened bryophyte species (P<0.001), and total woody species (P<0.001), as well as nonnative species (P<0.01) [52].
In its nonnative US and Canadian range, glossy buckthorn is often referred to as a "problem" species. In 2004, the eastern region of the Forest Service ranked glossy buckthorn as "highly invasive" due to its potential or known invasion of natural habitats and "replacement" of native species [88]. In Vermont, a 1998 report characterized glossy buckthorn as "highly invasive" and noted it was displacing native plants on local and widespread scales [91]. In a survey of The Nature Conservancy staff, glossy buckthorn was identified as a "serious threat" to multiple sites or large geographical areas [11]. In the Leitrum Albion Road wetlands in Canada's Ottawa-Carleton area, glossy buckthorn has been referred to as a "menace" [19]. Glossy buckthorn made the top 10 list of priority nonnative invasive plants in southern Canada considered to have a "major impact" on native vegetation [8].
While most studies have focused on glossy buckthorn's impact on native vegetation, there are other potential impacts on agricultural crops, birds, insects, succession, and fire ecology. Glossy buckthorn is an alternate host for the fungus that causes oat rust disease [14] and for alfalfa mosaic virus (Marani and Giunchedi 1976 as cited in [81]). A study conducted by Milwaukee's Zoological Society found that common buckthorn and glossy buckthorn supported fewer arthropods than did 11 species of native trees and shrubs [15]. In an abandoned horse paddock in New England where glossy buckthorn dominated to the near exclusion of any understory species, the loss of the herbaceous understory led to changes in moisture, temperature, humidity, and light on the site, altering grasshopper, mouse, vole, and snake habitats. Dense glossy buckthorn stands also decreased the recruitment of native shrubs and the diversity of bird foods [12]. One researcher suggested that "timely high-quality native food (was) replaced by easily accessible junk food"; glossy buckthorn was referred to as junk food because of its laxative effect, which may be a net energy loss (MacDougall, Land Manager for Essex County Greenbelt in Massachusetts, as cited in [12]).
Allelopathic potential probably does not play a part in glossy buckthorn's negative impacts. In laboratory experiments, glossy buckthorn root and leaf extracts did not inhibit germination and stimulated seedling growth of the test species. Researchers cautioned, however, that field conditions could produce results different from the laboratory setting [53].
Long growth periods and densely packed glossy buckthorn stems are often suggested as partly responsible for glossy buckthorn's replacement or exclusion of native species. Glossy buckthorn may have a competitive advantage, since it leafs out earlier and retains leaves longer than most associated native species [74]. Glossy buckthorn is described as "having a serious impact on the ecology of our Midwestern forests". Dense glossy buckthorn thickets shade native tree species, decreasing tree seedling establishment, and early leaf production by glossy buckthorn restricts the light available to native spring ephemerals (McDougall, USDA/FS forester for the Northeastern Area State and Private Forestry, personal communication cited in [10]). However, not all research has shown a negative relationship between glossy buckthorn and native vegetation. When the composition of 58 wetlands in southeastern Ontario was studied, researchers found no evidence that nonnative wetlands species, including glossy buckthorn, excluded native species. Native species richness and nonnative species richness were positively correlated, and nonnative species were no more likely than native species to be dominant. This study, however, involved only a single year of study and did not evaluate change over time [40].
In some wetland habitats, glossy buckthorn threatens the persistence of rare or threatened species. In Ottawa, glossy buckthorn occurs in low-nutrient wetlands such as fens and shores where many rare and endangered species are concentrated [5]. Since the arrival and spread of glossy buckthorn in the Gavin bog of Illinois, abundance of endangered highbush blueberries (Vaccinium corymbosum) and sensitive-listed bog birches (Betula pumila) has decreased [86].
