The Biogeography of the Scotch Broom
(Cytisus scoparius)
by Shelly Sheperson, student in
Geography 316 Spring 2005
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our site. This web pages was written by a student in Geography 316: Biogeography
and edited by the instructor, Barbara Holzman, PhD. All photos and maps
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on these web pages. The students have tried to be as accurate as possible with
the information provided and sources and references are cited at the end of each
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Kingdom Plantae (Plants) Subkingdom Tracheobionta (Vascular Plants) Superdivision Spermatophyta (Seed Plants) Division Magnoliophyta (Flowering Plants) Class Magnoliopsida (Dicotyledons) Subclass Rosidae Order Fabales Family Fabaceae (Pea Family) Genus Cytisus Species Cytisus scoparius
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Figure 1: Picture of Scotch Broom,
Source: http://www.extendinc.com/weedfreefeed/index.htm |
Description:
Cytisus scoparius, or scotch broom, is a deciduous perennial shrub that grows one to two meters tall (The Nature Conservancy, 2005) and can live up to 17 years (Bossard, 1990). The orientation is almost completely vertical with few horizontally growing branches. Scotch broom is a member of the pea family and its fruit and flowers reflect the relation.
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FLOWERS The bush produces many tiny yellow flowers that bloom between March and May in the northern hemisphere. The flowers cluster near the base of the leaves and the stem (Figure 2). The pedicels, the part of the bush that attaches the flowers to the stem, are less than 1.5 cm in length. The calyx, the part that holds the flower to the pedicel, is composed of three sepals. The flower itself appears to have four external yellow petals, with a single petal that appears to be two petals fused together and is spread out to form a backdrop for the front three petals. The front three petals are arranged to lie parallel to each other, with the two out petals spread out and arching outward while the middle petal remains flat. The flowers contain a light fragrance. Figure 2 at right is a close up picture of the flowers.
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| Figure 2: Close up of Scotch Broom, Source: | |
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LEAVES AND BRANCHES The flat leaves are trifoliate, which means that three leaflets are combined in a single connection to form a leaf. The leaflets themselves vary in shape from egg-shaped to oblanceolate, which means they are lance-shaped but broadest above the middle and tapering toward the base (Figure 3). The entire stem includes leaflets grows 6 to 12 mm long. The leaves are covered in microscopic hairs that are barely visible to the naked eye but can be felt if you run your finger along the leaf. The leaves grow in the spring after the appearance of the flowers and then drop off in the fall (Bossard, 2000). As a young plant, the branches are green and flexible with five to seven ridges that run parallel the length of the stem. The branches lose their ridges and turn brown and lose flexibility as the plant ages. The young branches have fine hairs on them that fall off as the plant matures (Bossard, 2000). As would be expected, the leaves contain photosynthetic tissues; however half of the photosynthetic tissues are in the stem itself. The ability to photosynthesize in the stem allows for greater survivability in times of stress (Bossard and Rejmanek, 1992).
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| Figure 3: Close up of stem of Scotch Broom, Source: Shelly Sheperson | |
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FRUIT Scotch broom bears fruit in the summer in the form of black/brown pea pods that are one to two inches in length and have tiny hairs along the seam of the pod. The pods contain 3-12 green/brown to black seeds per pod which are shiny and round to oval in shape (Bossard, 2000). The fruit itself is inedible.
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| Figure 4: Close up of seed pods (fruit) of Scotch Broom, Source: Shelly Sheperson |
Natural History:
REPRODUCTION
Scotch broom can reproduce either vegetatively or by seed and is extremely prolific. Scotch broom begins producing seed pods in its second year, and can produce as many as 12,000 seeds per year in a mature plant (Bossard, 2000). The seeds are initially released through ballistic dispersal where the seeds can be carried by the wind over long distances. The seeds can also be dispersed by rain wash, wind and animals. The seeds themselves are tolerant of rain wash scrubbing and drought. As long as the seeds remain in shallow earth, no deeper than 10 cm, the seeds have been known to survive for five, possibly up to 30 years (Carson, 1998). With massive seed production and long seed life, Bossard (2,000) states that the seed bank can build to over 2,000 seeds per square foot. The seeds germinate from November to June inland and from January to July along the coast (Bossard, 1993). Most of the growth occurs from May to July and the shrub can live for 20 years.
The vegetative reproduction seen in America may be due to the relatively recent introduction of the organism, so few pollinators exist to perform the pollinator service (Suzuki, 2003).
ECOLOGY
It grows best in dry sandy soils with full sunlight and can grow in soils that vary in pH from 4.5 to 7.5 (The Nature Conservancy, 2005). Scotch broom also performs well in nitrogen-poor soils because of its special mutualistic relationship with rhizobium to fix nitrogen into the soil. Scotch broom grows best in disturbed areas such as along road cuts, river edges and forest clear cuts.
