Gorse responds to fire by sprouting from the basal stem region and by establishing from seed in the soil seed bank [5,42,74,92,98,99]. Postfire regeneration of gorse can be prolific and rapid [42,80,82].
Immediately following experimental burns in an area of gorse scrub in northwest Spain, gorse stem phytomass was reduced by approximately 318 g/mÂ² over prefire levels. An increase in aboveground gorse biomass of 245.5 g/mÂ² and 1366.6 g/mÂ² were recorded in the 1st and 2nd postfire years, respectively . The percentage of ground covered by gorse quickly and steadily increased after these fires :
Months after fire
Gorse cover (%)
The rapid regeneration of gorse was primarily from new shoots on old root systems. After 3 years, the main difference in cover between the control plots and burned plots was that there was greater overlayering of vegetation on the control plot .
Johnson  suggests that sprouting is more important than seedling establishment for postfire regeneration of gorse. After a mid-summer wildfire on a southern New Zealand peatland, gorse sprouted from lower portions of old stems within 4 months of burning, and seed germinated the following spring. Height growth was more rapid on sprouts than on seedlings. Gorse seedlings reached 1.2 inches (3 cm) tall (maximum) 10 months after fire, while sprouts reached 12 inches (30 cm) tall at that time. Fifteen months after fire, sprouts were 35 inches (90 cm) and seedlings 26 inches (65 cm) tall. Cover of gorse on these sites averaged about 2.5% four months after fire, 5% ten months after fire, and 15% fifteen months after fire. Twenty-two to 28 months after fire, gorse cover was about 35%; and 39 to 120 months after fire gorse cover was 70% to 85% .
Gorse seed in the soil seed bank germinates slowly, over a long period, in undisturbed sites dominated by gorse  (see Germination). Observational and experimental evidence from New Zealand indicates that clearing vegetation from gorse-dominated stands results in a flush of germination of gorse both with [73,74] and without fire . Postclearing flush of gorse germination may be due to changes in light intensity or greater temperature fluctuations at the soil surface, but this remains unclear. For example, observations suggest that light is not essential for gorse seed to germinate (Zabkiewicz personal communication cited by ).
Reviews suggest that changes in soil temperatures during fire may scarify gorse seed or volatilize organic compounds in the seed environment that had inhibited germination and, when extreme, may kill gorse seed [74,97]. Laboratory tests indicate that germination of gorse is poor without pretreatment (e.g., soaking in boiling water for 30 seconds or cutting the seed coat), and may be enhanced by exposure to heat. Dry heat of 220 Â°F (105 Â°C) for 5 minutes resulted in at least 79% of gorse seeds germinating (Butler 1976, as cited by ). After exposure to wet or dry heat of 212 Â°F (100 Â°C) for 5 minutes or less or to 180 Â°F (80 Â°C) or 140 Â°F (60 Â°C) for any duration germination rates were similar to mechanically scarified seed. Reduced germination or increased time to complete germination occurred when seeds were exposed to 212 Â°F (100 Â°C) for 5 minutes or more. Virtually total seed sterilization could be attained by exposure to 300 Â°F (150 Â°C) for 1 minute. Wet heat became lethal to gorse seed at lower temperatures or shorter exposures than the equivalent dry heat treatment .
Temperatures recorded in the field during fire in gorse stands range between 390 Â°F (200 Â°C) and 1,100 Â°F (600 Â°C) in the surface litter 0.2 inch (0.5 cm), and decline markedly with increasing depth. Maximum temperatures in the litter (top 0.2 inch (0.5 cm)) during a burn of felled gorse and broom teatrea in New Zealand were 390 Â°F (200 Â°C), 480 Â°F (250 Â°C), and 1,100 Â°F (600 Â°C) at lightly, moderately, and heavily burned sites, respectively. At the heavily-burned site, where most of the surface litter was destroyed, temperatures of 410 Â°F (210 Â°C) and 280 Â°F (140 Â°C) were recorded in the litter at 1 and 2 inch (2.5 and 5.0 cm) depths, respectively. During another fire in gorse, temperatures above the surface reached 570 Â°F (300 Â°C) to 1,500 Â°F (800 Â°C), while soil temperatures were less than 212 Â°F (100 Â°C) (, and references therein). Rolston and Talbot  report soil temperature changes over time (in the zone where gorse seeds occur) during burning of gorse. Lower overall plant and soil moisture levels appear to be associated with higher soil temperatures during the burn. On the 2 sites where high temperatures were recorded, temperatures 1 to 2 mm below the soil surface exceeded 220 Â°F (105 Â°C) for 153 and 156 seconds, respectively, and exceeded 300 Â°F (150 Â°C) for 102 seconds on both plots. Even though high temperatures were recorded above the surface and at 1 to 2 mm below the surface, only small changes in temperature were recorded at greater depths. These observations suggest that soil temperatures are unlikely to become lethal to gorse seed at depths greater than 0.4 inch (10 mm) . Temperatures recorded at 2 inches (5 cm) depth during fire in gorse-dominated shrubland in Spain were less than 120 Â°F (50 Â°C) in all plots (Diaz Fierros and others 1990, as cited by ).