Several studies have shown that glossy buckthorn can negatively affect native herbaceous cover, total species richness, and overstory recruitment. In eastern white pine stands on southeastern New Hampshire old fields, high glossy buckthorn basal area was associated with low woody seedling density (P<0.001), low herbaceous cover (P=0.048), and low groundlevel species richness (P=0.01). Generally, the variability in these factors was low with high glossy buckthorn basal area, and high with low glossy buckthorn basal area. Species on sites where glossy buckthorn dominated the midstory started growth early in the spring and/or had clonal growth habits [25,26,27]. Total herbaceous layer cover, total species richness, and woody plant seedling richness were significantly lower on plots invaded by glossy buckthorn than on uninvaded plots in a riparian savanna near Salmon Creek in the Allegheny National Forest (P<0.002). Nonwoody species richness was not significantly different between invaded and uninvaded plots, but the dominant on invaded plots was bearded shorthusk (Brachyelytrum erectum) and on uninvaded plots was wrinkleleaf goldenrod (Solidago rugosa) [68]. In eastern white pine forests in Lebanon, New Hampshire, sugar maple, red maple, and white ash (Fraxinus americana) saplings had lower growth under a glossy buckthorn subcanopy than in areas where the subcanopy was cleared. Over a 9-year period, stem growth of the native tree saplings was significantly lower and, over a 5-year period, radial growth was significantly lower under the subcanopy than in cleared areas (P<0.05). Average sapling mortality under glossy buckthorn was 10.4% and in the open was 5.0%. Fewer than 10% of native saplings were expected to penetrate the subcanopy [20]. During a 2-year field experiment in eastern white and red pine stands in southern New Hampshire, first-year native tree seedling density was significantly lower on plots with glossy buckthorn than on plots where the glossy buckthorn canopy was removed (P=0.048) or on uncut plots (P=0.042). Presence of glossy buckthorn did not significantly affect herbaceous cover or groundlayer species richness, but researchers noted that short experiment duration, low sample size, and/or the very low cover of herbaceous vegetation in the study area may have affected results. Native tree seedling suppression may have resulted from competition for light, space, nutrients, and/or moisture [24,26].
Control: Studies have shown variable success in the control of glossy buckthorn. "Vigorous" sprouting following top-kill makes control difficult and prolongs management. Integration of multiple methods may be most effective. Efforts to eradicate glossy buckthorn from Wisconsin in the 1950s were unsuccessful [14]. In the Gavin bog of Illinois, glossy buckthorn dominates in the tamarack forest zone and tall shrub zone. Researchers predicted that eliminating glossy buckthorn and replacing it with native species was unlikely under any management involving fire, cutting, and herbicide treatments [86].
It is likely that successful management of glossy buckthorn will involve prioritization, site-specific strategy development, and a long-term commitment [12]. Control efforts targeting glossy buckthorn seedlings are likely to succeed [10]. Studies in a mined peat bog in Delafield, Wisconsin, showed that glossy buckthorn growth rates were highest in late spring or early summer. Control efforts may be more effective during this time [96]. Many potential control methods are discussed in a review by Converse [13].
While control of glossy buckthorn is seen as a priority in southern Canada [8], successful control may eliminate an important food source and developmental habitat for Henry's elfin near the northern limit of its range in Ottawa-Carleton, Ontario [4].
Water level manipulation: In some wetland systems, glossy buckthorn dominance has coincided with low water levels. Restoration of flooding or high water tables may provide some control. In the Leitrim Albion Road wetlands in Ottawa-Carleton, aerial photos and other management records showed that increased glossy buckthorn growth coincided with draining and ditching operations [19]. Several researchers suggest that flooding or high water tables may eliminate glossy buckthorn in areas where water tables were artificially lowered [15,39,82].
Prevention: Probably the most effective and economical management of glossy buckthorn would involve preventing its establishment. Prevention methods would likely include minimizing disturbances, restricting the sale and use of glossy buckthorn, and public education. In 11 early-seral sites in southeastern New Hampshire, glossy buckthorn was positively correlated with the abundance of agricultural fields and other disturbed lands (R²=0.176, P=0.057) [45]. In the late 1980s in eastern Massachusetts, glossy buckthorn transplants were used to vegetate artificially created wetlands [43]. Restrictions on the use and sale of glossy buckthorn as well as increased education about its impacts would likely promote prevention.
Physical and/or mechanical: Even when done repeatedly, pulling, mowing, and/or cutting glossy buckthorn rarely provides substantial control, but some researchers note that associated native plants may benefit. In Lincoln, Massachusetts, pulling young glossy buckthorn plants from a very small area appeared to benefit sundews (Drosera spp.) and pitcher plants (Sarracenia spp.) [12]. However, 1 year after glossy buckthorn shrubs were pulled from a site in northern Massachusetts, differences before and after were not discernible (MacDougall, Land Manager for Essex County Greenbelt in Massachusetts, as cited in [12]). Glossy buckthorn was generally unaffected by 3 consecutive years of June or July mowing in a sedge meadow with high shrub density in the Kettle Moraine region of southeastern Wisconsin. Glossy buckthorn density before mowing was 6.1 to 12.3 stems/m² and after was 5 to 6 stems/m². Before mowing, the frequency of glossy buckthorn was 66% to 86% and after was 73% to 77% [59]. Researchers suggested that mowing increased the density of glossy buckthorn roots and may have increased glossy buckthorn's resistance to other control methods [12].
