Cytisus scoparius (L.) Link

More info for the terms: density, direct effects of fire, fire intensity, fire severity, prescribed burn, prescribed fire, resistance, severity, wildfire

The direct effects of fire on Scotch broom depend on fire severity. A study in Australia indicates that wildfire caused complete mortality in Scotch broom on 2 of 3 sites. The site judged to have experienced the lowest-severity burn was the only site where postfire sprouting of Scotch broom (from 3% of burned stumps) was observed [36].

Several reports indicate responses ranging from increased germination to mortality of Scotch broom seeds following heat treatments of increasing temperatures in the laboratory [14,107,121,130]. Field observations of Scotch broom seedling emergence following fire [17,85,97,110,138] support the contention that germination of Scotch broom seed is stimulated by heat from fire. Portuguese broom seeds also had increased germination rates following heat treatments [53].

Scotch broom seeds were heated in an oven at 150, 212, 300, and 390 °F (65, 100, 150, and 200 °C) for 1 to 2 minutes to simulate temperatures and durations measured at about 1 inch (2 cm) below the soil surface during a prescribed burn at El Dorado National Forest (Bossard unpublished, cited in [14]). Temperatures at or above 300 °F (150°C) for 2 minutes killed Scotch broom seed. Temperatures of 212 °F (100 °C) or more for 1 minute increased seed susceptibility to fungal pathogens; and temperatures of 150 °F (65 °C) for 2 minutes significantly (P<0.05) increased germination and did not decrease fungal resistance [14]. Scotch broom seeds, collected in France from plants that had been subjected to fire and had recovered, were heated in an oven to a range of temperatures from 120 to 300 °F (50-150 °C) and a range of exposure times from 1 to 15 minutes. No germination was observed at temperatures of 270 °F (130 °C) or higher when the exposure time was 5 minutes or more. However, moderate heat treatments at 160 and 212 °F (70 and 100 °C) significantly increased the rate of germination relative to controls. Scotch broom had higher germination rates following 5-minute exposure to 212 °F (100 °C) and 1 minute exposure to 270 °F (130 °C) than after mechanical scarification [130]. Similarly, tests on Portuguese broom seed collected in 1994 from a population in southern Spain that was last affected by fire in August 1970, were tested at treatment temperatures of 190, 250, 300 °F (90, 120, and 150 °C) for 1, 5, and 10 minute durations; and 120 °F (50 °C) for 10, 15, 30, and 60 minutes and 160 °F (70 °C) for 5, 10, 15, and 30 minutes. Heating at 190 °F (90 °C) for 5 and 10 minutes enhanced germination in Portuguese broom. Additionally, of the 7 legume species tested, Portuguese broom had the highest heat resistance with 76 + 8.48% germination after heating at 250 °F (120 °C) for 10 minutes, and 2% germination after heating at 300 °F (150 °C) for 10 minutes. Mechanical scarification and hot water treatment also significantly (p<0.05) increased germination rates of Portuguese broom seeds over those of untreated controls. Seeds treated at 120 and 160 °F (50 and 70°C) at all durations, and at 190 and 250 °F (90 and 120 °C) for 1 minute did not have germination rates significantly different from controls [53].

In another laboratory study, Oregon white oak and Scotch broom were planted together in 1) pots containing soil that had been heated to 140 °F (60 °C) for 10 minutes before planting; 2) pots with about 1 inch (2 cm) of ash added to the top of the potted soils; and 3) control pots with no treatment. Scotch broom stem count was significantly (p<0.001) less than the control with the addition of ash and significantly greater than the control in heated soil. Heating the soil and adding ash significantly increased the mean stem count of Scotch broom above the control but was both significantly greater than the ash treatment alone, and significantly less than the heat treatment alone. Results suggest that the heat generated by prescribed burning would likely increase germination of Scotch broom, and that ash would reduce germination of all seedlings tested, but would have the least effect on Scotch broom [106,107]. Stem densities per pot were [106]:

* Stem density of all species includes Scotch broom and other oak-
competitor plants (not oak seedlings)

Field studies support the contention that heat from low- to moderate-severity fire stimulates germination in Scotch broom seeds in the soil seed bank. Robertson and others [110] recorded seedling emergence for 3 years after a prescribed fire in subalpine snowgum (Eucalyptus pauciflora) woodland in Australia. They found 60 to 280 seedlings per 100 m² in burned plots 12 months after burning, and no seedling emergence in unburned plots. There was almost no seedling emergence in either burned or unburned plots 2 and 3 years after burning [110].

A comparison of postfire seed banks and seedling densities at 3 sites in Australia showed reductions in seed bank density after fire. A correspondence between fire intensity (generally, height of crown scorch) and soil seed bank reduction was not consistently observed [36]:

*data from [110]

Some postfire seed bank depletion can be attributed to germination, and on 2 of 3 burned sites, seedling density was proportionately higher in burned plots than in unburned plots 6 months after fire, suggesting that seeds may be scarified by fire. Seedling density was not higher after fire on the high severity site [36].

