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Cytisus scoparius (L.) Link
IUCN
NCBI
EOL Text
Cytisus scoparius (Broom; syn. Sarothamnus scoparius) is a species in the pea family Fabaceae. It is native to much of Europe, from the British Isles east to southern Scandinavia, south to Iberia, and east to Belarus and Romania. Further northeast, its range is limited by its lack of tolerance of severe winter cold, with temperatures below around -25° to -30°C killing the stems. It is a woody shrub with green photosynthetic shoots, and small caducous leaves present only in spring and early summer. The leaves are simple or trifoliate, 5-15 mm long. Young shoots remain green for several years, silky-hairy at first, and have up to five small longitudinal ridges. Older stems have finely flaky to stringy grey-brown bark. The flowers are bright yellow, 1-2 cm long in bud opening to 2-3 cm long, with the typical pea flower structure; they are produced in mid spring to early summer and are pollinated by bees. The seeds are 3-4 mm diameter, produced in a 2-5 cm long pod, green ripening black. Seed dispersal starts with explosive pod splitting in hot sunny weather, and is continued further by ants, which feed on the small fleshy peduncle at the base of the seed. The seeds are long-persistent in the soil (up to 20-30 years); this can enable the species to survive periodic bush fires, and also to survive in colder regions of northeastern Europe (southern Scandinavia, Poland, etc.) where periodical severe winters may kill the entire adult population.
There are two subspecies, which differ mainly in growth habit:
* Cytisus scoparius subsp. scoparius (Common Broom). An erect shrub, growing to 2-3 metres (rarely 4 m) tall; shoots thinly hairy at first, soon becoming glabrous. This is the common form, occuring throught most of the species range.
* Cytisus scoparius subsp. maritimus (Rouy) Heywood (Prostrate Broom). A prostrate, ground-hugging shrub, not exceeding half a metre in height; shoots densely silky-hairy. It is restricted to the Atlantic coasts of southern Ireland, west Wales, southwestern England, and northwestern France.
Broom (primarily subsp. scoparius) is widely cultivated as a garden plant, and for wildlife benefit. The Andreanus Group cultivars are particularly popular, selected for their bright orange-red to pink flowers. It is also naturalised, and sometimes an invasive weed species, in parts of Australia, New Zealand, India, and North America.
The English name derives from its historical use in the manufacture of brooms, as the harvested twigs retain a degree of flexibility in use without becoming brittle. A number of other English names, some of them offensive, have been applied to the species outside of its native range.
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Rights holder/Author | Michаel Frаnkis, Michаel Frаnkis |
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Forest plantations and degraded forest areas
More info for the terms: competition, cover, density, fern, fire exclusion, fire frequency, fire intensity, fire management, fire regime, fire suppression, forb, forbs, frequency, fuel, fuel loading, low-severity fire, natural, nonnative species, prescribed fire, presence, presettlement fire regime, restoration, severity, shrub, shrubs, top-kill
Fire adaptations: Scotch broom sprouts from the stem after top-kill from fire [17,130,137] or mechanical removal [13,58]. Its ability to sprout seems to vary with season and severity of damage, although this relationship is unclear and deserves further investigation (see Fire Effects and Physical/mechanical control).
Several reports indicate a postfire flush of Scotch broom germination from the soil seed bank [17,85,97,138]. Several studies also indicate increased germination of Scotch broom seeds following heat treatments in the laboratory [14,107,121,130]. These results suggest that seeds of this species are well adapted to postfire germination.
FIRE REGIMES: There is no information available on the FIRE REGIMES in which the brooms evolved in their native range. However, Scotch broom and Genista florida, a close relative of French broom, were early successional species following fire in their native range in Spain [52].
It is unclear how the presence of brooms may affect FIRE REGIMES in invaded communities. In general, in ecosystems where broom replaces plants similar to itself (in terms of fuel characteristics), brooms may alter fire intensity or slightly modify an existing fire regime. However, if broom invasion introduces novel fuel properties to the invaded ecosystem, they have the potential to alter fire behavior and potentially alter the fire regime (sensu [20,31]). A review of Scotch broom in Australia [88] suggests that presence of Scotch broom creates a fire hazard in forest areas in Australia and California, although the source of this assertion is not given. Where Scotch broom invades subalpine eucalypt woodland in Australia it forms a dense shrub layer, overtopping and depleting the grass layer, thus altering fuel structure such that fire intensities fueled by shrubs in the invaded community would likely be higher than those fueled by grasses in an uninvaded community [36]. Scotch broom invasions are also said to increase fire intensity and frequency in invaded Oregon white oak communities [23,138]. According to Tveten [138], where Scotch broom has invaded prairie and Oregon white oak woodlands on Fort Lewis in western Washington, it forms dense stands and increases fire hazard by creating extensive areas with large amounts of dead wood.
Scotch broom and Portuguese broom occur in a variety of ecosystems in North America that represent a range of historic FIRE REGIMES. In many areas where brooms occur, historic FIRE REGIMES have been dramatically altered due to fire exclusion and to massive disturbances associated with human settlement. The historic FIRE REGIMES of native communities in which brooms sometimes occur range from high frequency fires in grasslands to high frequency, low-severity fires in open ponderosa pine forests; and moderate frequency, high-severity fires in California chaparral. Brooms did not occur in these communities at a time when historic FIRE REGIMES were functioning, but has established since fire exclusion and habitat alteration began. It is unclear how historic FIRE REGIMES might affect broom populations.
It is also unclear how the use of fire to control broom in these communities might impact native species. Plant adaptations to fire are usually to a particular fire regime, or combination of fire frequency, intensity, extent, and season. When fire is used to control nonnative species, the frequency, intensity, and season of burning must be carefully chosen to avoid damaging native species. Prescribed fire may have undesirable effects if introduced into an ecosystem that has undergone shifts in species composition, structure, and fuel characteristics outside a natural range of variability in these attributes [1]. When the natural fire regime is altered, even highly fire-adapted plant communities may be vulnerable to competition from nonnative species [63].
According to Swezy and Odion [129], fire is an effective management tool for French broom, but is used primarily in mixed evergreen forest and grassland communities in California, where repeated annual burning for broom control "appears to have no unwanted side effects." Prescribed fire is used less frequently in chaparral communities where frequent burning or burning outside the natural fire season may have adverse effects on native communities.
Herbaceous communities dominated by nonnative annual grasses and forbs of Mediterranean origin occur throughout the Coast Ranges and foothills of the Cascade Range and the Sierra Nevada. A review by Keeley [63] indicates that much of the nonnative annual grassland in the Coast Ranges of central and southern California derives from a fire-induced type conversion of shrublands. The herbaceous communities that have long dominated these landscapes were largely created by anthropogenic burning by Native Americans, and were further maintained by intensive land use with fire and livestock grazing by European-Americans. In recent decades, however, grazing has been eliminated in some areas and anthropogenic fires reduced such that woody vegetation is reestablishing. Along with native shrubs, nonnative shrubs such as Scotch broom, French broom, and gorse colonize these sites. Nonnative shrub colonization of grasslands may decrease fire frequency but increase fuel loads and alter fire behavior ([63] and references therein).