Glossy buckthorn stems that were cut twice in a single growing season for 2 to 3 successive years were less dense and shorter than stems on an untreated control site in a Wisconsin calcareous fen. Herbaceous ground cover was greater on sites cut twice than on sites cut once (Lovely 1983, personal communication as cited in [13]). While cutting did not remove glossy buckthorn from a dense thicket in Ottawa, it did increased overall native species cover. In the first growing season after cutting, overall native species cover was significantly greater on cut than on uncut plots (P=0.008). Following cutting, glossy buckthorn sprouting was "vigorous". The average number of sprouts ranged from 20 to 38/m², and their average height ranged from 6 to 13 inches (16-34 cm). Native species cover increases were likely due to the short-term removal of heavy shade [81].
Fire: See Fire Management Considerations.
Biological: No biological controls for glossy buckthorn have been released to date (2008), but testing is occurring, and releases may be made by 2010 [15].
Chemical: The effectiveness of herbicides to control glossy buckthorn may increase if used in conjunction with other control methods. In Massachusetts, herbicide treatment of cut stumps provided control for 2 to 3 years. Treated stumps produced sprouts after that time [12]. Herbicides considered useful in glossy buckthorn control are discussed by Solecki [82].
Integrated management: Most studies reporting some level of success in the control of glossy buckthorn involved more than one control method and repeated treatments. Heidorn [39] indicated that repeated cutting or girdling of glossy buckthorn followed by herbicide treatments was often effective. Since glossy buckthorn's leaf retention made it easy to identify and associated vegetation was dormant, fall treatments minimized nontarget effects [39]. In mixed-conifer and hardwood swamps in the Cedarburg bog in Wisconsin, just 3 of 150 glossy buckthorn plants survived after cutting and stump herbicide treatments. Sprouts from the 3 survivors were small and deformed at the end of the first posttreatment growing season [73]. When sites were plowed and seeded with little bluestem (Schizachyrium scoparium) in Lincoln, Massachusetts, glossy buckthorn abundance decreased [12].
Some studies suggest that the use of fire with cutting or revegetation may control glossy buckthorn. Prescribed fire and mowing were used to control glossy buckthorn in the Wolf Road Prairie in Illinois. No treatment comparisons were provided, and the effort was ongoing [54]. Northeastern Illinois oak woodlands at Cap Sauers Holding were managed starting in 1989 with spring and/or fall prescribed fire and cutting and removal of nonnative woody vegetation. Unmanaged woodlands on the McClaughry Springs Forest Preserve also in Illinois were used as a comparison to evaluate the effectiveness of burning and cutting treatments. Fires occurred 1 to 4 times in 7 years; frequency of cutting was not provided. Glossy buckthorn density in managed woodlands decreased significantly between 1988 and 1995 (P=0.0001), but cover of glossy buckthorn increased, although not significantly, from 2.6% in 1988 to 3.3% in 1995. Researchers suggested that fall fires may have been less successful than spring fires. In unmanaged woodlands, the density of glossy buckthorn increased from 1,090 stems/ha in 1992 to 5,590 stems/ha in 1995 [55]. On sites where stems are cleared and stumps are herbicide treated, fire and postfire seeding may limit glossy buckthorn reestablishment. In oak woodlands in University of Wisconsin's arboretum, 4 follow-up treatments were compared on sites cleared of glossy buckthorn [79]:
Number of glossy buckthorn seedlings 1 year after follow-up treatments on cleared plots [79] Treatment Low-severity, November prescribed fire and postfire seeding* November prescribed fire only Seeding only No additional treatments Glossy buckthorn density (seedlings/144-m² plot) 6 99 143 191 *7 grass and 18 forb woodland species.Birds, Henry's elfin butterflies, and small mammals feed on glossy buckthorn. A number of bird species feed on glossy buckthorn fruits and are important in seed dispersal [31,64,92]. In southern Ontario, American robins, Bohemeian waxwings, cedar waxwings, rose-breasted grosbeaks, and European starlings were observed eating glossy buckthorn fruits (Darbyshire others, personal communications in [5]). The laxative effect of fruits may, however, limit their nutritional value [34].
