Botanical Studies (2012) 53: 1-8.
REVIEW PAPER
A review of the biology of the weedy Siberian peashrub, Caragana arborescens, with an emphasis on its potential effects in North America
Katelyn B. SHORTT and Steven M. VAMOSI*
Department of Biological Sciences, University of Calgary, Calgary, Canada T2N1N4
(Received June 9, 2010; Accepted December 29, 2011)
ABSTRACT. The introduction and establishment of non-native species has been recognized as one of the most significant threats to the maintenance of native biodiversity in most taxa, including angiosperms. The Siberian peashrub, Caragana arborescens Lam. (Fabaceae), is native to Eurasia, but was introduced to North America in the mid-1700s. In the past 250 years, the species has become established in almost all of Canada, and approximately half of the states in the USA. However, the literature on its potential effects on native eco­systems is relatively sparse and scattered. To complement the Caragana Control Trials Project initiated by the City of Calgary (Alberta, Canada) in 2009, we review the biology, ethnobotany, ecosystem effects, and candidate control methods of C. arborescens. Perhaps unsurprisingly, we find evidence for both positive and negative effects and uses of C. arborescens. We caution that continued habitat degradation and climate change may facilitate C. arborescens becoming an invasive or noxious species in more areas with time. Finally, we advocate that more attention be paid to C. arborescens throughout its range, with special focus on habitat frag­ments, recently deforested areas, and wetlands impacted by human activities.
Keywords: Biological control; Caragana arborescens; Ecosystem; Fabaceae; Invasive species.
CONTENTS
INTRODUCTION...................................................................................................................................................................2
METHODS..............................................................................................................................................................................2
SPECIES BIOLOGY...............................................................................................................................................................2
Evolutionary history............................................................................................................................................................2
Phenotypic characteristics...................................................................................................................................................3
Chemical characteristics......................................................................................................................................................3
Enemies...............................................................................................................................................................................4
ETHNOBOTANY....................................................................................................................................................................4
Medicinal applications........................................................................................................................................................4
Cultural uses........................................................................................................................................................................4
EFFECTS.................................................................................................................................................................................5
Colonization and spread in exotic habitats..........................................................................................................................5
Potential invasiveness..........................................................................................................................................................5
CONTROL METHODS..........................................................................................................................................................5
Physical control...................................................................................................................................................................5
Chemical control.................................................................................................................................................................6
Biological control................................................................................................................................................................6
CONCLUSIONS......................................................................................................................................................................6
LITERATURE CITED.............................................................................................................................................................6
*Corresponding author: E-mail: smvamosi@ucalgary.ca; Tel: +1-403-210-8508; Fax: +1-403-289-9311.
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INTRODUCTION
ows (Zhang, 2005; Zhang et al., 2009).
The relationships among the species in the genus Cara-gana, as well as their origin and spread, have been the sub­ject of some debate since the beginning of the last century. Komarov (1908) established the first classification of the genus in a monograph (Zhang, 2005; Zhang et al., 2009). The genus was proposed to have originated in East Asia. Furthermore, C. sinica (Buc'hoz) Rehd. was proposed to be the ancestral species of the group. By examining the leaf morphology of C. sinica, characterized by its pinnate leaves that contain two pairs of leaflets, Komarov (1908) determined that two distinct groups of species evolved from C. sinica. The radiation of species with pinnate leaves and many pairs of leaflets was accompanied by dis­persal into North China, whereas the radiation of species with palmate leaves and two pairs of leaflets was accom­panied by dispersal into Central Asia (Zhang, 2005). From this evidence, as well as the distribution range of C. sinica, Komarov (1908) proposed the 'East Asian Mongolian flor-istic migration hypothesis', suggesting that the genus dis­persed northwards and westwards (Komarov, 1908; cited in Moore, 1968; Zhang, 2004, 2005; Zhang et al., 2009).
The introduction of non-native species is one of the leading threats to biodiversity and natural ecosystems, with numerous and sometimes irreversible effects (Wit­tenberg and Cock, 2001). Caragana arborescens Lam. is native to Russia and China, and was introduced to North America in 1752, where it is considered exotic, but largely not noxious (AKEPIC, 2005; Dietz et al., 2008). How­ever, in localized areas, such as the Weaselhead Natural Environment Area, a 620 ha wilderness area in the heart of the City of Calgary, it is deemed of potential concern to the ecological integrity of local ecosystems. The purpose of this review is to determine the potential for C. arbore-scens to become invasive in North America and to assess candidate control methods. We focus on four main topics, namely: (1) species biology, (2) ethnobotany, (3) effects, and (4) control methods.