Variation in seed bank depletion following fire is influenced by several factors, including the initial size of the Scotch broom seed bank relative to that of native species, as well as interactions of fire severity and soil moisture content and their effect on heat penetration into the soil and duration of heating. Soil temperatures during fire do not exceed 212 °F (100 °C) until soil water is vaporized, while fire over dry soil can produce temperatures of several hundred °C in the top few centimeters of soil, where the majority of broom seeds are present. Soil moisture was very low when the wildfire burned at Majors Creek, whereas fires conducted at 'The Lanes' were conducted the day following rain. During 10 years of using fire to control Scotch broom at Barrington Tops, it was concluded that Scotch broom reduction would be maximized during the hottest, driest months ([36] and references therein).

These results suggest that broom seeds are adapted to postfire germination. Thus, prescribed burning in areas infested with broom could increase germination and decrease seed bank density at the soil surface, depending on duration of temperatures reached during the fire. Changes in soil temperature could also play a key role in the flush of new germinating seeds when established plants are removed. Increased solar radiation could heat the soil and stimulate seed germination [34].

"Well-branched shrub; branches 5-angled. Leaves 3-foliolate at base, simple above, subsessile, adpressed-pubescent; leaflets elliptic-oblong to obovate, 18 x 4 mm. Flower axillary, solitary, 3 x 2 cm; pedicel 1 cm, glabrous. Calyx glabrous, half as long as pedicel; lobes 5 mm, abaxial lobe 2-toothed. Corolla bright yellow, shortly clawed; standard 2 x 2 cm; wings and keels 2 x 1 cm. Stamens 10, monadelphous; sheath 1 cm; 6 stamens longer, 4 shorter. Ovary 1 cm, densely white pubescent with elongate hairs; style 2 cm, curved. Pod to 3.5 x 0.8 cm, black, hairy along sutures."

More info for the terms: cover, forbs, shrubs

Mule deer, elk, and black-tailed jackrabbits were observed browsing on Scotch broom at 2 California sites (mule deer and elk at one site, mule deer and black-tailed jackrabbits at the other) [13,16]. On a prairie site among redwood forest habitat, mountain quail, blue grouse, and harvest mice are known to eat Scotch broom seed [11]. Scotch broom seeds were also eaten by mountain quail in the southwestern Cascade Range of Oregon [101].

Scotch broom supports a rich insect fauna in England where it is native. Prior to introduction of insects for biological control of Scotch broom, a survey of insect fauna on Scotch broom at several sites in California and British Columbia found relatively fewer total insects, less diversity of species, and a total absence of seed and pod insects compared with Scotch broom in England. Larger numbers of 2 species were found on some sites. Observations suggest that this may be due to lack of herbivory of brooms in North America [148]. Even after the introduction of biological control agents, no substantial vertebrate or invertebrate herbivory was found at any life stage of a California foothills population; however, vertebrate herbivory significantly (p<0.05) decreased biomass of Scotch broom on a northern coastal site [13].

Several species of ant eat elaiosomes of Scotch broom seed, and thus aid in seed dispersal, with varying numbers and species at different sites [11].

Palatability/nutritional value: A review from Australia suggests that Scotch broom seeds and mature shoots are unpalatable and even toxic to ungulates. Shoots are not used for forage except by rabbits in the seedling stage [88]. The source of this information is not given. A field experiment in New Zealand found that broom was moderately preferred by domestic goats and sheep in comparison to 5 other leguminous shrubs and forbs, 3 nonleguminous species, and 2 grasses [80].

An examination of the chemical composition of various browse species in New Zealand indicates that Scotch broom has high potential as a browse species for domestic goats and sheep. Nitrogen and element concentrations in leaf and stem material are presented here [66]:

*Units are % dry matter for macro-elements (N, P, K, S, Ca, Mg, Na) and mg/kg dry matter for micro-elements

Researchers in Spain indicate that Scotch broom may represent a high quality food source for grazing ruminants, particularly during the dry season. Chemical composition (g/kg dry matter) of leaves, stems, flowers, and fruits of Scotch broom at different maturity stages are presented below. Plant material was collected from upland sites in northwest Spain [5].

ªFerulic acid equivalents/kg dry matter

In-vitro digestibility and extent of degradation in different plant parts averaged across sampling dates is presented below. In general digestibility was highest at the first sampling date (May) and tended to decrease with later sampling dates [5].

Cover value: No information is available on this topic.

More info for the terms: adventitious, ground residual colonizer, shrub

POSTFIRE REGENERATION STRATEGY [123]:
Small shrub, adventitious bud/root crown
Ground residual colonizer (on-site, initial community)