Studies in the Wicken Fen in Cambridgeshire, England, showed that mice harvested, consumed, and cached glossy buckthorn fruits or seeds [30].
In Ottawa-Carleton, Ontario, which is near the northern limit of Henry's elfin's range, glossy buckthorn provides important rearing habitat and food. As glossy buckthorn abundance has increased in this area, so have Henry's elfin populations. Populations of Henry's elfin in the area have increased 7 fold from 1980 to 1996 [4].
Glossy buckthorn forms thickets and occupies similar habitats in its native
and nonnative ranges. Reviews report that glossy buckthorn in its native range
occurs in calcareous wetlands, alder (Alnus spp.) thickets, heath-oak (Erica-Quercus ssp.)
woodlands, and pine and spruce (Pinus and Picea spp.) forests
[12,13]. In native European habitats, "gregariousness"
is common for glossy buckthorn [31].
In North America, glossy buckthorn is commonly described in wet to moist
habitats including bogs, fens, marshes, riverbanks, pond margins, and disturbed
areas of larch (Larix spp.) and Atlantic white-cedar (Chamaecyparis thyoides)
swamps but is also found in drier sites such as sand
forests, roadsides, and prairies [42,60,92]. In these habitats, glossy buckthorn
may rapidly form dense even-aged thickets [12,13]. Glossy buckthorn has been described as "aggressive" and
as a "serious pest" in damp to wet habitats [60,92].
In southern Ontario, glossy buckthorn occupies habitats ranging from early-seral
old fields to mature woodlands and forests. Fens,
sedge (Carex spp.) marshes, and swamps with red maples (Acer rubrum),
Atlantic white-cedars and/or alders are prime
habitats. In very dense, light-limited forests, glossy buckthorn may be restricted to
forest edges [5]. In Gavin bog in Lake County, Illinois, glossy buckthorn
occurs in the understory of the tamarack (L. laricina) forested zone
and as a nearly "impenetrable" thicket in the tall-shrub zone [86].
In Pennsylvania's Allegheny National Forest, glossy buckthorn occurs in grass-shrublands,
riparian savannas, wet to mesic roadsides, riverine big bluestem (Andropogon
gerardii) grasslands, and less commonly in mesic hardwood forests [68].
Glossy buckthorn dominates the shrub layer of alluvial swamp communities
such as red maple-silver maple-swamp white oak (A. saccharinum-Q.
bicolor) on eastern Massachusetts floodplains [47]. On old fields in Durham,
New Hampshire, the wrinkleleaf goldenrod-northern dewberry (Solidago rugosa-Rubus flagellaris)-glossy buckthorn
understory occurs in partially open eastern white pine forests [41].
Glossy buckthorn is often associated with disturbances. In central and western
Massachusetts, researchers found that previously
plowed sites were more than twice as likely to have glossy buckthorn than pastures or
woodlots. Areas with a greater proportion of forest cover were less likely to
have glossy buckthorn (P=0.085). There was also a trend of finding glossy
buckthorn on sites where soils had low carbon:nitrogen ratios; however, researchers noted
that soil properties were likely related to land use and vegetation type [61]. Glossy buckthorn and disturbed
sites are discussed more in Successional Status.
Glossy buckthorn reproduces by seed production and seedling establishment [86,90] and regenerates from root crown sprouts following top-kill [31,60,92].
Pollination and breeding system: Glossy buckthorn produces perfect flowers [75]. Based on studies conducted in glossy buckthorn's native European habitats, flowers are insect-pollinated and self-incompatible [36,63]. Flowers were visited by bees, wasps, flies, and beetles [31].
In southern Spain, the details of glossy buckthorn's reproductive biology were evaluated. Glossy buckthorn flowers had nectaries, but nectar volume was low (0.2 µl/flower, n=8). Sugar concentration averaged 9.8% in shady sites and 26.6% in sunny sites. About 21 of the 47 insect species that visited glossy buckthorn flowers were likely pollinators. When more than 4,300 flowers were bagged, just a single fruit was produced, indicating that self-fertilization did not occur. Flowers contained in mesh bags also failed to produce fruits, indicating the wind was not a suitable pollination agent [63].
Seed production: Most sources indicate that glossy buckthorn produces "abundant" seed each year ([34,92], Hubbard 1974 as cited in [13]). However, weather conditions and seed predators may affect seed production and survival. In southern Britain, Godwin [31] reported that glossy buckthorn seed is "set very freely indeed every year", but that in dry conditions, flowers drop before mature, and seed crops are reduced.