Most gorse seed occurs within the top 0.8 to 2.4 inches (2-6 cm) of soil, although density and distribution of gorse seed in the soil is highly variable, both within and between sites [39,74,99]. Some research indicates greater than 60% reduction of gorse seed in the top 4 inches (10 cm) of soil after fire [74,99] (see Seed banking). In a dense, 13- to 16-foot (4-5 m) tall gorse thicket in New Zealand, Ivens  measured 10,000 gorse seeds/mÂ² in the top 6 inches (15 cm) of soil, about half of which occurred in the litter or top inch (2.5 cm). Among soil samples collected from several sites dominated by gorse among 5 districts in New Zealand, most (90% on average) gorse seed occurred in the top 2.4 inches (6 cm) of soil, both before and after burning. Density of gorse seed was reduced about 66% on average after fire. Burning did not affect viability of gorse seed remaining after fire, and most gorse seed (>80%) in the top 35 inches (30 cm) of soil was viable before and after burning . Rolston and Talbot  measured 2,660 gorse seeds/mÂ² in the top 4 inches (10 cm) of soil, with 71% of the seed in the top inch (2.5 cm) before spraying and burning. Five weeks after burning gorse seed density had declined by 62% to 1,000 seeds/mÂ² in the top 4 inches (10 cm) and by 59% in the top inch (2.5 cm). The authors suggest that most of the decline in seed numbers can be attributed to germination of buried seed after fire. Germination rates of buried gorse seed before and after burning were 73% and 79%, respectively. There appears to be no relationship between soil temperatures achieved during the fire and the germination capacity of remaining gorse seeds. Even where temperatures immediately below the soil surface exceeded 300 Â°F (150 Â°C) for 102 seconds there was no reduction in germination capacity . Most of the gorse growth after combined herbicide and burning treatments is from seedling establishment and not from sprouting (Zabkiewicz and Gaskin unpublished, as cited by ).
Burning may accelerate germination of gorse seeds, and in some instances can kill or consume gorse seed . For example, Miller (1992, as cited by ) reported that fire at an Oregon site reduced the number of viable gorse seeds in the soil by 54% (from 2,883 to 1,318 seeds/mÂ²). Because lethal temperatures rarely penetrate the soil below 1 cm, loss of gorse seed from the soil seed bank is due mainly to germination and not mortality . An exception may be high-severity ground fires where seed is contained in the surface organic horizons and is consumed by the fire. This might explain the absence of gorse regeneration after a ground fire in a heathland in northern Spain  (see Plant Response to Fire).
Aboveground gorse biomass may be consumed by fire, depending on fire behavior. Fire-killed stems may remain upright for several months , and some stems may survive . Most gorse plants survive fire, and postfire sprouting from the basal stem region (coppicing) is common (see Plant Response to Fire) .
Gorse seeds are either killed, scarified, or unaffected by fire [98,99], depending on depth of burial and fire severity.
There are no known conservation measures specifically for U. europaeus, but the species is currently known to occur in many protected areas across it snatural range. Samples of seed of U. europaeus are stored in the Millennium Seed Bank as an ex situ conservation measure.