METHODS
We collated information on C. arborescens by searching the ISI Web of Science research database and the Natural Resources Conservation Service database of the United States Department of Agriculture (hereafter, USDA). Our efforts revealed that much of the research concern­ing C. arborescens has been conducted in its home range (especially China and Russia), which limited the number of articles available in English. Therefore, some of the information presented in this review has been taken from secondary sources, which have their own interpretations of the primary sources. Because very little published research has been conducted in North America regarding its inva-siveness potential, our limited coverage of this topic relies on relatively few sources and inferences made from the biology of C. arborescens in its native range.
Moore (1968) adopted a different approach in charac­terizing the ancestral species of Caragana. Specifically, he questioned the basal position of C. sinica within the group. Chromosome counts for 17 species of Caragana were obtained and used to produce the first estimate of phylogenetic relationships among Caragana species, also incorporating rachis development (deciduous to persistent) and foliage condition (pinnate to variable to palmate). Caragana sinica was found to be a triploid hybrid, and the species group containing pinnate leaves with many pairs of leaflets were mostly diploid. Moore (1968) inferred that the species group with a diploid chromosome number of 2n = 16 was basal. By examining the distribution of spe-cies with different chromosome numbers, Moore (1968) proposed that Lake Balkhash (Kazakhstan) was the centre of origin for the genus. From this origin, the species were then thought to have dispersed eastwards towards the Pacific Ocean and westwards to southern Europe and Rus-sia (Moore, 1968).
SPECIES BIOLOGY
Evolutionary history
Fabaceae (Leguminosae) is the third largest family of flowering plants, with approximately 19,400 species (Stevens, 2001 onwards). Traditionally, the family has been divided into three major subfamilies: Caesalpin-ioideae, Mimosoideae and Papilinoideae (Faboideae) (Wojciechowski et al., 2004). Papilinoideae is considered the most diverse and widespread, with 476 genera and 13,860 species, containing all familiar domesticated foods and crops that are found globally (Gepts et al., 2005). Caragana Fabr., a genus in the subfamily, is composed of approximately 100 species distributed within north­ern Eurasia from the Black Sea to southeast Siberia, and southward to eastern and south-western China, Nepal, Afghanistan and Turkmenistan (Zhang et al., 2009). Cara-gana has a temperate Asian distribution and commonly occurs in arid regions that can reach extremely cold tem­peratures (Zhang, 2004, 2005; Zhang et al., 2009). Species also occur in forests, grasslands, deserts and alpine mead-
More recently, chromosome counts were obtained for 11 additional Caragana species, as well as descriptions of the pollen morphology for 34 species (Zhang et al., 1996; Zhang, 1998; see also Zhou et al., 2002). From these new data, these authors regarded eastern Siberia as the origin of the genus and C. arborescens as the basal species (Zhang, 2004, 2005; Zhang et al., 2009). Zhang (2004, 2005) additionally provided evidence that vicariance, rather than dispersal, was the main driver of the speciation patterns observed.
Given the lack of a fossil record for Caragana (Zhang, 2005), there is some uncertainty about when it first origi-nated. Because Caragana is naturally isolated to tem-perate Asia, it has been proposed to be a comparatively young genus, possibly having originated after the Miocene (Zhang, 2004, 2005). Zhang (2004, 2005) based this infer-
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ence on the observation that no species of Caragana are native to North America, hypothesizing that some species would have dispersed to North America if the genus had originated prior the existence of the Bering Land Bridge. However, given evidence that the land bridge was present during the Pleistocene, and may have even persisted into the Holocene (Ager, 2003), this hypothesis would require origination rates (i.e., ~100 species in <15,000 yr) that are orders of magnitude greater than are characteristic of famous adaptive radiations. For example, it is estimated that the 33 species constituting the Hawaiian silversword alliance diversified over the last 5-6 million years (Bald­win and Wagner, 2010). Similarly, ecological character displacement has led to only two species of threespine stickleback fish in several southwestern British Columbia lakes since the retreat of glaciers (i.e., ~13,000 yr; Vamosi, 2003). It was also suggested that Caragana diversified into two phylogenetically distinct "directions" influenced by the paleogeographical and paleoecological histories of the Tibetan Plateau and Central Asia: species inhabiting the Tibetan Plateau being characterized by pinnate leaves and adaptations against cold temperatures, and those found within Central Asia being characterized by palmate leaves and adaptations against drought (Zhang, 2005). Some ma­jor events in the Earth's history hypothesized to have con­tributed to the evolution of the group include: the uplifting of the Tibetan Plateau, Pleistocene glaciations and increas­ing aridification of Central Asia (Zhang, 2005).
with a limited number of rhizobial strains (Gregory and
Allen, 1953).