Glossy buckthorn reaches reproductive age early [30]. McClain [60] reported that plants less than 3 feet (0.9 m) tall produced fruit. Sprouts of mature top-killed plants have produced fruit in their first growing season [73].
Studies conducted in southern Spain provided estimates of glossy buckthorn's reproductive potential, which was reduced in dry conditions. Glossy buckthorn produced an estimated 10,200 to 36,600 flowers/plant. On average, fruits contained 1.5 seeds. Calculations based on the proportion of available ovules that developed into viable seeds suggested that medium- to full-sized glossy buckthorn individuals could potentially produce between 430 and 1,560 offspring/year, but seedlings and juveniles were rare in the study area [63]. The end of the flowering period was triggered by summer drought, and successful fruit production was confined to times of peak pollinator abundance. The flowering period was shorter and seed production was 50% lower in 2001―when conditions were drier, warmer, and windier―than in 2000 [36].
Seed predation: Studies in Germany and England indicate that small mammals feed on glossy buckthorn seed [30,49]. In southwestern Germany, level of seed predation varied by vegetation type and stature. When a known quantity of seeds was left out for a single night, predation of glossy buckthorn seed was 0% in a mixed 30- to 50-year-old shrubland dominated by 13- to 20-foot-tall (4-6 m) vegetation. In stands dominated by glossy buckthorn shrubs less than 15 years old and 5 to 10 feet (1.5-3 m) tall, 27% of seeds were removed; and in an abandoned meadow 47% of seeds were removed. Feces and seed processing evidence found near the seed trays suggested that rodents were the primary seed predators [49].
Seed dispersal: Many researchers indicate that birds are important dispersers of glossy buckthorn seed [31,64,92], but other potential dispersal agents include small mammals, gravity, and water [31,35]. In European habitats, glossy buckthorn seedlings often occurred beneath the parent tree, and clumps of seedlings germinating from abandoned mouse caches were also encountered [31]. In the Wicken fen in Cambridgeshire, England, the majority of glossy buckthorn fruits fell when ripe. Experiments and field observations suggested that mice harvested and cached seed. As many as 30 to 50 glossy buckthorn seedlings germinated from abandoned caches [30].
Glossy buckthorn produces fruits throughout the growing season, and seeds have a laxative effect on birds, making seed dispersal continual and effective [34,39,64]. In southern Ontario, researchers observed American robins, Bohemian waxwings, cedar waxwings, rose-breasted grosbeaks, and European starlings feeding on glossy buckthorn fruits (Darbyshire and others, personal communications as cited in [5]), making them likely dispersers. Since introductions of glossy buckthorn and European starlings were nearly synchronous in northeastern Ohio, researchers speculated that European starlings may have been instrumental in glossy buckthorn dispersal [42].
The importance of water in the dispersal of glossy buckthorn in North America is unclear. Some researchers report that glossy buckthorn fruits may float for weeks in water (Ridley 1930 as cited in [86]), and that naked seeds may float for up to a week in water (Praeger 1913 as cited in [31]). However, ripe fruits collected in Ontario sank immediately in water [5]. Along small mountain streams in southern Spain, birds were important dispersers of glossy buckthorn seed and took an average of 53% of seeds [37], but water was an important secondary dispersal method [35].
Seed banking: Sources suggest that glossy buckthorn seed remains viable in the seed bank for at least 2 years and perhaps longer ([15], Kinzel 1926 as cited in [31]). Following removal of a glossy buckthorn canopy, glossy buckthorn seedlings are common and are likely the result of germination from the seed bank [24,26,79,87]. However, seed bank studies are generally lacking. In deciduous forests of Van Cortlandt and Pelham Bay parks in the Bronx, New York, glossy buckthorn occurred in aboveground vegetation plots but did not emerge from soils samples taken from these plots [50].
Germination: Fresh glossy buckthorn seeds germinate more readily than dried seeds, and warm temperatures may be necessary to encourage germination. Most available studies, however, are either experimental or based on observations made in glossy buckthorn's native habitats, and their relevance to North American specimens and field conditions is unknown. In European habitats, spring germination of glossy buckthorn from seed sources in wet surface litter has been described as "abundant" [31]. From collections made in England's Wicken fen, 90% to 100% of glossy buckthorn seeds from fleshy fruits germinated under controlled conditions, but when fruits or seeds were dry, germination was "very greatly" reduced [30]. Glossy buckthorn seeds collected from open-grown trees near Copenhagen, Denmark, reached 46% to 79% maximum germination/plant. Seeds were stratified for about 20 weeks, and germination trials occurred in the laboratory [67].