In the Puget Trough of Washington and adjacent parts of British Columbia, native plant communities were once a mosaic of Oregon oak woodlands, wetlands, and fescue prairies. This mosaic is said to have been maintained by Native Americans who used fire to maintain conditions favorable to the growth of common camas and bracken fern (Pteridium aquilinum). These frequent fires removed shrubs and killed small Oregon oaks and Douglas-fir, maintaining a low density of woody species. A plethora of impacts following in the wake of Euro-American settlement, including fire suppression, grazing by livestock, introduction of nonnative species, landscape fragmentation, recreation, and other management impacts, have changed the structure and composition of these plant communities ([23,107,144] and references therein). Exclusion of fire from these communities has changed the regeneration pathways of Oregon white oak and increased densities of Douglas-fir, ponderosa pine, and understory shrubs. The natural vegetation associations of Oregon white oak are threatened by these interrelated conditions [1]. Scotch broom has become an important nonnative species in Oregon white oak habitats. It forms dense canopies 3 to 9 feet (1-3 m) tall, interfering with native species. Altered fuel structure and increased fuel loading result from invasion of Scotch broom and other nonnative species such as colonial bentgrass [23,134,137].
The relationship of Oregon white oak communities and fire is critical to any restoration effort [106]. If fire is used to reduce the occurrence and spread of Scotch broom in these ecosystems, consideration must be given to presettlement fire regime characteristics. Oak woodlands and associated prairies evolved with frequent, low-severity surface fires [1]. "Seasonal burning" in lowland prairies in Washington reportedly discourages Scotch broom invasion [108]. Spring burning on a 3- to 5-year rotation in Weir Prairie, Washington, causes little change in native prairie vegetation and maintains open Oregon white oak stands. Conversely, 50 years of annual burning in one area has eliminated Scotch broom and restricted Douglas-fir establishment, but has changed the native perennial bunchgrass prairies to introduced forb and annual grassland. On the other hand, fire suppression is more harmful to prairie vegetation than excessive burning, allowing Douglas-fir and Scotch broom to invade prairies and Oregon white oak woodlands. Closed stands of these species eliminate nearly all native prairie species. Sampled plots indicate that no native prairie species remained after 12 years of closed Scotch broom cover. Fall burning is recommended to remove Douglas-fir and Scotch broom from heavily infested areas, along with follow-up fires to kill dense Scotch broom postfire seedling establishment [138].
Reintroducing fire to these communities as a means of rehabilitation and restoration is complicated by the increased fuel loads associated with long-term fire exclusion and nonnative species invasion. On oak-prairie margins, fire used to control Scotch broom can pose risks to oaks unless it is used frequently enough to prevent excessive accretion of fuel [23]. High fire severities associated with high fuel loads increase mortality of Oregon white oak seedlings and saplings. Thysell and Carey [134] describe areas at Fort Lewis where mature oaks appear to have been killed by severe fire that was fueled by Scotch broom. Mechanical removal of Scotch broom and Douglas-fir before burning may reduce the potential for negative effects on oaks [23,134,137].
A single intense fire can reduce Scotch broom cover, but is likely to encourage germination of Scotch broom from the seed bank [17,85,97,138] and at least temporarily reduce cover of native perennials such as Idaho fescue. A second fire is necessary to kill broom seedlings within 2 to 3 years, before Scotch broom seedlings are reproductively mature (see Fire Management Considerations) [17]. Many native plants in these native communities thrive after a single low-severity fire, but may be adversely affected by repeated burning [1]. Additionally, nonnative species may be favored over native species if fire is too frequent [23,137]. Spot treatments using a flame thrower in the winter, when grasses are green and fire will not spread, can remove residual Scotch broom plants missed in previous fire treatments [1].
A prescribed fire program has been used to manage prairies and oak woodlands in some areas of Fort Lewis since the 1960s and 1970s. Fires are mostly set in February and March, and occasionally in the fall, on a 3 to 5 year rotation. The primary objective of the program is fuel reduction. While the program has been successful at maintaining some of the "best prairie and Oregon white oak woodland vegetation in western Washington," it has not completely stopped prairie encroachment by Douglas-fir and Scotch broom. Spring fires often fail to burn under dense Scotch broom or young Douglas-fir. So whenever possible, heavily invaded areas are burned under drier conditions than are open prairies. Even under drier conditions, however, spring fires often fail to burn through stands of dense Scotch broom or young Douglas-fir, leaving clusters of these species to reinvade burned areas [138].
The complex relationships among oak woodlands, wetlands, prairies, Douglas-fir forests, introduced nonnative plants, and intensively developed urban, suburban, and agricultural areas suggest that both a comprehensive set of conservation objectives and a comprehensive assessment of techniques for promoting indigenous species and techniques for controlling nonnative species are needed [23].
The following list provides fire return intervals for plant communities and ecosystems where Scotch and/or Portuguese broom may be important. It may not be inclusive. If you are interested in plant communities or ecosystems that are not listed, see the complete FEIS Fire Regime Table.
Community or Ecosystem | Dominant Species | Fire Return Interval Range (years) |
silver fir-Douglas-fir | Abies amabilis-Pseudotsuga menziesii var. menziesii | > 200 [6] |
California chaparral | Adenostoma and/or Arctostaphylos spp. | < 35 to < 100 |
California montane chaparral | Ceanothus and/or Arctostaphylos spp. | 50-100 [93] |
California steppe | Festuca-Danthonia spp. | 93,125] |
western juniper | Juniperus occidentalis | 20-70 [93] |
jack pine | Pinus banksiana | 37] |
Jeffrey pine | Pinus jeffreyi | 5-30 |
Pacific ponderosa pine* | Pinus ponderosa var. ponderosa | 1-47 [6] |
interior ponderosa pine* | Pinus ponderosa var. scopulorum | 2-30 [6,8,67] |
red-white-jack pine* | Pinus resinosa-P. strobus-P. banksiana | 10-300 [37,51] |
quaking aspen (west of the Great Plains) | Populus tremuloides | 7-120 [6,49,73] |
coastal Douglas-fir* | Pseudotsuga menziesii var. menziesii | 40-240 [6,75,109] |
California mixed evergreen | Pseudotsuga menziesii var. menziesii-Lithocarpus densiflorus-Arbutus menziesii | < 35 |
California oakwoods | Quercus spp. | 6] |
oak-hickory | Quercus-Carya spp. | 147] |
coast live oak | Quercus agrifolia | 2-75 [48] |
blue oak-foothills pine | Quercus douglasii-P. sabiniana | <35 |
Oregon white oak | Quercus garryana | 6] |
California black oak | Quercus kelloggii | 5-30 [93] |
redwood | Sequoia sempervirens | 5-200 [6,41,126] |
elm-ash-cottonwood | Ulmus-Fraxinus-Populus spp. | 37,147] |
*fire return interval varies widely; trends in variation are noted in the species review
The scientific name for Scotch broom is Cytisus scoparius (L.)
Link [47,54,55,62,104,111,124,131,146,153,159]
and for Portuguese broom is C. striatus (Hill) Rothm. [54,62,131].
Both are in the pea family (Fabaceae).
In North America, there are 2 varieties of
Scotch broom, distinguished by their flower
color: C. scoparius var. scoparius and C. scoparius var.
andreanus (Puiss.) Dipp. The former is the more widely distributed variety, and
the latter occurs only in California [62]. This review
does not distinguish between these varieties. No infrataxa are described for
Portuguese broom.
