Fire Effects Information System (FEIS)

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Table of Contents

• Introductory
• Distribution and occurrence
• Botanical and ecological characteristics
• Fire ecology
• Fire effects
• Management considerations
• References

INTRODUCTORY

AUTHORSHIP AND CITATION:
Zouhar, Kris. 2005. Spartium junceum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us/database/feis/plants/shrub/spajun/all.html [].

FEIS ABBREVIATION:
SPAJUN

SYNONYMS:
None

NRCS PLANT CODE [53]:
SPJU2

COMMON NAMES:
Spanish broom

TAXONOMY:
The currently accepted name for Spanish broom is Spartium junceum L. (Fabaceae) [27,30,50].

LIFE FORM:
Shrub-tree

FEDERAL LEGAL STATUS:
None

OTHER STATUS:
Spanish broom is classified as a noxious weed in Hawaii and Oregon, and as a "Category A" nonnative species in Washington [54]. See the Invaders database for more information.

DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• ECOSYSTEMS
• STATES/PROVINCES
• BLM PHYSIOGRAPHIC REGIONS
• KUCHLER PLANT ASSOCIATIONS
• SAF COVER TYPES
• SRM (RANGELAND) COVER TYPES
• HABITAT TYPES AND PLANT COMMUNITIES

Spanish broom was introduced into the California ornamental trade in 1848 in San Francisco. Beginning in the late 1930s, it was planted along mountain highways in southern California. By 1949, Spanish broom had escaped cultivation and established populations in Marin County ([38], and references therein). It now occurs in the north coast counties of California, the San Francisco Bay region, the Sacramento Valley, through the south coast counties into northern Baja California [27,37], in the western Transverse Ranges, and the Channel Islands [38]. It also occurs on dry slopes in the eastern half of the Santa Monica Mountains [15]. Of the invasive brooms in California, Spanish broom is less widespread and is considered less of a problem than Scotch broom (Cytisus scoparius) and French broom (Genista monspessulana) [28]. There is no information in the literature on distribution of Spanish broom in Oregon, Washington, or Hawaii. Plants database provides a state distribution map of Spanish broom.

Spanish broom is 1 of 4 nonnative invasive broom species that occur in North America. Spanish broom, Scotch broom, Portuguese broom (C. striatus), and French broom occur in similar habitats. Common gorse (Ulex europaeus) is another leguminous shrub that occurs in similar habitats.

The following lists include vegetation types in which Spanish broom is known to be or thought to be potentially invasive, based on reported occurrence and biological tolerances to site conditions from studies of Spanish broom in California. There is no information about Spanish broom distribution or site tolerances outside California; therefore, these lists are somewhat speculative and may be imprecise.

Spanish broom seems to be most common in disturbed areas, especially along roadsides [15,35,38], where it was seeded in the early 1900s [38]. In 1958, Hellmers and Ashby [26] stated that Spanish broom has been planted along roads for 20 years, where it survives and grows well, but has not been able to invade the adjoining stands of chaparral. It has since become invasive in chaparral in southern California [10], where it was seeded for revegetation after fires in the early 1900s [3].

According to the California Invasive Plant Council [8], Spanish broom occurs in coastal scrub, grassland, wetlands, and oak (Quercus spp.) woodland throughout California, and forests in the northwestern part of the state. Spanish broom is associated with coyote bush (Baccharis pilularis) in the interior Santa Cruz Mountains, with a large monospecific stand of French broom located downslope [36]. Spanish broom also occurs in redwood (Sequoia sempervirens) forests [8,43].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

GENERAL BOTANICAL CHARACTERISTICS:
Spanish broom is 1 of 4 nonnative invasive broom species that occur in North America. All are perennial, leguminous shrubs. DiTomaso [16] provides a table of characteristics to distinguish among broom species and common gorse. Spanish broom, French broom, Scotch broom and Portuguese broom have some similar morphological characteristics, while common gorse is morphologically distinct from the brooms. Spanish broom is not as widely distributed nor as common as Scotch and French brooms (see Distribution and Occurrence), and less is known about its biology and ecology. According to DiTomaso [16] Spanish broom may have ecological characteristics similar to Scotch broom.

The following description of Spanish broom provides a summary of the range of characteristics described in reviews [16,38] and florae [15,27]. It provides characteristics that may be relevant to fire ecology, and is not meant for identification. A key for identification is available in Hickman [27].