Germination was best (81% after 170 days and 91% after 272 days) for glossy buckthorn seed sown in the greenhouse immediately after collection. Collections were made in early September, but the collection area was not given. Glossy buckthorn seed germination was 26% after 255 days when fresh seed was sown outdoors immediately after collection. When seeds were dried and stored over the winter, germination 90 days after spring sowing ranged from 34% to 37% regardless of storage conditions [1].
Seedling establishment/growth: Most studies and observations suggest that glossy buckthorn produces abundant seedlings, and growth and survival are best in open conditions. Glossy buckthorn seedlings were common on Sphagnum hummocks throughout Gavin bog in Lake County, Illinois, where glossy buckthorn was dominant in the tall shrub zones and in the understory of tamarack zones [86]. Densities of glossy buckthorn seedlings and saplings were 1,166/ha and 200/ha, respectively, in river birch (Betula nigra)-dominated stands in the Matanzas Prairie Nature Preserve in central Illinois [90].
During a 2-year field experiment in eastern white and red pine (Pinus resinosa) stands in southern New Hampshire, the density of glossy buckthorn seedlings was 5 times greater where more than 90% of glossy buckthorn cover was removed than in uncut plots. Seedlings were likely the result of seed bank germination. Glossy buckthorn seedling density decreased sharply from the 1st to the 2nd year after canopy removal [24,26].
In a review, glossy buckthorn seedling establishment was considered best on exposed soil in areas where light levels were high [13]. High light levels and herbivory protection were most conducive to seedling growth and survival in the Mt Toby Demonstration Forest, Massachusetts. Growth and survival were greater for glossy buckthorn seedlings transplanted to plots where the canopy was removed than for seedlings transplanted to plots with a closed canopy of 14-year-old birch (Betula spp.) and pin cherry (Prunus pensylvanica). Protection from herbivory increased seedling height and diameter growth but did not increase survival. Unprotected seedlings failed to flower, but 81% of protected seedlings flowered [48].
Growth and survival of glossy buckthorn with and without canopy cover and herbivory protection [48] Canopy removed (n=7-8) Protected Unprotected Absolute height growth (cm/yr) 65.17 6.66 Absolute diameter growth (mm/yr) 6.55 1.88 Percent survival (2000-2002) 52 57 Canopy intact (n=4-8) Absolute height growth (cm/yr) -0.11 0.87 Absolute diameter growth (mm/yr) 0.13 0.07 Percent survival (2000-2002) 29 41Growth: A review reports that glossy buckthorn grows rapidly and has a long growing season [13]. Site conditions that affect water table depth and water table fluctuations may affect glossy buckthorn growth. See Site Characteristics for details.
Vegetative regeneration: Glossy buckthorn sprouts "prolifically" from its roots and and/or root crown after being wounded or top-killed [31,60,92]. In England, glossy buckthorn is "strikingly characteristic" of burned and grazed areas [31].
A review reports that glossy buckthorn sprouts can be "prolific" and almost immediately fecund. Mature glossy buckthorn plants cut to the stem base early in the growing season may produce sprouts up to 6.6 feet (2 m) tall in the same year [13]. Sprouts may produce fruit in their first growing season (Brue 1980 as cited in [13]). A glossy buckthorn shrub with stems ranging from 2.8 to 4.3 inches (7-11 cm) in diameter produced 50 sprouts after cutting [13]. In mixed-conifer-hardwood swamps in Wisconsin's Cedarburg bog, 100% of girdled or cut stems sprouted from their stumps. As stem diameter increased, so did the length of the sprout. The number of sprouts/plant ranged from 13 to 47, and length of the longest sprout/plant ranged from 3 to 7 feet (0.9-2.1 m). Some sprouts produced fruit in their first year [73].
In its native European habitats, glossy buckthorn is described as "on the whole intolerant of shade". Glossy buckthorn is typical in the understory of open woodlands, is often abundant following logging and/or grazing, but is rare in late-seral, closed-canopy forests [31]. Based on studies and observations made in the Wicken fen of Cambridgeshire, England, glossy buckthorn was an early species in fen succession. It was one of the first to dominate the shrub stage but was later replaced by common buckthorn [30].