The following botanical description of gorse is based on information compiled from florae [28,33,36,63,84] and reviews [15,20,37,70], unless otherwise cited. It describes characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g. [28,33,36]). Additionally, gorse resembles but is morphologically distinct from several invasive broom species that occur in similar habitats. DiTomaso  provides a table of characteristics to distinguish among broom species (Cytisus spp., Genista monspessulana, and Spartium junceum) and gorse.
Gorse is a medium to tall, densely branched, perennial shrub up to 16 feet (4.8 m) but usually less than 8 feet (2.5 m) tall in North America. Gorse is woody and evergreen. Twigs are hairy when young. On older plants rigid, strongly angled, intricately intertwined branches arise from the base and end in spiny tips. Seedlings are of variable morphology , though seedlings and young shoots near the ground have small alternate leaves with 3 leaflets. Leaves are reduced to scales or stiff spines, 1.8 to 2.6 inches (4.5-6.5 cm) long, on mature plants such that plants are densely covered with sharp spines. Gorse has persistent, pea-like flowers, up to 1 inch (2.5 cm) long. Flowers are usually borne singly in leaf axils or concentrated near branch tips on 2nd-year twigs. The fruit is a legume, 0.4 to 0.8 inch (1-2 cm) long, bearing a variable number of seeds (1 to 8). Gorse seeds bear elaiosomes .
The root system of gorse growing in a chalk heath in England was described as shallow, with the majority of roots occurring in the top 4 inches (10 cm) of soil, and a tap root growing to at least 12 inches (30 cm) (Grubb and others 1969, as cited by [12,70]). Observations of gorse growing in New Zealand suggest that plants that have been repeatedly burned have a very dense network of roots, and when these plants are removed they can leave a hole up to 3 ft (1 m) across and 1.5 to 2 feet (46-60 cm) deep . No description of a gorse root system growing in North America or in other substrates is available. However, according to reviews, gorse has an extensive, multi-branched lateral root system with nitrogen-fixing root nodules , and is supplemented by a fine mat of adventitious roots that descend from lower branches . The source of this information is not given.
According to reviews, gorse has photosynthetic stem tissue , and photosynthesis occurs mainly in the epidermis of the stems and spines .
Growth form and stand structure: Some authors identify 2 variants or ecotypes of gorse in New Zealand as "short spine" gorse and typical or "wild" gorse. Short spine gorse has shorter spines and a denser bush than typical or wild gorse [48,59].
Other authors describe 2 ecotypes of gorse as prostrate and erect. Prostrate types occur in exposed, windy areas. Erect plants described in British Columbia attain heights of 7 to 10 feet (2-3 m), on average, with a crown diameter of up to 13 feet (4 m), and a maximum height of 16 feet (4.8 m). In areas with very dense vegetation, gorse generally produces a single main stem, and on more open sites grows multiple stems ( and references therein). In middle-aged stands in New Zealand, a mean canopy height of 13 feet (4 m), a maximum height of 23 feet (7 m), and a maximum diameter of 8.5 inches (21.7 cm) at 3 feet (1 m) aboveground were recorded (Lee and others 1986 as cited by ).
In areas where it is invasive, gorse is often described as growing in dense stands [5,13,22,24,32,39,64] and impenetrable thickets [26,63] that cover large areas and produce a substantial amount of aboveground biomass [20,21,22,64]. Mature gorse stands in New Zealand had stem densities of 60,000 stems/ha, and a mean basal area of 51 mÂ²/ha (Lee and others 1986 as cited by ). Similar stem densities have been observed in British Columbia .
Measurements in New Zealand indicate rapid biomass accumulation in gorse-dominated stands. On one site, gorse was the dominant species 10 years after fire and annual dry matter accumulation averaged 10,000-15,000 kg/ha. In older stands other species shared dominance with gorse and biomass accumulation slowed to 2,000 to 4,000 kg/ha/year . In another area, where fire had burned a closed-canopy stand of gorse, postfire communities with greater than 80% cover of gorse averaged over 65,000 kg/ha 6.5 years after fire . In 8-year-old gorse shrubland in northwest Spain with approximately 90% cover of gorse and mean height of vegetation about 4 feet (1.2 m), total biomass was about 5.4 kg/ha [80,82]. However, biomass can reach about 40,000 to 60,000 kg/ha with stratified layers of vegetation . Relative amounts of woody to green material increase as gorse plants age [22,65] (see Fire hazard potential).