There are no known naturally occurring hybrids of either Scotch broom or
Portuguese broom. There are,
however, a number of ornamental hybrids. Some hybrids have escaped cultivation
in Australia, although none are thought to be invasive [9,59,97].
More info for the terms: competition, frequency, natural, resistance, tussock
General landform/land use type: Scotch broom is well adapted to dry hillsides (often on steep slopes), pastures, and forest clearings [29,88,116]. In eastern North America, Scotch broom typically occurs near areas where it was purposely planted [9,47,104,124,146], in open woods [104,111], old fields and pastures [19,111], along roadsides, and in other disturbed areas [104,111,159]. Its occurrence is also noted on sandy substrates such as dunes and beaches [47,116].
In western North America, Scotch broom occurs on similar sites [29,58,100,102]. It is invasive on coastal sites from Monterey, California, to Washington state [77], especially in areas where it was planted for dune stabilization [61]. In California, Scotch broom has spread extensively in grassland areas on open hills and invades chaparral and lower montane habitats in the San Bernardino Mountains [122]. It also invades upland areas in the Central coast. Scotch broom is also common in riparian areas, including riparian sites in the Sierra Nevada [38], and cobble bars on the Olympic Peninsula, Washington [33]. It is also invasive in undisturbed prairie remnants in western Washington (see Succession: Disturbance) [84].
Scotch broom does not grow well in forested areas but invades rapidly following logging, land clearing, and burning [74]. Scotch broom has become a serious pest in logged areas replanted with conifer seedlings [29], and is commonly found on clearcuts and on open sites disturbed and opened by logging, roads, or fire. It also invades natural meadows and open forest [33,97,100].
In New Zealand and Australia, Scotch broom invades similar land types, including pastures and cultivated fields, dry scrubland and "wasteland," native grasslands, previously forested hill country, rocky sites, roadsides, dry riverbeds, other waterways [96,157], and both disturbed and undisturbed woodland and open forest [59]. Regression analysis of aerial map data from a national park in Australia indicates that sparse vegetation and flat terrain are among the variables contributing most to expansion of Scotch broom populations [79].
The limited available literature on Portuguese broom seems to indicate similar site tolerances as Scotch broom; however, Portuguese broom's distribution is more limited [4].
General climate: Scotch broom inhabits maritime to submaritime cool mesothermal climates with dry summers in moderate to high rainfall areas of humid temperate regions. As elevation and continentality increases the frequency of Scotch broom decreases ([97] and references therein). Distribution of Scotch broom indicates that it is well-adapted to the coastal climate and the drier climate of the Sierra foothills. Scotch broom thrives in coastal areas due, in part, to mild winter temperatures that allow it to photosynthesize and fix nitrogen into the winter [155]. Optimum temperatures for stem photosynthesis range from 68 to 77 °F (20-25 °C), while lower temperatures cause a decrease in stem photosynthesis [76]. Scotch broom has some drought resistance, but does not survive in arid regions [34,58].
Scotch broom is native to the Mediterranean, where its southern distributions are limited by drought and its northern by winter cold ([96], and references therein). Several reviews [29,96,97] indicate that the brooms do not tolerate extreme high or low temperatures. Scotch broom does not do well in areas with very cold winters. Seedlings and young plants are especially sensitive to frost, while mature plants can tolerate fairly severe frosts. Frost appears to have little direct effect on its total height growth as the damaged tips are replaced by growth from lateral buds, but plants may experience considerable dieback after very severe winters [96,97]. Some large Portuguese broom individuals can be killed by an unusually long freeze, perhaps limiting its geographic distribution (Alvarez, personal observation in [4]).
The distribution of Scotch broom in North America is limited in the north and inland by cold winter temperatures. In eastern North America, severe winter conditions and more intense competition from local flora may limit the growth and distribution of Scotch broom more than in western North America. On the Atlantic coast of Canada, Scotch broom occurs on disturbed sites in coastal areas south of 46 °N latitude [97]. In Nova Scotia, Scotch broom does not seem to persist in colder regions inland or northward [111]. On the Pacific coast, Scotch broom occurs in disturbed areas in the southwest coastal British Columbia mainland and adjacent islands. Except for the latter, Scotch broom generally does not occur north of 51 °N in western Canada [97]. The altitudinal limit of broom in New Zealand also appears to be limited by winter cold or winter drought [96].
With sparse leaves and photosynthetic stems bearing sunken stomata beneath thick epidermal wax, Scotch broom is adapted to the high-sunlight, seasonally droughty, Mediterranean environment. Scotch broom is considered to be drought tolerant and can have a water potential of -0.259 Mpa or lower without showing any apparent signs of stress (Bannister 1986, cited in [88]). Scotch broom can tolerate some level of drought stress and invade harsh riverbed environments with mid-summer drought in New Zealand and have the ability to set seed over a wide altitudinal range and under conditions of moisture stress [96]. Nonetheless, Scotch broom seedlings may die in sunny locations during rainless periods, even in cool and humid environments. In Australia, Scotch broom occurs mainly in cool temperate areas. In drier climates, it is restricted to the edge of watercourses and along drainage lines. Scotch broom also invades and persists in treeless vegetation such as subalpine grassland and cleared pasture land. In open areas, tussock grasses protect Scotch broom seedlings from drought and grazing [59].
In North America, brooms occur on sites with moderately dry to very dry moisture regimes (Klinka and others 1989, cited in [97]), but do not survive in extremely arid regions in the southwestern United States [34]. Scotch broom is occasional east of the Cascade Range, where precipitation is above 20 inches (500 mm) per year. In California, water-use efficiency of Scotch broom remained unchanged between spring and summer seasons [76], and daily carbon gain remained constant throughout the year [77]. Although Scotch broom is difficult to grow in the hot valleys of California [74], its occurrence in dry habitats and value for stabilizing sand dunes [61] implies a certain degree of drought resistance [58].
Soils: Literature reviews [17,34,58,59,96,97] indicate that brooms can survive under a wide range of soil conditions and have few constraints to growth on almost any medium. They seem to prefer coarse textured, seasonally dry, well-drained soils and a pH range of 4.5 to 7.5. They do well on sites with low to moderate fertility, but grow only rarely on highly calcareous soils. In the San Francisco Bay area Portuguese broom is particularly common on noncalcareous soils [4].
There is very little empirical information on site tolerances of Scotch broom, and some of the observational or anecdotal information is contradictory. In general, Scotch broom seems to be invasive in dry sandy soils in full sunlight [17,61,148,157], and strongly drained, water-shedding sites (Klinka and others 1989, cited in [97]). However, it may grow best on moist, fertile soils [59]. In Australia, broom occurs on soils derived from a wide variety of substrates, particularly basalt, and is rarely found on undisturbed skeletal sandy soils [59].
Scotch broom also seems to tolerate acid soils, but it is unclear whether is tolerates lime soils. In Europe it is found on moderately leached heathland soils, acidic grasslands, and in sand dunes (Bicher and Larsen 1958, cited in [58]). At 1-year-old mine spoil sites in Kentucky, Scotch broom was among several woody species tested for revegetation. Scotch broom was an average of 3.4 feet (1 m) tall after 4 growing seasons at Site 1 but did not survive at Site 2. Differences in site conditions were as follows [99].