In the United States and Canada, glossy buckthorn is described in many early-seral nonnative habitats. Colonization of newly disturbed or abandoned sites by sprouting is nearly guaranteed if glossy buckthorn was present before the disturbance. Colonization by seed dispersed onto disturbed sites is also common. Contrary to reports from its native European habitats, reports from the United States and Canada indicate that glossy buckthorn tolerates both full sun and heavy shade [15,93], suggesting it may be less restricted to open-canopy woodlands and disturbed sites in North American habitats. In southern Ontario, glossy buckthorn occurs in old fields, shaded ravines, and dense woodlands, but may occupy only edge habitats in very dense forests [5,83].
Although possible in shady sites in North America, glossy buckthorn growth and reproduction may be reduced in low light conditions. In New England, glossy buckthorn fails to produce seed in densely shaded sites but may persist vegetatively for up to 50 years [12]. In the Mt Toby Experimental Forest, glossy buckthorn seedling survival and growth were compared in clearings and in the understory of a 14-year-old mixed-hardwood stand. Seedling survival, height, biomass, leaf area, and relative height growth were greater in clearings than in the understory [77]:
Differences in the characteristics of glossy buckthorn seedlings growing in the open and in a woodland understory [77] Characteristic measured Survival (%) Height (cm) Biomass (g) Leaf area (cm²) Relative height growth rates (cm/m/d) Open 62a 43.9a 4.97a 421a 1.62a Understory 48a 12.5b 0.14b 13.7b 0.10b Values within a column followed by different letters are significantly different (P=0.05).
Frangula alnus, commonly known as alder buckthorn, glossy buckthorn, or breaking buckthorn, is a tall deciduous shrub in the family Rhamnaceae. Unlike other "buckthorns", alder buckthorn does not have thorns. It is native to Europe, northernmost Africa, and western Asia, from Ireland and Great Britain north to the 68th parallel in Scandinavia, east to central Siberia and Xinjiang in western China, and south to northern Morocco, Turkey, and the Alborz in Iran and the Caucasus Mountains; in the northwest of its range (Ireland, Scotland), it is rare and scattered. It is also introduced and naturalised in eastern North America.[3][4][5][6][7]
Alder buckthorn is a non-spiny deciduous shrub, growing to 3–6 m (10–20 ft), occasionally to 7 m (23 ft) tall. It is usually multistemmed but rarely forms a small tree with a trunk diameter of up to 20 cm (8 in). The bark is dark blackish-brown, with bright lemon-yellow inner bark exposed if cut. The shoots are dark brown, the winter buds without bud scales, protected only by the densely hairy outer leaves.
The leaves are arranged alternately on 8–15-millimetre (5⁄16–19⁄32-inch) petioles. They are ovate, 3–7 cm (1+1⁄4–2+3⁄4 in) long by 2.5–4 cm (1–1+5⁄8 in) wide (rarely to 11 cm or 4+1⁄4 in by 6 cm or 2+1⁄4 in). They have 6–10 pairs of prominently grooved and slightly downy veins and an entire margin.
The flowers are small, 3–5 mm (1⁄8–3⁄16 inch) in diameter, star-shaped with five greenish-white acute triangular petals, hermaphroditic, and insect-pollinated, flowering in May to June in clusters of two to ten in the leaf axils.
The fruit is a small black berry 6–10 mm (1⁄4–13⁄32 inch) in diameter, ripening from green through red in late summer to dark purple or black in early autumn, containing two or three pale brown 5-millimetre (3⁄16-inch) seeds. The seeds are primarily dispersed by frugivorous birds, which readily eat the fruit.[3][5][6][7][8][9]
Alder buckthorn was first formally described by Carl Linnaeus in 1753 as Rhamnus frangula. It was subsequently separated by Philip Miller in 1768 into the genus Frangula on the basis of its hermaphrodite flowers with a five-parted corolla (in Rhamnus the flowers are dioecious and four-parted); this restored the treatment of pre-Linnaean authors, notably Tournefort.[10] Although much disputed historically, the separation of Frangula from Rhamnus is now widely accepted, being supported by recent genetic data[11] though a few authorities still retain the genus within Rhamnus (e.g., the Flora of China[7]).