The flowers of C. arborescens are bisexual and self-compatible, emerging from April to late June (Gregory and Allen, 1953; Dietz et al., 2008). The flowers form linear pods during June and July. Caragana arborescens is well known for its prolific seed production (Martine et al., 2008). A sigmoidal relationship exists between seed pod production and age (Henderson and Chapman, 2006). Seed pods are 2-5 cm in length and each contain around six seeds that vary in shape from oblong to spherical (Dietz et al., 2008). While ripening, the pods change from yellow to an amber or brown colour. When fully grown, the seed pods crack and burst, releasing the seeds. Seed dispersal of C. arborescens begins in July and is typically completed by mid-August (Dietz et al., 2008).
Caragana arborescens is known for its tolerance of many environmental conditions including droughts, tem-peratures to -38°C, infertile soils, sunny sites, high winds, alkaline soils and saline conditions (Henderson and Chap-man, 2006; Dietz et al., 2008; Martine et al., 2008; USDA NRCS, 2010). This tolerance for cold and dry sites is almost certainly associated with its much greater spread through North America compared to other exotic legumes, such as Scotch broom (Cytisus scoparius L.) and common gorse (Ulex europaeus L.). Gorse, for example, is cur-rently restricted to nine states and one province that are all either coastal (US: Hawaii, Washington, Oregon, Califor-nia, Massachusetts, New York, Virginia; Canada: British Columbia), or adjacent to coastal states (Pennsylvania, West Virginia) (USDA NRCS, 2010).
Although the aforementioned events likely have con­tributed to the history of this group, we caution that these interpretations relied partly on the notion that Caragana is a very young group and on a phylogenetic hypothesis that posited C. arborescens as the basal species (Zhang, 2005). Subsequent molecular phylogenies (Zhang et al., 2009; Zhang and Fritsch, 2010) reveal the latter species to be nested well within the crown group of section Caragana, which itself is not recovered as a basal group. Further phylogenetic and phylogeographic work is needed, which may yield interesting insights into the origins and spread of the genus and its member species.
Chemical characteristics
Caragana species contain many chemical compounds that may prolong its lifespan and increase dispersal. At least ten species of Caragana contain esters, cardic glyco-sides, steroids, terpenoids and phenolic compounds, but almost no traces of alkaloids (Wang et al., 2005). Phenolic compounds have been found in the water of drainage basins near C. arborescens populations. The compounds, due to their inhibitory properties, may disturb vital func­tions in plants such as Agropyron repens, supporting evi­dence that C. arborescens is detrimental to the growth of many grass species (Zolotukhin, 1980).
Phenotypic characteristics
Caragana arborescens is a deciduous perennial shrub or small tree that can reach heights of 3-5 m when fully mature (USDA NRCS, 2010). It takes approximately 10 years to mature and has a very rapid growth rate (Hender-son and Chapman, 2006; USDA NRCS, 2010). A curvilin-ear relationship exists between height and age (Henderson and Chapman, 2006). Additionally, C. arborescens has the ability to resprout if damaged (USDA NRCS, 2010).
When young, the bark is smooth and olive green in colour, which gradually fades with age (USDA NRCS, 2010). The foliage is green and has alternate pinnate leaves, each being 5-10 cm long. Each leaf has eight to twelve paripinnate leaflets that end in a spine or bristle (Zhang et al., 2009; USDA NRCS, 2010). When forming nodules on its roots, C. arborescens establishes symbioses
As noted above, C. arborescens is tolerant of alkaline soils and saline environments. It is especially tolerant to potassium chloride (KCl) and potassium sulfate (K2SO4) in soil (Redmann, 1986). Soils in prairie regions are often naturally rich in potassium, which may serve as a partial barrier to successful colonization of intolerant species. The current range of C. arborescens in North America (Figure 1) corroborates Redmann's (1986) finding that this species is not noticeably limited by soil potassium concentrations.