Parent material | pH | Soluble salts | Available phosphorus | |
Site 1 | Gray and black shales | <4.5 | <0.5 mmhos/cm | <7 ppm in about 60% of the samples |
Site 2 | Mixture of sandstone and shale | 50% of blocks <4.5
50% of blocks >7.0 |
<0.25 mmhos/cm | < 3ppm at 95% of sample sites |
Scotch broom can survive on nitrogen-deficient soils and those with very low levels of inorganic phosphorus, but also responds rapidly to increases in phosphorus [16,17,89,96]. Scotch broom prefers nitrogen-medium soils at sites in British Columbia (Klinka and others 1989, cited in [97]).
Elevation: Elevation ranges reported for Scotch broom are as follows:
Place | Elevation Range |
California | below 4,000 feet (1,300 m) [17,115] |
California (northern and central) | up to 3,300 feet (1,000 m) [54,122] |
California (El Dorado County) | 500 to 6,500 feet (150-2,000 m) [74] |
Australia | 0 to 4,800 feet (0-1,450 m) [121] |
Portuguese broom is found on sites less than 980 feet (300 m) in California [54].
Altitudinal limit of Scotch broom in New Zealand appears to be determined by winter cold or winter drought. Scotch broom grows more rapidly at low altitudes in New Zealand. Growth (cumulative shoot elongation) was highest on sites at 2,800 feet (854 m) and decreased with increasing elevation. Scotch broom plants flowered as high as 4,600 feet (1,400 m) [96].
More info on this topic.
More info for the terms: cover, density, litter, succession, tree
Brooms are early seral colonizers. The dispersal of photosynthetic tissue throughout the crown of Scotch broom, in long stems and small leaves, makes it well adapted to the open environments of early succession [157]. Some authors suggest that broom is shaded out once native species are established (e.g. [112]). Bossard [17] expresses concern, however, that their vigorous growth, along with acidification of the soil (see Impacts), may inhibit establishment of other species.
Disturbance: Broom stands commonly establish and spread after soil and/or vegetation disturbance (e.g. from roads, logging, herbicide treatments, or fire) in both their native range in Europe [52,92,148] and in introduced areas, such as New Zealand [157], Australia [59,88,121], and North America [11,34,60,68,97,112,115]. Scotch broom populations may be perpetuated by continued disturbance [121]. Scotch broom can also colonize undisturbed grassland, shrubland, and open canopy forest [17,58,59,84,88,100,115]. A review by Hosking and others [59], for example, suggests that Scotch broom can invade undisturbed vegetation in Australia, with seedlings establishing in open microsites such as along wallaby tracks.
Conflicting results from research conducted in North America on the role of disturbance in Scotch broom seedling establishment [11,84] suggests that the response of Scotch broom populations to disturbance is site-specific. Bossard [11] found that soil disturbance favored Scotch broom seedling establishment at 1 California site, and results were inconclusive at a 2nd site. Parker [84] compared populations of Scotch broom growing in urban fields to those growing in prairie remnants in western Washington. Populations in prairies expanded more rapidly, although urban populations were more fecund, due to greater numbers of pollinators (see Pollination). The author noted that while much of Scotch broom habitat is disturbed areas (e.g. urban fields, highway rights-of-way, abandoned lots, landfills, etc.), the most rapid rate of spread in this study occurred in the most pristine habitats. Results of this research suggest that these undisturbed prairie sites are more hospitable for invasion of Scotch broom due to the beneficial influence of native cryptogams [84].
Once stands of Scotch broom are established, small Scotch broom individuals in the understory seem to require the removal of a large dominant plant in order to grow into a large size class [84].
Shade tolerance: Scotch broom's common role as an early seral colonizer [59,157] and its tendency to establish in forest environments following vegetation disturbance that opens the canopy [74,88] imply shade intolerance.
Several reviews indicate that Scotch broom survival is best in high-light areas. Scotch broom is generally intolerant of shade and will not grow in heavily shaded places [34,59,97]. Others suggest that it will usually be shaded out once native species are established [17,157] or forest canopy closes [112]. However, Scotch broom can continue to grow and compete for moisture, space, and nutrients under a partial tree canopy on some sites [97]. It can also invade eucalypt-dominated vegetation where the tree foliage protective cover is less than about 50% (Waterhouse 1988, cited in [59]).
Williams [96] suggests that under low light conditions (10%-30% full sunlight) Scotch broom plants tend to form a single upright shoot and produce sparse foliage and few flowers. In a laboratory study by Vallardes and others [145], seedlings of Scotch broom had greatest survival (~82%) in moderate shade (30% full sunlight), ~30% survival in 100% full sunlight, and ~12% survival in deep shade (3% full sunlight). Rates of net photosynthesis were significantly (P<0.005) higher in moderate shade versus full sun, and dark respiration was significantly higher in full sun than in moderate shade. Scotch broom was tentatively classified by the authors as shade intolerant, although further tests are needed for this to be definitive [145].
Individual life span/stand longevity: Estimates of Scotch broom's life span range from 10 to 15 years in its native range [149], 15 years in communities in New Zealand [157], 17 years in California [15], and over 23 years at Barrington Tops, New South Wales, Australia [121]. Long-term studies in England found that Scotch broom plants kept free of insects grew larger and more quickly, and had lower death rates at 10 years than plants exposed to insects [149]. A comparative study of life expectancy between regions found mean maximum ages were 14.4, 12.8, and 14.4 years for New Zealand, Australia, and Europe, respectively. Death rates and the mean persistence of populations did not vary significantly (P>0.5) between native and nonnative populations. However, few of the Scotch broom populations examined in this study contained senescent individuals, so the maximum ages recorded do not necessarily indicate the maximum longevity of Scotch broom [90].
Smith (1994, as cited by [97]) outlined 4 growth stages of Scotch broom in Australia: Stage 1 comprises the first 2 years of age when seedlings establish in the spring. This stage is a period of high summer mortality and susceptibility to browsing by wildlife. Stage 2 covers the next 2 years and is characterized by rapid vertical growth (up to 6.5 feet (2 m)) and canopy establishment of a dense population of many young, erect plants. Reproductive activity usually begins by the time the plant is 3 years old. At Stage 3, plants are mature and are fully able to reproduce. Scotch broom typically forms dense stands with a nearly closed canopy, with branches leaning increasingly outwards as the plants age. Some dieback may occur in the lower, shaded branches. Stage 3 lasts from age 5 to about 9 years, with Scotch broom reaching its full height of 6 to 13 feet (2-4 m) before plants begin to lean and topple over. Stage 4 is characterized by an increasing number of prostrate, spreading branches, giving Scotch broom a leggy appearance. There is little or no establishment of Scotch broom seedlings during the latter stage, unless disturbance creates gaps in the broom canopy (Smith 1994, cited by [97]). These findings are supported by research in Washington [84] and New Zealand [157].
After Stage 4, Scotch broom stem density declines rapidly with stand age. Stems may become denuded of foliage and twigs, and eventually the plants may topple over and die. As the canopy of Scotch broom opens up, seeds falling from parent plants can grow and germinate, and in this way a stand of Scotch broom can perpetuate itself for many years, effectively excluding other vegetation. Scotch broom is said to retard the establishment and spread of many rare and endemic species in invaded Oregon white oak communities in the Pacific Northwest ([97] and references therein). Conversely, Williams [157] suggests that because Scotch broom leaves drop in late summer, and litter is sparse and readily decomposable, later successional species are able to establish and persist in broom stands.