Spanish broom is a tall shrub to small tree, up to 10 to 15 feet (3-5 m) tall. Its long, slender stems are erect with few branches. Stems are cylindrical, rush-like, and green when young, maturing into woody branches with bark. Mature plants have 1 to several trunks. Spanish broom leaves are small, 0.5 to 1 inch (2-2.5 cm) long, oval, and smooth-margined. Leaves are ephemeral, remaining on the plant for 4 months or less. The inflorescence is an open terminal raceme with several flowers located on current-year shoots. Flowers are large, pea-like, up to 1 inch long, and grow on short stalks on both sides of the main stem. Fruit is a linear, dehiscent legume, 2 to 4 inches (5-10 cm) long and 5 mm wide, with 10 to 15 seeds.

At the time of this writing (2005), no descriptions of Spanish broom root structure or morphology are available for plants growing in North America. Chiatante and others [9] describe root morphology of Spanish broom growing in 3 different rooting environments in Italy: terrace, plane, and 40° slope. The authors observed that the overall architecture of the root system was modified on a slope by an increase in the length and number of root apices of 1st-, 2nd-, and 3rd-order lateral roots. This suggests that Spanish broom reinforces its anchorage strain by changing the organization of its root system, particularly in the up-slope direction [9].

Although the leaves of both Spanish broom [36] and Scotch broom [5,36] are ephemeral, their canopies have a constant positive carbon balance due to stem photosynthesis. Both species have positive stem assimilation resulting in approximately 200 mmol per m˛ per day carbon dioxide assimilation on study sites in California. Although these species grow in different habitats with different vapor pressure and temperature, assimilation response to vapor pressure is similar between species. Water-use efficiency is higher and intercellular carbon dioxide is lower for Spanish broom compared to Scotch broom. The constant carbon gain throughout the year, from stem assimilation, may enhance the growth capacity of both species in disturbed habitats [36].

Botanical traits of Spanish broom vary somewhat between cloned individuals and those grown from seed. In a greenhouse study, several mean growth traits were significantly (P<0.05) different between ramet and genet populations, and the variance in these traits tended to be higher in genet populations. Seedlings had consistently greater whole plant dry mass per shoot length, a higher percentage of total biomass in leaves, and more roots compared with cloned individuals. In contrast, few mean physiological traits differed between ramet and genet populations, and variance was similar between the 2 population types. Environmental variance accounts for a large proportion of the variance in physiological traits, and about 33% of the variance in growth traits [37].

Growth form and stand structure: According to a review by DiTomaso [16], dense broom infestations produce substantial dry matter that can create a serious fire hazard. While this is particularly true for gorse and French broom [16], Nilsen [38] also suggests that mature Spanish broom stands should be considered a fire hazard during the dry season, because patches can be dense and may contain a large amount of dead wood.

Spanish broom spreads by producing abundant seeds. No research has been conducted on Spanish broom seed banks, germination, or seedling recruitment [38].

Breeding system: Spanish broom plants are monoecious and outcrossed [37].

Pollination: Spanish broom flowers are pollinated by bees [38].

Seed production: Seed production begins when Spanish broom plants are 2 to 3 years old. Each inflorescence produces 10 to 15 pods containing approximately 15 seeds each. One plant can produce 7,000 to 10,000 seeds in one season [38].

Seed dispersal: Spanish broom seeds fall near the parent plant and are subsequently moved by erosion, rain wash, and possibly ants [38].

Seed banking: According to Nilsen [38], Spanish broom seeds remain viable for at least 5 years, suggesting that a large seed bank may be present in Spanish broom stands. The source of this information is not given, nor is there any additional information in the literature about seed banking in Spanish broom. More research is needed in this area.

Germination: Spanish broom seeds, collected in the Santa Cruz Mountains of California and germinated in the greenhouse, had 100% germination rates [37]. Similarly, Spanish broom seeds from Israel that were used in an experiment in California "germinated readily with no pretreatment" [26]. However, scarification is said to result in "greater" germination rates (Cabral 1954, as cited by [28]). More research is needed on germination and seed bed requirements of Spanish broom.