Caragana arborescens is a nitrogen-fixing plant and, furthermore, it initiates nitrogen fixation at temperatures of 3-5°C, which is considerably lower than in many other species (Hensley and Carpenter, 1979). With the ability
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include: Alternaria alternate, Cladosporium herbarum, Leptoxyphium fumagu, Oidium spp. and Phyllosticta cara-ganae (Tomoshevich, 2009). If infected on the roots, the nodules create a cylindrically-shaped infection zone, and the plant will restrict gas and liquid exchange to that area (Allen et al., 1955).
Finally, C. arborescens is also susceptible to several fungi, including Erysiphe palczewskii, a powdery mildew species found in Asia, Europe, Canada and the USA (Va-jna, 2006; Lebeda et al., 2008a,b; Tomoshevich, 2009). Powdery mildew severely affects the leaves and shoots of C. arborescens (Vajna, 2006). Other fungal species com-monly found on C. arborescens leaves include: Micros-phaera trifolii, Uromyces cytisi and Ascochyta borjomi (Lebeda et al., 2008b; Tomoshevich, 2009).
Figure 1. Distribution of the introduced species, C. arbores­cens, in North America; areas (states, provinces, territories) confirmed to contain C. arborescens are shown in gray, whereas areas from which it is not known to occur are shown in white (USDA NRCS, 2010).
ETHNOBOTANY
Medicinal applications
There is a long history of using Caragana species to treat ailments such as headaches, asthma, cough, nose­bleeds and strain-induced fatigue in East Asian folk medicine (Meng et al., 2009). Caragana arborescens has also been used to treat menoxia, fatigue, rheumatoid arth-ritis, asthenia and uterine, cervical and breast cancer. The USDA describes C. arborescens as being used medicinally for breast and uterine cancer and other female anatomy problems (Meng et al., 2009). The two main chemical classes thought to contribute to the medicinal properties of C. arborescens are flavonoids and lectins (Wang et al., 2005; Meng et al., 2009).
to initiate nitrogen fixation at such low temperatures, C. arborescens has a greater northern hardiness limit than most other studied species (Hensley and Carpenter, 1979).
Finally, C. arborescens is a prolific producer of the toxic non-protein amino acid, L-Canavanine (Rosenthal, 2001), which is an allelochemical that provides a barrier to herbivore predation and pathogen uptake. L-Canavanine is structurally similar to L-Arginine and is taken up by in-sects that cannot differentiate between the two amino acids (Rosenthal, 2001).
Flavonoids are phenolic compounds that protect plants from UV radiation and play a part in sexual reproduction (Koes et al., 2005). They are beneficial to humans be­cause they can act as anti-oxidant, anti-inflammatory, anti-cancer, anti-viral and anti-bacterial chemical compounds (Deng et al., 1997; Meng et al., 2009). The flavonoid found in C. arborescens is isoquercetin, which possesses hypoglycemic properties in vitro and has potential as an anti-diabetic agent (Meng et al., 2009).
Enemies
Despite being tolerant of harsh environmental condi­tions, C. arborescens is susceptible to various herbivores, allelopathic chemicals, pathogens and fungi. Grasshoppers, birds, beetles, moths and deer are known herbivores of C. arborescens (Rosenthal, 2001; Henderson and Chapman, 2006). Two species of aphids, Therioaphis tenera and Aphis craccivora, have been found living on many species of Caragana, and have previously been recorded with C. arborescens (Ripka, 2004). An energy trade-off has been identified for C. arborescens between growth of defensive spines and reproduction, which may initiate the growth of grazing-resistant individuals that are shorter, have smaller leaves, and produce fewer seed pods (Zhang et al., 2006).
Two types of lectins, which function to bind nitrogen-fixing bacteria to their root systems (Barondes, 1981), have been identified in C. arborescens. These lectins can be used as contraception and prophylaxis against sexually transmitted infections (STI) (Meng et al., 2009). C. arbor-escens lectins in vitro selectively eliminated cells infected with the human immunodeficiency virus (HIV) (Meng et al., 2009), suggesting their potential use in the fight against HIV and other sexually transmitted infections. Evidence to date demonstrates the potential of C. arborescens as a medicinal plant; however, more research on its efficacy in treating and eliminating specific conditions is needed.
Under short-term lab conditions, C. arborescens was susceptible to the allelopathic chemicals of the black wal­nut tree, Julans nigra (Rietveld, 1983). Seed germination of C. arborescens was inhibited by juglone, an allelopathic chemical released by the black walnut tree. However, these results were based on high concentrations of juglone, which may not be attained in nature.
