Frangula alnus P. Mill.

More info for the terms: bog, cover, density, fen, fresh, litter, perfect, root crown, shrub, shrubs, top-kill, tree

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: 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.