Seedling establishment/growth: Results from an experiment in California indicate that ambient temperature affected Spanish broom growth form. Spanish broom plants grown for 24 weeks at day/night temperatures of 73/79 °F (23/26 °C), 86/39 °F (30/4 °C), and 86/63 °F (30/17 °C) were weak and did not stand erect; whereas Spanish broom plants grown at cooler temperatures (63/39 °F (17/4 °C), 63/63 °F (17/17 °C), and 73/39 °F (23/4 °C)) had shorter and thicker stems that were able to support their own weight. Leaves were retained on Spanish broom plants grown at cooler temperatures. At higher temperatures the leaves dropped soon after they were formed. All Spanish broom plants had green stems, were branched, and had a very bushy appearance, especially at temperatures higher than the 17/4 temperature condition. The roots were nodulated and branched, and permeated the entire medium in a 1-gallon can at the end of the growth period [26].

Asexual regeneration: According to Nilsen [38] Spanish broom is "an effective stem sprouter," suggesting that Spanish broom may sprout from stumps or root crowns following damage or destruction of aboveground biomass.

Spanish broom was planted along roadsides [10,26] and seeded in chaparral sites for revegetation after fires in California in the early 1900s [3]. Populations of Spanish broom have persisted and spread along roads [8,15,35] and in other disturbed areas such as eroding slopes, riverbanks, and abandoned or disturbed lands [27,35,38]. Earlier accounts indicate that Spanish broom is not invasive in native habitats [26], (McClintock 1985, as cited by [47]). It escaped cultivation and invaded chaparral in southern California, particularly after fire [10,38].

According to Conrad [10], Spanish broom was planted along roadsides below 6,900 feet (2,100 m), while Hickman [27] states that it occurs below 2,000 feet (600 m). Spanish broom commonly occurs on steep slopes [38].

The invasive brooms are successful in high irradiance, disturbed habitats, most likely due to their photosynthetic stems, rapid growth, and ability to fix nitrogen. Drought stress severely inhibits photosynthesis of brooms. Photosynthetic stems allow them to utilize a deciduous leaf phenology (to avoid water stress) and still maintain a large photosynthetic surface area in summer and fall after leaves have abscised. On interior mountain sites where Spanish broom occurs, there is a large difference between summer and winter climate compared with coastal sites [36]. Although the leaves have twice the photosynthetic rate of stems [36], photosynthesis in stems provides most to the whole plant carbon gain because of their longer life span and larger surface area (Nilsen and Bao 1990, cited in [38]).

Shade tolerance: Seedlings of Spanish broom had greatest survival (~97%) in moderate shade (30% full sunlight), ~70% survival in 100% full sunlight, and ~10% survival in deep shade (3% full sunlight). Rates of net photosynthesis were somewhat (although not significantly) higher in full sun versus moderate shade, and dark respiration was significantly (P<0.005) higher in full sun than in moderate shade. Spanish broom was tentatively classified by the authors as a shade avoider, being neither highly tolerant nor intolerant of shade, although further tests are needed for this to be definitive [55].

According to Nilsen [38] Spanish broom flowers in late March to early April in California, while other authors [10,15,35] indicate flowering in Spanish broom occurs from April to June in the Santa Monica Mountains in southern California. Spanish broom flowers remain when flowers of most associated native species have faded and folded [10,15,35]. Spanish broom pods mature from late May through early July, depending on location, after leaf abscission [36,38].

No information is available on seasonal development in other areas of North America where Spanish broom occurs.

Spano and others [46] gathered phenological data, derived threshold temperatures for the computation of degree-days, and evaluated the sensitivity to weather variations of 9 plant species, including Spanish broom, at an experimental garden on the Mediterranean Coast in Italy. Results were as follows [46]:

FIRE ECOLOGY

• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

Fire regimes: No information is available on fire regimes in plant communities where Spanish broom evolved.

It is unclear how the presence of Spanish broom might affect fire regimes in invaded communities. In general, in ecosystems where it replaces plants similar to itself (in terms of fuel characteristics), it may alter fire intensity or slightly modify an existing fire regime. However, if Spanish broom invasion introduces novel fuel properties to the invaded ecosystem, it has the potential to alter fire behavior and potentially alter the fire regime (sensu [6,13]). Given this perspective, it seems unlikely that Spanish broom will alter fire regimes where it is invasive in California chaparral communities. It is unclear which other plant communities, and to what extent, Spanish broom invades see Habitat types and plant communities). Some examples of potential fire regime changes brought about by Scotch broom and French broom are reviewed in FEIS.