Expansion of existing Scotch broom stands appears to be slow, having been measured as 6 to 13 feet (2-4 m) over a 7-year period at Barrington Top, Australia. Spread is more rapid in ungrazed open pasture, where it can reach 10 to 16 feet (3-5 m) a year ([59], and references therein).
Scotch broom
Portuguese broom
English broom
scotchbroom
striated broom
"Notes: Western Ghats, Naturalized, Native of Mediterranean Region"
More info for the terms: cover, forb, forbs, invasive species, natural, nonnative species, shrubs, tree
The following description of habitat types and plant communities in which Scotch
broom and Portuguese broom occur is taken from examples found in the literature. The objective is to
provide examples of vegetation types in which these species occur, and is not
meant to imply that they are restricted to these types within these areas. For
Scotch broom most
examples come from areas where Scotch broom is most widespread and most invasive. In some
areas, particularly the eastern U.S., there is little to no information on
vegetation types in which Scotch broom occurs. There is very little information in the
literature on vegetation types in which Portuguese broom occurs.
Scotch broom threatens endangered
ecosystems such as Oregon white oak (Quercus garryana) woodlands in
southwestern British Columbia, adjacent Washington, and Oregon by interfering
with the establishment and spread of many rare and endemic species found in
these endangered ecosystems ([97] and references therein).
In British Columbia Scotch broom dominates many sites in the Cowichan
Garry Oak Reserve. The Reserve is a highly fragmented Oregon
white oak savanna ecosystem extending along the coastal areas of southeastern
Vancouver Island, and supports many nonnative, invasive species. The most
abundant native species include common camas (Camassia quamash), mosquito
bills (Dodecatheon hendersonii), long-stolon sedge (Carex inops), and
Idaho fescue (Festuca idahoensis). The most abundant nonnative species include
Kentucky bluegrass (Poa pratensis), orchard grass (Dactylis glomerata), and
sweet vernalgrass (Anthoxanthum odoratum) [69].
Scotch broom may be associated with coast Douglas-fir (Pseudotsuga menziesii var. menziesii),
Pacific madrone (Arbutus menziesii), common gorse, and Himalayan blackberry (Rubus
discolor) [97]. It also occurs in the coast forest area dominated by western hemlock (Tsuga
heterophylla), Sitka spruce (Picea sitchensis), and broadleaf maple (Acer
macrophyllum) [148].
Scotch broom is invasive in several plant communities in the Puget Trough area of
western Washington, including Idaho fescue grasslands and Oregon
white oak woodlands. These plant communities are heavily impacted by nonnative
species, among which Scotch broom is one of the most common and widespread. Native
animal and plant species that are threatened with habitat loss include several
federally listed species and species of concern: the western gray squirrel, the
western pocket gopher, several species of birds, reptiles, amphibians, and
butterflies including the mardon skipper and
valley silverspot, and several plant species including Columbian whitetop aster
(Seriorcarpus rigidus), longhair sedge (Carex comosa),
greenfruit sedge (Carex interrupta), smallflower wakerobin (Trillium parviflorum), Torrey's pea (Lathyrus
torreyi), and golden Indian paintbrush (Castilleja levisecta) [23].
In one grassland community at Fort Lewis Military Reservation in the Puget
Trough, stands of Scotch broom are replacing a prairie community dominated by
Idaho fescue and small herbaceous perennials such as common camas and rigid whitetop aster (Symphyotrichum
retroflexum) [50]. In many areas, most native prairie species have low cover values or
are absent. Shade tolerant grasses and forbs such as colonial bentgrass (Agrostis
capillaris), Kentucky bluegrass, and Pennsylvania sedge (Carex
pensylvanica) are the dominant graminoids, and wall bedstraw (Galium parisiense)
is the dominant forb under Scotch broom. Other nonnative species that have higher cover under
Scotch broom compared to uninvaded prairie include common St. Johnswort (Hypericum perforatum),
common velvetgrass (Holcus lanatus), and blue fieldmadder (Sherardia
arvensis). The only species that did not differ in cover between Scotch broom-invaded and uninvaded sites was
another nonnative species, oxeye daisy (Leucanthemum vulgare).
Many bare areas occurred under dense Scotch broom cover. Most native prairie species
are present as occasional relics under gaps in the Scotch broom canopy. As gaps close with stand age, the
remaining native prairie plants are lost, leaving only colonial bentgrass and
nonvascular cryptogams as the dominant understory components [138].
Another study at Fort Lewis found Scotch broom in all site types identified within Oregon white oak communities,
but most commonly on sites with an oak overstory and an understory characterized by long-stolon sedge and blue wildrye
(Elymus glaucus), and by the nonnatives colonial bentgrass and
common St. Johnswort. Several other nonnative species
were also common on these sites, which had the lowest
diversity and evenness values of the 3 site types identified [134]. At Nature Conservancy preserves in Washington,
Scotch broom occurs in lowland prairies
where it displaces native species and may impact threatened species such as
golden Indian paintbrush, and Whulge checkerspot
and Mardon skipper butterflies [108].
A prairie site in the Willamette Valley, Oregon, contains both native
wetland dominated by tufted hairgrass (Deschampsia
caespitosa) and upland prairie vegetation invaded by Scotch broom [25].
Where prairies in the Willamette Valley have been invaded by woody species
including Scotch broom, lupines (Lupinus spp.) and other nectar plants
for the Fender's blue butterfly are shaded out, thus impacting habitat for this
endangered species [114]. At Nature Conservancy preserves in northwest Oregon,
Scotch broom occurs in grassland
and forest communities where it may interfere with threatened species such as
Columbian whitetop aster (Aster curtus) and upland larkspur (Delphinium
leucophaeum) [108]. Scotch broom occurs in Douglas-fir-western hemlock stands
in western Oregon [7], but is infrequent along roads and streams on the western
slope of the Cascade Range, where native vegetation is dominated by western
hemlock at lower elevations and Pacific silver fir (Abies amabilis) at higher elevations.
Scotch broom occurrence had no detectable trends with site type (type of road),
but was more common along roads than streams [81]. At Myrtle Island Research
Natural Area in Oregon, Scotch broom is frequent in red alder-Oregon ash (Alnus
rubra-Fraxinus latifolia)/Himalayan blackberry/reed
canarygrass (Phalaris arundinacea) and Oregon white oak/Scotch broom/redtop
(Agrostis gigantea) plant community types [132].