The genus name Frangula, from Latin frango "to break", refers to the brittle wood. Both the common name alder buckthorn and specific epithet alnus refer to its association with alders (Alnus) on damp sites. Unlike other "buckthorns", alder buckthorn does not have thorns.[12][8] Other recorded names include glossy buckthorn and breaking buckthorn; historically, it was sometimes called "dogwood" through confusion of the leaves with those of dogwood Cornus sanguinea.[12]
Alder buckthorn grows in wet soils in open woods, scrub, hedgerows and bogs, thriving well in sunlight and moderate shade, but less vigorously in dense shade; it prefers acidic soils though will also grow on neutral soils.[6][12][9]
Frangula alnus is one of just two food plants (the other being Rhamnus cathartica) used by the common brimstone butterfly (Gonepteryx rhamni). The flowers are valuable for bees, and the fruit an important food source for birds, particularly thrushes.[8]
Frangula alnus was probably introduced to North America about 200 years ago, and in Canada about 100 years ago. It was planted for hedgerows, forestry plantings, and wildlife habitat, but has become an invasive species, invading forests in the northeastern United States and wetlands and moist forest in the Midwestern United States.[13][14][15] It is predicted to continue to expand its North American range with time.[13] Its invasiveness is assisted by its high adaptability and pollution tolerance.[16] It is one of three species of buckthorn that occurs without cultivation in eastern Canada.[17]
It invades forests and grows in the understory in spots with a lot of light. These areas, usually where a tree has fallen, normally allow locally native tree seedlings to grow and eventually fill in the gap in the canopy. But when Frangula alnus invades and grows in these locations, its dense canopy prevents light from reaching the ground and therefore prevents other seedlings from growing.[16] It tends to grow more densely and with larger individuals in lower topographical areas with moist, fertile soils, and is very problematic for land managers. Uplands forests are not invaded as easily as lower lying ones. Hemlock-oak stands, which tend to be older stands of trees, are much less suitable for Frangula alnus because the density of the tree canopy creates a more shady environment that is not as suitable for Frangula alnus. Eastern white pine stands are easily invaded because they allow more light to reach the forest floor, and tree stands that are cut are very quickly invaded while undisturbed stands are rarely invaded.[18]
Although considered an invasive species in North America, In other places, such as Northern Ireland, Frangula alnus has suffered greatly from habitat loss.[19] This is due to the decline in its preferred habitats, which are boglands.[19][20] It is currently only found on the southern side of Lough Neagh, but it the past, it was also found on the northern side of the lake.[21] Compared to other populations of Frangula alnus, the population in Ireland also has lower genetic diversity.[19][22]
Small saplings can be hand-pulled, but control of larger examples is best achieved using herbicides.[23] Frangula alnus and the related species Rhamnus cathartica have been banned from sale, transport, or import to Minnesota[24] and Illinois.[25] It is considered invasive, but not banned, in Connecticut.[26]
Alder buckthorn has limited decorative qualities without conspicuous flowers or bold foliage, and is mainly grown for its conservation value, particularly to attract Brimstone butterflies. A variegated cultivar Frangula alnus 'Variegata' and a cultivar with very slender leaves 'Asplenifolia' are sometimes grown in gardens as ornamental shrubs. The cultivar 'Tallhedge' has been selected for hedging.[27]
Galen, a Greek physician of the 2nd century A.D., knew of alder buckthorn, although he did not distinguish clearly in his writings between it and other closely related species. All of these plants though, were credited with the power to protect against witchcraft, demons, poisons, and headaches.
The bark (and to a lesser extent the fruit) has been used as a laxative, due to its 3–7% anthraquinone content. Bark for medicinal use is dried and stored for a year before use, as fresh bark is violently purgative; even dried bark can be dangerous if taken in excess.[3][27]
Alder buckthorn charcoal is prized in the manufacture of gunpowder, being regarded as the best wood for the purpose. It is particularly highly valued for time fuses because of its very even burn rate.[3][12] The wood was formerly used for shoe lasts, nails, and veneer. The bark yields a yellow dye, and the unripe berries furnish a green dye.[27]
Frangula alnus, commonly known as alder buckthorn, glossy buckthorn, or breaking buckthorn, is a tall deciduous shrub in the family Rhamnaceae. Unlike other "buckthorns", alder buckthorn does not have thorns. It is native to Europe, northernmost Africa, and western Asia, from Ireland and Great Britain north to the 68th parallel in Scandinavia, east to central Siberia and Xinjiang in western China, and south to northern Morocco, Turkey, and the Alborz in Iran and the Caucasus Mountains; in the northwest of its range (Ireland, Scotland), it is rare and scattered. It is also introduced and naturalised in eastern North America.