The following list provides fire return intervals for plant communities and ecosystems where Spanish broom is important. It may not be inclusive. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

FIRE EFFECTS

• IMMEDIATE FIRE EFFECT ON PLANT
• DISCUSSION AND QUALIFICATION OF FIRE EFFECT
• PLANT RESPONSE TO FIRE
• DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
• FIRE MANAGEMENT CONSIDERATIONS

See FEIS reviews on French broom and Scotch broom for more information on fire effects on these species.

Preventing postfire establishment and spread: The USDA Forest Service's "Guide to Noxious Weed Prevention Practices" [52] provides several fire management considerations for weed prevention in general that may apply to Spanish broom.

Preventing invasive plants from establishing in weed-free burned areas is the most effective and least costly control method. This can be accomplished through careful monitoring, early detection and eradication, and limiting invasive plant seed dispersal into burned areas by [23,52]:

• re-establishing vegetation on bare ground as soon after fire as possible

• using only certified weed-free seed mixes when revegetation is necessary

• cleaning equipment and vehicles prior to entering burned areas

• regulating or preventing human and livestock entry into burned areas until desirable site vegetation has recovered sufficiently to resist invasion by undesirable vegetation

• detecting weeds early and eradicating before vegetative spread and/or seed dispersal

• eradicating small patches and containing or controlling large infestations within or adjacent to the burned area

In general, early detection is critical for preventing establishment of large populations of invasive plants. Monitoring in spring, summer, and fall is imperative. Managers should eradicate established Spanish broom plants and small patches adjacent to burned areas to prevent or limit postfire dispersal and/or spread onto the site [23,52].

The need for revegetation after fire can be based on the degree of desirable vegetation displaced by invasive plants prior to burning, and on postfire survival of desirable vegetation. Revegetation necessity can also be related to invasive plant survival as viable seeds or root crowns [23].

Managers can enhance the success of revegetation (natural or artificial) by excluding livestock until vegetation is well established (at least 2 growing seasons) [23]. See Integrated Noxious Weed Management after Wildfires for more information.

When planning a prescribed burn, managers should preinventory the project area and evaluate cover and phenology of any Spanish broom and other invasive plants present on or adjacent to the site, and avoid ignition and burning in areas at high risk for Spanish broom establishment or spread due to fire effects. Managers should also avoid creating soil conditions that promote weed germination and establishment. Weed status and risks must be discussed in burn rehabilitation plans. Also, wildfire managers might consider including weed prevention education and providing weed identification aids during fire training; avoiding known weed infestations when locating fire lines; monitoring camps, staging areas, helibases, etc., to be sure they are kept weed free; taking care that equipment is weed free; incorporating weed prevention into fire rehabilitation plans; and acquiring restoration funding. Additional guidelines and specific recommendations and requirements are available [52].

Fire as a control agent: While prescribed fire is sometimes used in management of French broom and Scotch broom, no information is available on using fire to control Spanish broom.

MANAGEMENT CONSIDERATIONS

IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Spanish broom provides poor forage for native wildlife [38], and presumably poor forage for livestock as well. Domestic goats may eat young Spanish broom plants [28].

Palatability/nutritional value: No information is available on this topic.

Cover value: No information is available on this topic.

As a nitrogen-fixing plant, Spanish broom may enrich soil nitrogen levels in invaded communities. Although nitrogen fixation has not been studied in Spanish broom, Scotch broom is capable of fixing nitrogen throughout the year in regions with mild winters [58]. The ability of the brooms to fix nitrogen increases the total amount of nitrogen and the way in which nitrogen cycles in invaded communities [25]. Nitrogen enrichment is unlikely to benefit native plants and may reduce species diversity [14], except in ecosystems dominated by nitrogen-fixers. This may have implications for restoration and rehabilitation efforts [25].

Control: There is little information on controlling Spanish broom. Nilsen [38] presents a summary of possible control approaches based on the biology of the plant, rather than on information derived from controlled experiments [38]. See FEIS reviews on Scotch broom and French broom for information on controlling these similar species.

It is likely that the success of any control method will vary with site characteristics (topography, soils, climate), age and density of plants in the stand, and the availability of human and technical resources. Since a large and persistent seed bank is predicted for this species, it is likely that seedlings will establish rapidly following fire or mechanical removal of aboveground biomass [38].