In California Scotch broom occurs in the northwest humid coast belt or redwood (Sequoia
sempervirens) transition life zone, the Sierra foothills gray pine (Pinus
sabiniana)-chaparral belt, and the lower part of the ponderosa pine (Pinus ponderosa) belt of the
Sierra Nevada [16,148]. Scotch broom also invades coastal
grasslands. Scotch broom and French broom are persistent problems along
roadsides in the redwood region of California. Scotch broom establishes in
redwood forests after clearcutting and persists until the canopy closes
[112]. Scotch broom also occurs on prairie sites,
in the Bald Hills area of Redwood National Park, with mixed annual
grasses and forbs [16]. Scotch broom invades valley grassland and foothill oak (Quercus spp.)
woodlands in California, occurring up to middle elevations in the Sierra
foothills, and there is concern that it and other invasive species will continue
to move upslope in the Sierra. Scotch broom is found in several California
counties within the Sierra Nevada, primarily around the ponderosa pine
forest-chaparral transition, but also along roadsides across both of these
community types [115]. It occurs on an oceanside slope at
Point Reyes National Seashore, sharing dominance with coyote bush (Baccharis pilularis), on a
site previously used for cattle grazing [77]. Scotch broom occurs, interleaved
with French broom, on hillslopes dominated by nonnative grasses, with patches of
coyote bush and several nonnative forbs and shrubs including bigleaf periwinkle (Vinca major),
silverleaf cotoneaster (Cotoneaster
pannosus), narrowleaf plantain (Plantago lanceolata), evergreen
blackberry (Rubus laciniatus), and poison hemlock (Conium maculatum) at Mt. Tamalpais
State Park. Overall cover of Scotch broom and French broom at this site was
about 15% and 30%, respectively. At China Camp State Park, Scotch broom and
French broom grew in patches and as scattered individuals, with a total cover of
about 15% and 35%, respectively, and associated with coyote bush, toyon (Heteromeles arbutifolia),
poison-oak (Toxicodendron diversilobum), nonnative musk thistle (Carduus nutans), and several nonnative grasses
including Uruguayan pampas grass (Cortaderia selloana) [87].
There is very little habitat information available for Scotch broom in eastern North America,
probably because "it is a poor competitor with the local lush vegetation of the
eastern states" [148]
(see Site Characteristics).
Shanks and Goodwin [117] report that it is well established in a sandy field in
Mendon Ponds Park, New York.
In Prince Edward Island and Nova Scotia, Scotch broom may be associated with annual grasses,
forbs, and other weedy plants such as goldenrod (Solidago canadensis) and
thistles (Cirsium spp.) as well as trees and shrubs found in open
woodlands [97].
There is no habitat information available for Portuguese broom in North America. In its native range,
Portuguese broom occurs in mixed maquis shrubland with strawberry tree (Arbutus unedo),
briar root (Erica arborea), gum rockrose (Cistus ladaniferus),
Erica australis, Phillyrea angustifolia, and Halimium ocymoides
in the western half of the Iberian Peninsula [53].
More info for the terms: cohort, cover, density, elaiosome, fresh, interference, litter, monoecious, perfect, presence, root crown, shrub, shrubs
Scotch broom and Portuguese broom reproduce and spread from abundant seeds and both can sprout from stumps or root crowns following damage or destruction of aboveground biomass [4,17,34].
Breeding system: Scotch broom flowers are perfect. Vivipary has not been reported [97]. Scotch broom is monoecious and does not show appreciable levels of selfing [87,128]. Portuguese broom is monoecious [54].
Pollination: The large yellow flowers of Scotch and Portuguese broom are pollinated by several species of bee [34,97], and possibly other insects [121]. Only a small proportion of flowers develop into fruit (<50%) [34,121]. Parker and others [83,87] demonstrated pollinator limitation in both Scotch broom and French broom, underscoring the potential importance of pollinators to the fecundity and spread of these species. Because nonnative honeybees are often the most common pollinators of broom [86,128], potential negative impacts of beekeeping on broom management have been suggested [86].
Scotch broom produces flowers that are "tripped" open when pollinated and has a nearly obligatory relationship with bumblebees and honeybees in Washington. Less than 1% of untripped flowers produce fruits, and outcross-pollinated flowers produce 4 times as much fruit as self-pollinated flowers [83].
Suzuki [127,128] studied pollination biology in Scotch broom in Japan and found 40.26% of flowers were visited by effective pollinators, 28.91% were pollinated, and 13.75% matured fruits (N=2,600). Therefore, 71.8% of visited flowers were pollinated and 47.58% of pollinated flowers matured fruits. Pollinator visitation rate was higher for plants in sunny habitats than for those in shade. Nineteen species of insects visited flowers. The author concluded that pollinator limitation is the most limiting factor for Scotch broom fruit production and that resource limitation was secondary, yet more severe in larger plants [127]. Similarly, on study sites in Washington, mean pollinator visitation rate was low, ranging from 3% to 30% among 4 Scotch broom populations over 3 years. Urban populations received higher numbers of visits than did 2 native prairie sites. Hand-pollination experiments revealed significant (P<0.0001) pollinator limitation in all populations over 2 years. Prairie populations were more pollinator limited than urban populations. Significant correlations were found between pollinator visitation and whole-plant fruit production in all 3 years. Simulations over a short time scale (10-30 years) demonstrated little effect of pollen limitation on urban populations, but a potentially large effect of increasing pollinator visitation in the rapidly invading prairie populations [83].
Seed production: Broom seed production varies from year to year, and can vary among species and site conditions. Scotch broom becomes reproductive on reaching an age of 2 to 3 years and a height of about 2 to 3 feet (0.6-1 m) [17,59,97], unless damaged or otherwise suppressed [121]. Portuguese broom becomes reproductive at 2 to 3 years of age at a height of 1.5 to 3.3 feet (40-100 cm) [4]. In an Australian study, comparing recruitment of Scotch broom among sites with different disturbance histories, time between recruitment and flowering varied between 2 and 5 years, depending on habitat. Seed production per plant was unaffected by site, stand maturity, grazing, disturbance, or cohort, but was related to plant density [119]. On some sites a second, less prolific flowering occurs in summer [88,96].
Several estimates of seed production in Scotch broom plants under various site conditions in both its native range and in invaded communities are available in the literature. Mature Scotch broom shrubs produce anywhere from a few hundred to over 7,000 pods per plant, with a range of zero to 22 seeds per pod (about 5 to 8 on average) [59,97,121,149]. One medium-sized Scotch broom shrub can therefore produce several thousand seeds per year [13,16,59]. Factors that may affect seed production in Scotch broom include predation by insects, site conditions, plant size, climatic conditions (i.e. drought), plant age, and individual plant physiology.
Predisperal predation by insects may reduce Scotch broom seed yield by as much as 75% in its native habitat. In its native range, Scotch broom plants at least 3 years old produced 1,242 to 7,104 pods per bush in sprayed (to eliminate insect herbivores) shrubs, and 319 to 2,718 in unsprayed shrubs. The number of seeds per pod ranged from 6.9 to 9.84 in sprayed and from 4.54 to 8.04 in unsprayed shrubs [149]. Unripe seeds and pods are largely free from predation in North America, New Zealand, and Australia [34,121]. Dense stands of Scotch broom can produce 4,000 to 20,000 seeds/m²/year ([34] and references therein). Seed production under broom canopies beneath and outside a eucalyptus overstory on a site in New South Wales was 107 and 8,885 seeds/m², respectively (A. Sheppard and P. Hodge, unpublished data cited by [59]).
