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).