A comprehensive monitoring of control effectiveness is critical because there is no scientifically based knowledge about control of Spanish broom. Experimental manipulations should be monitored at least annually. Each monitoring visit should determine the number of new plants and the size or age distribution of the recovering populations. Attention should be placed on the proportion of new individuals coming from the seed bank or sprouting from old plants. Monitoring should continue for at least 5 years after control treatment [38].

Prevention: The most effective method for managing invasive species is to prevent their establishment and spread. Some methods of prevention include limiting seed dispersal, containing local infestations, minimizing soil disturbances, detecting and eradicating weed introductions early, and establishing and encouraging desirable competitive plants [44]. One way to help prevent continued introductions of Spanish broom into wildlands is to prevent its sale as a horticultural species.

Integrated management: A particularly effective control combination for Spanish broom may be saw cutting followed by application of herbicide to the cut stem to kill adult plants. Spanish broom seedlings are likely to establish from the soil seed bank so monitoring and follow-up treatments of new seedlings is necessary for several years [38].

Physical/mechanical: In general, physical and mechanical control methods are likely to be effective only when Spanish broom is young [38]. The Nature Conservancy's Element Stewardship Abstract on Spanish broom provides a general overview of physical and mechanical control methods that may be effective for controlling infestations [28].

Pulling with weed wrenches is effective for small broom infestations or in areas where an inexpensive, long-duration labor source is dedicated to broom removal [51]. Hand-pulling Spanish broom plants may be most practical and effective when the stand is 1 to 4 years old, and plants are small enough, as long as roots are removed and follow-up treatment of seedlings is done. The optimal season for pulling may be July to September when plants are experiencing water stress [36]. When plants have matured to small tree size, they cannot easily be removed with hand tools [38].

Nilsen [38] suggests that machines such as brush hogs are probably impractical for Spanish broom removal, since it commonly occurs on steep slopes, and because the trunks of Spanish broom grow rapidly to a size outside the range of effectiveness for this technology. Saws can be used to cut plants with larger stems; however, Spanish broom has a great facility for sprouting from a saw cut even when the cut is close to the ground. When brush hogs or saws are used to cut Spanish broom stems, sprouting should be expected. Among all the mechanical methods, saw cutting is least likely to be effective in preventing sprouting [38].

Fire: See the Fire Management Considerations section of this summary.

Biological: Biological control of invasive species has a long history, and there are many important considerations before the implementing a biological control program. Tu and others [51] provide general information and considerations for biological control of invasive species in their Weed control methods handbook. Additionally, Cornell University, Texas A & M University, and NAPIS websites offer information on biological control.

As of this writing (2005) there are no USDA approved biological control agents for Spanish broom. In greenhouse situations plants are susceptible to mealy bugs and show evidence of viral depression of growth [38]. An insect purposely introduced for control of Scotch broom, the Scotch broom bruchid (Bruchidius villosus) [11], also attacks Portuguese broom, Spanish broom, and French broom. See Coombs and others [12] for more information on this insect, its distribution, and effects.

Domestic goats are said to be effective at controlling reestablishment of broom [28].

Chemical: Herbicides are effective in gaining initial control of a new invasion (of small size) or a severe infestation, but are rarely a complete or long-term solution to invasive species management, as they do not change conditions that allow infestations to occur [7]. Herbicides are more effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. See the Weed control methods handbook [51] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals and adjuvants.

Spanish broom is sensitive to applied pesticides. In greenhouse situations only mild pesticides can be used without detrimentally affecting the plants. Therefore, it is highly likely that application of chemicals such as glyphosate or triclopyr will drastically reduce population size. The ramifications of applying herbicides to a plant community must be carefully considered, because effects on nontarget species are likely, especially when foliage spray methods are used [38]. Rusmore and Butler [42] compared the efficacy of basal bark applications of varying rates of triclopyr on different size Spanish broom shrubs, at 3 phenological stages, under different moisture and shade conditions on a California riparian site. Small differences were observed among treatments, although results were not statistically significant. The kill rate averaged over 90% across all treatments [42].

See The Nature Conservancy's Element Stewardship Abstract on Spanish broom for a more detailed review of chemical control [28].

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