Results presented by Williams [96] indicate variation in seed production among sites in New Zealand, with a trend toward lower seed production in higher elevation sites (above 2,800 feet (854 m)) and on poorly developed floodplain soils:
Site | Soil | Elevation | Shrub age (yrs) | % fruiting | Pods/bush |
Seeds/pod |
|
1st flowering | 2nd flowering | ||||||
gardens | recent, silt loam | 27 m | 2 | 100 | 2,200 + 1,500a | 9.3 + 2.0 | 3.62 + 2.7 |
river bed | coarse alluvium | 137 m | 2 | 0 | 0 | -- | -- |
river bed | coarse alluvium | 274 m | 2 | 20 | 1 + 1 | -- | -- |
river bed | coarse alluvium | 274 m | 5 | 100 | -- | 8.64 + 3.2 | -- |
high country | brown earth, silt loam | 854 m | 2 | 100 | 84 + 60 | 8.12 + 2.0 | 3.62 + 2.4 |
high country | brown earth, silt loam | 1082 m | 2 | 18 | 2 + 2 | 6.6 + 3.9 | 4.36 + 2.6 |
high country | brown earth, silt loam | 1311 m | 2 | 20 | 1 + 1 | 7.7 + 4.7 | -- |
astandard deviation
A study of Scotch broom in California found drought and plant size affected seed production. The following data are from the first 2 years of a 3-year study at 2 California sites where 5 Scotch broom shrubs over 3 feet (1 m) tall were included in each sample. "Damage" is from seed weevils. No information is given on time periods when early and late flowering occurred [16].
Early flowering shrubs |
Late flowering shrubs |
|||
Year |
1987 |
1988 |
1987 | 1988 |
Mean # of pods/shrub | 2,891 | 1,287 | 3,876 | 1,400 |
Mean # of seeds/pod | 8.1 | 2.9 | 8.1 | 2.4 |
% of seed damaged | 5 | 5 | 80 | 91 |
Mean # pods/shrub/year (early + late flowering shrubs for 1987-1988) | 2,363 | |||
Mean # seeds/pod (early + late flowering shrubs for 1987-1988) | 5.37 | |||
Total mean # seeds/shrub/year | 12,701 |
The number of pods and seeds decreased significantly (p<0.01) for both early and late flowering shrubs during 1988 (a drought year) compared with 1987 and 1989. The number of pods per shrub had a linear positive dependence on shrub volume for both early and late flowering shrubs during the 3 years of study [13]. Similarly, Parker [84] found that fruit production increased dramatically with plant size in Scotch broom, while invasion stage did not have an impact on fecundity.
Waloff and Richards [149] cite evidence that Scotch broom produces fewer pods per plant (by weight) as plants age. However, Smith and Harlen [121] observed that the proportion of flowers that develop into fruits, as well as the number of flowers overall, tends to increase with shrub age and size. They also found that seed production per unit area did not change with stand age between 6 and 24 years [121].
A biennial cycle of relatively low and high pod density was observed in Scotch broom in its native range. Since the timing of these cycles did not coincide between treatment plots on the same site, seed production was thought to be independent of climatic conditions and reflected more the physiology of individual plants [149].
Seed dispersal: Short distance transport of broom seeds is similar to that of many other legume species and involves "ballistic" dispersal. As pods mature and dry, the 2 pod halves split and wrap in alternate directions, audibly snapping apart and catapulting the seeds [17,97,100,121]. Ballistic dispersal of seeds resulted in a mean dispersal distance of 3 feet (96 cm), a median distance of 44 inches (112.8 cm), and a maximum distance of 213 inches (540 cm) at 2 California sites [16]. In Australia, Smith and Harlen [121] found that most Scotch broom seed falls within 3.3 feet (1 m) of parent plants, and rarely beyond 10 to 13 feet (3-4 m) without assistance from wind or insects. Robertson and others [110] found that Scotch broom seed was dispersed as far as 33 feet (10 m) from the edge of thickets, although the means of dispersal was not studied.
Scotch broom seeds bear elaiosomes that attract ants [94]. After seeds disperse from the pods, ants gather them and typically carry them back to their nest, where they eat the elaiosome. Removal of the elaiosome does not kill the seed nor affect seed germination, and Scotch broom plants may be found in high density around ant nests on some sites. Foraging behavior of ants varies, and not all ants carry seed to their nests [14,16]. Bossard [16] measured mean dispersal distance by ants at 3 feet (96 cm) and maximum dispersal distance at 207 inches (525 cm) at California study sites. Ant dispersal in Australia has only been measured for short distances, up to 3.3 feet (1 m) [121]. Birds and animals may also play a part in broom seed dispersal, although seed predation by birds and other animals is considered negligible [34]. When Scotch broom seeds were fed to domestic goats, 8% of the seeds remained viable following ingestion [57]. Smith and Harlen [121] also suggest evidence that Scotch broom seed may be dispersed in the feces of a variety of animals such as horses.
Spread of Scotch broom along riverbanks strongly suggests seed dispersal by water [34,96,121]. Williams [96] indicated that the hard seed coat of Scotch broom can survive long distance transport in rivers and streams, and viable seed was recovered from stream sediments up to 160 feet (50 m) downstream from Scotch broom shrubs [121].
Scotch broom distribution patterns along roads and in work sites suggest transport by vehicles and equipment. Additionally, samples taken from recreationist's vehicles (inside and out) and shoes held many Scotch broom seeds [121]. Construction crews can disperse broom seeds long distances by transporting contaminated soil or gravel [34,97].
Portuguese broom probably spreads like Scotch broom, with the same seed dispersal mechanisms. Portuguese broom seeds are released ballistically from the pod, then further dispersed by animals and water runoff [4].
Seed banking: Portuguese broom and Scotch broom have hard seed coats that delay germination for months or years and enable seeds to survive in the soil seed bank for at least 5 years and possibly as long as 30 years ([17] and references therein). Other estimates of Scotch broom seed longevity indicate that some seed may survive as long as 80 years under conditions of dry storage ([34,59] and references therein).
Studies carried out on Scotch broom seeds from a foothill site on the El Dorado National Forest in California showed that fresh seed was 98% viable but that >65% of seeds were dormant. About 7% of seed remained ungerminated after 3 years at 1.5 inches (4 cm) below the soil surface [14]. In Australia, 69% to 83% of seeds displayed dormancy in an experiment using seeds collected in the field and stored in the lab for 6 months. Seed buried for 182 to 1,371 days were 90% dormant upon recovery. Viability was also retained for similar periods by seeds stored under water [121]. Williams [96] found an average of 68 Scotch broom seeds per m² in the top 4 inches (10 cm) of soil and 51 seeds per m² in the 4 to 8 inches (10-20 cm) depth along rivers in New Zealand. Of these, a highly variable number (0-100%) was dormant at the time of sampling [96].
Scotch broom seed dormancy and longevity contribute to large soil seed banks. Seed bank density under mature broom stands is highly variable [119]. In its native range, soil seed banks below a mature broom canopy vary from 430 to almost 20,000 seeds per m² [2,39,59,92]. Scotch broom seed density on a grassland site in England was not affected by exclusion of insects, mollusks, or rabbits [39]. Soil seed banks below mature broom canopies in Australia varied from 190 to 2,700 per m² in the Adelaide Hills, 1,100 to 12,300 per m² at Barrington Tops, and 4,630 to 27,400 per m² around Braidwood. Less than 2% of the seeds extracted from the soil were nonviable ([59,121] and references therein). Broom seeds were usually most abundant in the top 1 inch (2.5 cm) of soil [121]. Average densities ranged from 0 to 2,649 seeds per m² under Scotch broom in New Zealand, and seed viability was 90% to 100% [3].
No consistent relationship is found between seed bank density and stand age. Patterns observed at Barrington Tops, Australia, suggest an increase in seed bank density with stand age for several years [121]. However, in the absence of seed rain the seed bank declined by about 50% over 1 year at another site (Sheppard and Hodge, unpublished data in [59]). Seed bank densities under Scotch broom plants of varying age class in New Zealand tended to be highest under plants between 6 and 10 years old, but varied among sites [3].