Scotch broom is an example of an ideal invasive species because it can adapt to growing in a variety of weather, sunlight and soil conditions and because it is extremely prolific reproductively. Once introduced, this invasive can outcompete the native flora in short period of time. In disturbed areas, the fast growth of young scotch broom can shade out native tree saplings.
The shrub’s invasive behavior has garnered the classification title of a noxious weed, which means that it can move into an area and displace native flora. Scotch broom’s specific classification in California is a Class C noxious weed, which means that the state is working to retard its spread but there is no enforcement for introduction (Kartesz, 2005). By comparison, a Class A noxious weed is managed at the state level for eradication, quarantine and containment (Kartesz, 2005).
It is also helps spread crown fires, which are the most dangerous types of fire in a forest. Although trees can be damaged in any fire, crown fires are the most dangerous for tree survivability (Parsons, 1992).
Another reason that scotch broom might do very well in California is because there appears to be few creatures that can feed on it. The seeds are inedible and only young foliage can be eaten by rabbits. The foliage can cause digestive disorders in horses.
Evolution:
Angiosperms, or flowering plants (which is the division that Cytisus scoparius falls into), are most closely related to Gymnosperms (Campbell, et al, 2002). Both Angiosperms and Gymnosperms are thought to have evolved from an extinct group of plants that are commonly called progymnosperms who went extinct approximately 245 million years ago (Campbell, et al, 2002). The oldest known angiosperm ancestor is 130 million years old (Campbell, 2002). The closest living relatives to the angiosperms are thought to be Amborella and water lilies such as Nymphaea (Campbell, 2002). At the end of the Cretaceous period, around 65 million years ago, angiosperms radiated out and became the dominant plants on Earth (Campbell, et al, 2002).
Cytisus scoparius is found in the class, Magnoliopsida, which contains all of the dicotyledons, whose members possess two embryonic seed leaves, or cotyledons (Campbell, 2002). The next level of organization down is to the subclass, Rosidae (Tree of Life, 2002). Based on DNA sequences of the chloroplast rbcL gene, all nitrogen fixing root symbioses occur in Rosidae (Solitis, et al, 1995).
Bacterial rhizobial symbiosis is formed within a nodule on the root in most, but not all, species of the order, Fabales. The symbiotic relationship yields nitrogen for the plant and carbon for the bacteria (Fitter, et al, 1997).
The order Fabales, contains four families; Fabaceae, Polygalaceae, Quillajaceae and Surianaceae. Fabaceae is considered the pea family and contains 169 genera in the family (Tree of Life, 2002) (Figure 5).
Figure 5: Cladogram of Scotch Broom
Distribution::
Scotch broom grows best in dry sandy soils with full sunlight and can grow in soils that vary in pH from 4.5 to 7.5 (Gill and Pogge, 1974). Scotch broom also performs well in nitrogen-poor soils because of its special mutualistic relationship with rhizobium to fix nitrogen into the soil (Bossard, 2000). It tolerates a wide range of soil moisture conditions and is competitive in low-fertility soils (Bossard, 2000). Scotch broom grows best in disturbed areas such as along road cuts, river edges and forest clear cuts.
Scotch Broom is native to Europe and West Africa. In Europe, it extends from England in the west to Ural Mountains (where is this) in the east and from Sweden in the north to the Mediterranean in the south. In Europe, it is found on moderately leached soils in heathlands, acidic grasslands and inland dunes (Bocher and Larsen, 1958).
Scotch broom was originally admired for its bright yellow flowers and was planted as an ornamental. (Bossard, 2000). It has been introduced to the United States, New Zealand and Australia. In the western United States, scotch broom has become established along the inland valleys of the Pacific Northwest, from British Columbia to central California (Hitchcock and Cronquist, 1973). Its northern limits are probably due to low winter temperatures, the southern limits due to summer drought (Williams, 1981).
In California, the two major concentrations of the shrub are in: (1) Sonoma, Marin, Contra Costa, Santa Cruz, San Francisco and Monterey counties and (2) Sierra, Nevada, Placer, El Dorado and Calaveras counties (Bossard, 2000). Scotch broom was introduced in the Sierra Nevada foothills of California in the 1850s as an ornamental plant and was later introduced in coastal California to prevent erosion and stabilize sand dunes (Geickey, 1957, Schwendimann, 1977).