Managers should be alert to the possibility of Scotch broom seed in the soil seed bank, even in areas where broom is no longer present in aboveground vegetation. Loss of seeds from the soil seed bank is mainly through germination and not mortality. At a California foothills site, seed bank germination was substantial 3 years after the removal of mature broom plants. The mean number of seedlings establishing from the seed bank ranged from 120 to 161 seedlings per 0.25 m² at the end of the 2nd germination period, and 15 to 25 seedlings per 0.25 m² at the end of the 3rd germination period. There was no evidence of insect or vertebrate herbivory affecting the density, biomass, relative growth rate, or seedling germination of Scotch broom at this site [16]. Persistent, deeply buried seed banks of Scotch broom were found at 2 study sites in New Zealand, 1 of which lacked Scotch broom in aboveground vegetation. Scotch broom may establish from the seed bank following disturbance, especially fire [89], as improved seed permeability may be brought about by scarifying the seed coat during disturbance [97,121].
Germination: Scotch broom seeds imbibe water and swell to 3 times their original size before germination. However, not all imbibed seeds germinate; some return to their original size and remain dormant [59]. Bossard [14] found that seed germination occurs best after a period of dormancy at temperatures between 39 to 91 °F (4-33 °C), with 64 to 72 °F (18-22 °C) being optimal. Germination rates are highest in the top inch (2 cm) of soil, and Scotch broom seedlings do not emerge from below 3 inches (8 cm) [14]. In experimental treefall gaps in New Zealand, Scotch broom seeds germinated most successfully on sites exposed to direct solar radiation, with or without litter; percent germination increased as exposure increased, while litter had no significant (p<0.001) effect on germination [71].
Most seed (often 100%) produced by Scotch broom is viable, but a large percentage of fresh seed is dormant (see Seed banking) and requires scarification before germination can occur. Scarified seeds can germinate throughout the year under suitable conditions [14]. Conditions terminating dormancy in Scotch broom seeds are not entirely clear. Mechanical and chemical scarification may induce germination (e.g. [121]). Heat scarification also induces germination, as indicated in laboratory studies [14,121,130] and field observations of postfire broom seedling establishment (see Fire Effects). On some sites, soil disturbance may also scarify seed and/or terminate dormancy and induce germination [11], although this relationship is not consistently observed among sites [85].
An Australian study found that scarification using fine sand paper induced 100% germination in Scotch broom seed, but with 8% to 12% mortality. Treatment of seeds with sulfuric acid also increased germination rates. Heat treatment (pouring boiling water over seeds in a flask then allowing it to cool to room temperature) led to 48% to 60% germination, although the same procedure using water at 176 °F (80 °C) had little effect [121].
Germination and seedling recruitment are sometimes favored by soil disturbance [17], but disturbance effects may vary among plant communities [85]. In California, Scotch broom germination at one site increased when soil was disturbed. At another site in this study, quail and blue grouse preferentially foraged in disturbed areas, and the interaction of these seed predators and seed dispersers (ants) modified the abiotic effects of habitat disturbance on seedling establishment, resulting in no significant differences in seedling recruitment between disturbed and undisturbed sites [11]. At Weir Prairie in Washington, however, undisturbed plots had significantly (P<0.004) higher germination rates and seedling establishment than scraped (cryptogams removed) or burned plots. Field observations and unpublished data cited by the author support the notion that soil disturbance does not favor Scotch broom seed germination in Washington prairies [85]. Similarly, Shepphard and others [119] found that soil cultivation following broom removal did not enhance germination rates of Scotch broom at 3 sites in Australia. Differences in Scotch broom seed germination in response to disturbance may be influenced by plant community composition and structure. Sites dominated by turf-forming grasses or mature stands of Scotch broom might be expected to show a positive effect of soil disturbance on Scotch broom establishment [35]. For example, in experiments in mature Scotch broom stands in France and England, cultivated plots showed much higher germination rates than control plots [85]. Effects of disturbance on seedling establishment are difficult to generalize from one population or habitat to another [11].
Portuguese broom seed germination requirements are probably similar to related broom species such as Scotch broom [4]; however, research is needed to test this assumption.
Seedling establishment/growth: Scotch broom seedling establishment and growth may be affected by several variables including depth of seed burial, soil type, soil moisture, stand density, and site disturbance.
Depth of seed burial and substrate texture affect Scotch broom seedling emergence. In greenhouse experiments, Williams [96] found that seedlings failed to emerge from seeds buried 4 inches (10 cm) deep. Seedlings emerged more rapidly and successfully from 1.2 inches (3 cm) than from 2 inches (5 cm); and more rapidly from fine substrate than coarse substrate at both these depths [96]. Soil moisture content was significantly correlated (r²=0.048 and 0.078; P< 0.05) with seedling establishment at 2 sites in California [11].
Seedling mortality differed with location within established Scotch broom stands. Survivorship was lower in the center of the population, where intraspecific density was very high, than at the edge. Germination rates were also lower in the center of the population and declined consistently from the edge to the center [84]. There was no evidence of density dependent seedling mortality in a study at Weir Prairie, and seedling establishment was influenced more by germination rates [85].
Scotch broom is said to readily colonize nitrogen-poor, seasonally moisture-stressed, disturbed areas like sand dunes, riverbeds, roadsides, and steep slopes [16,96]. In an Australian study, comparing recruitment of Scotch broom among sites with different disturbance histories and subjected to different disturbances (cutting mature plants with and without soil cultivation) between populations in Europe and Australia, seedling mortality rates were higher in Australia and were not affected by timing of establishment. More broom seedlings survived year-to-year in grazed vs. ungrazed plots; in cultivated (to 10 cm, resembling disturbance by pigs and wombats) vs. cut (mature broom removed) vs. undisturbed plots; and in immature vs. mature stands. The presence of other vegetation had little effect on Scotch broom seedling recruitment, age at flowering or the percentage cover of broom [119].
Rapid development of both roots and foliage enable Scotch broom to invade and persist in new habitats. The deep rooting habit, small leaf size, and rapid growth (height and girth) are morphological characteristics that help to enhance its survival. After 2 years, Scotch broom may be 8 feet (2.5 m) in height, with a stem diameter of 1 inch (2 cm). When grown under full light in greenhouse conditions, 65-day-old Scotch broom plants attained slow vertical growth, allocating 48% of weight gain to the roots. When light intensity was reduced to 30% full sunlight, the young plants were 3 times as tall and had a poorly developed and weakly nodulated rooting system [96].
Broom plants grow rapidly in the first 4 to 5 years, and growth slows considerably thereafter. Growth appears to be more vigorous in introduced habitats that lack native invertebrate predators [149]. However, in a comparison of Scotch broom populations in Europe, Australia, and New Zealand, growth rates did not differ among native and nonnative populations, and the tallest plant recorded was in its native range in Spain [90]. Interference can reduce the growth rate of broom seedlings in both native [92] and nonnative [119] habitats [90].
In New Zealand, growth of Scotch broom at low altitudes on moderately fertile soils is more rapid than on sites with nutrient-poor soils or at high altitudes [96].
Asexual regeneration: Broom plants can sprout from the root crown after cutting or freezing and sometimes after fire; particularly in the rainy season [4,17]. Sprouting varies with timing and degree of top growth removal [4,13] (also see Physical/mechanical control).