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United States Distribution
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Other interesting issues:
CONTROL
With a classification like “noxious weed,” one can only imagine the multitudes of suggested eradication methods. Because the plant can reproduce vegetatively, any root systems left in the soil can be a pathway for regeneration after an attempted eradication (Mountjoy, 1979). The shrub responds well to herbicides, but there are concerns for releasing poisons into the water and soil systems. Other suggested eradication techniques are grazing by sheep, but sheep do not distinguish between native plants and the target noxious weed. Other options being studied are using biological controls such as the introduction of small moths or pathogens but those methods have not proven to be more or less useful than other methods. Introducing new organisms to an ecosystem also includes the risk of introducing another species that can outcompete endemic flora and fauna. The best method for control appears to eradicate enough of the scotch broom to allow for native plants to take over because scotch broom has a difficult time invading in areas that are well established by other species (Bossard, 2000). However, the first step in preventing the spread of scotch broom should be the ceasing of selling this noxious weed in nurseries around the country as an ornamental plant.
Bibliography
Bellingham, P.J., Coomes, D.A. 2003. “Grazing and community structure as determinants of invasion success by Scotch broom in a New Zealand montane shrubland.” Diversity and Distributions 9: 19–28.
Bocher, T.W. and Larsen, K. 1958. “Secondary Polyploidy and Ecotypical Differentiation in Sarothamnus scoparius.” New Phytologist. 57: 311-317.
Bossard, C. 1990. “Tracing of Ant-Dispersed Seeds: A New Technique.” Ecology. 71: 2370-2372.
Bossard, C. 1991. “The Role of Habitat Disturbance, Seed Predation and Ant Dispersal on Establishment of the Exotic Shrub Cytisus scoparius in California.” American Midland Naturalist. 26 (1): 1-13
Bossard, C., Rejmanek, M. 1992. “Why Have Green Stems?” Functional Ecology 6(2): 197-205.
Bossard, C. 1993. “Seed germination in the exotic shrub Cytisus scoparius in California” Madroņo 40: 47-61.
Bossard, C. 2000. “Cytisus scoparius.” pp 145-149. In Bossard, C., Randall, M. and Hoshovsky, M. 2000. Invasive Plants of California’s Wildlands. Los Angeles: University of California Press, Ltd.
Campbell, N.A. and Reece, J.B. 2002. Biology. San Francisco, CA. Benjamin Cummings.
Fitter, A.H., and Moyerson, B. 1997. “Evolutionary Trends in Root-Microbe Symbioses.” Included in Plant Life Histories: Ecology, Phylogeny and Evolution. Silvertown, J., Franco, M. and Harper, J.L. 1997. Cambridge University Press, Cambridge, U.K.
Geicky, H.M. 1957. Weeds of the Pacific Northwest. Oregon Press, Portland, OR.
Gill, J.D. and Pogge, F.L. 1974. “Cytisus scoparius, Scotch Broom” in C.S. Schopmeyer edition. Seeds of Woody Plants in the United States. Washington, D.C.: USDA Press.
Hitchcock, C.L. and Cronquist, A. 1973. Flora of the Pacific Northwest. University of Washington Press: Seattle, WA.
Kartesz, J.K. 2005. “Plant Profile for Cytisus scoparius (scotch broom).” [Online]. Available: http://plants.usda.gov/cgi_bin/plant_profile.cgi?symbol=CYSC4.htm [March 13, 2005].
Mountjoy, J.H. 1979. “Broom a Threat to Native Plants.” Fremontia. 6 (4): 11-15.
Parsons, W.T. and Cuthbertson, E.G. 1992. Noxious weeds of Australia. Inkata Press, Melbourne, Australia.
Schwendiman, J.L. 1977. “Coastal and Sand Dune Stabilization in the Pacific Northwest.” International Journal of Biometeorology. 21:281-289.
Soltis, D.E., Soltis, P.S., Morgan, D. R., et al. 1995. “Chloroplast Gene Sequence Data Suggest a Single Origin of the Predisposition for Symbiotic Nitrogen Fixation in Angiosperms”. Proceedings of the National Academy of Sciences of the U.S.A. 92: 2647-2651.
Suzuki, N. 2003. ‘Significance of flower exploding pollination on the reproduction of the Scotch broom, Cytisus scoparius (Leguminosae).“ Ecological Research 18: 523–532.
The Nature Conservancy. (2005, January). “Element Stewardship Abstract for Cytisus scoparius and Genista mospessulanus.” [Online]. Available: http://theweeds.ucdavis.edu/esadocs/cytiscop.htm [March 13, 2005].
Tree of Life Web Project. 2002. “Life on Earth Content Group”. [Online] Available: http://tolweb.org/tree?group=Life_on_Earth&contgroup=.htm [April 23, 2005].
Williams, P.A. 1981. “Aspects of the Ecology of Broom (Cytisus scoparius) in Canterbury, New Zealand.” New Zealand Journal of Botany. 21: 237:247.
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