Cultural uses
Caragana arborescens has been globally cultivated for soil stabilization and mine and construction site improve­ment through nitrogen fixation (Wills, 1982; Meng et al.,
Caragana arborescens is susceptible to various patho­gens. Spot-forming pathogens that have been identified
SHORTT and VAMOSI ─Biology and control of Caragana arborescens
5
2009). In North America, it has been planted extensively as shrub buffer strips and shelterbelts on farms (Henderson and Chapman, 2006; Dietz et al., 2008). More recently, it has been used in residential areas for outdoor screening and ornamental hedges (Duke, 1983; Dietz et al., 2008; S. Vamosi, pers. obs.)., Although the federal government of Canada planted C. arborescens primarily for windbreaks on farms during the 1930's, it was also used for wildlife and erosion control (Henderson and Chapman, 2006). Caragana arborescens has also been implemented in re-vegetation programs to control weed growth in forest plantings (Zolotukhin, 1980; Dietz et al., 2008).
inant native species in its absence. Of the sixteen species considered, only four displayed significant neighbour as­sociations with C. arborescens, suggesting the difficulty of making sweeping generalization about its effects on native plants. However, several aspects of its biology show its considerable potential to negatively impact ecosystems.
Because the adult plants reach heights of 3-5 m, they may decrease light availability to native plants (Henderson and Chapman, 2006). Caragana arborescens has a long life history strategy, maturing at 10 years of age and living up to 90 years of age as a canopy dominant (Henderson and Chapman, 2006). Populations have large numbers of recruits per year, taking advantage of forest canopy gaps. Henderson and Chapman (2006) noted that C. arborescens may reach these gaps before many native canopy domin­ant plants (e.g., Populus tremuloides, Populus balsamif-era, Salix spp.). Finally, its long leaf-out period may lead to high recruitment rates and increased survival through increased intake of solar energy compared to many native species (Henderson and Chapman, 2006).
The pods and seeds of Caragana arborescens are edible and are cultivated as a vegetable (Meng et al., 2009). The plant is used as nutritional livestock forage and in the arc­tic/subarctic it is used as fodder for reindeer herds (Duke, 1983). Other uses include fuel for burning, fibre produc­tion and the production of an azure dye (Meng et al., 2009; USDA NRCS, 2010).
EFFECTS
Additionally, Caragana arborescens secretes phenolic compounds into the soil, which inhibit the growth and germination of native plants and are highly toxic to micro­organisms (Zolotukin, 1980; Whitehead et al., 1982). Phe­nolic compounds influence many physiological processes, including nutrient uptake, protein synthesis, respiration, photosynthesis, and membrane permeability (Reigosa et al., 1999). Nitrogen fixation also alters local soil character­istics, potentially altering normal successional pathways.
Colonization and spread in exotic habitats
In its native range, Caragana arborescens is distributed from China to southern Russia (Zhang, 2005; Dietz et al., 2008; USDA NRCS, 2010). It was first introduced into Europe during the mid-eighteenth century where it was cultivated for ornamental and hedgerow purposes (Moore, 1968). Caragana arborescens has spread extensively in North America since its introduction in 1752, and now oc-cupies 21 states in the United States of America, and nine provinces and territories in Canada (Figure 1). Since its introduction, it has been able to spread from shelterbelt plantings on farms to natural areas adjacent to it, invad-ing the natural forests of North America (Henderson and Chapman, 2006). Records show that 50 planted individu-als were able to grow to a population of approximately 60,000 plants over 75 years in the Great Plains of Canada, indicating a rapid growth rate and the risk of extensive colonization and displacement of native species in North America (Henderson and Chapman, 2006).
Perhaps not surprisingly, some animals have already altered their behaviour in the presence of C. arborescens. For example, C. arborescens is visited by various bumble­bee species for food and is an especially good plant for queens initiating nest building (Alanen, 2008). In the Northern Great Plains of Canada, C. arborescens shelter-belts have become an important nesting site for the Com­mon Grackle (Quiscalus quiscula) (Yahner, 1982; Homan et al., 1996). Whether such activities by these and other species will help or hinder the spread of C. arborescens over time remains an open question.
Potential invasiveness
CONTROL METHODS
Is C. arborescens a problem species in North America? At present, it is considered invasive only in three loca-tions: Minnesota, Manitoba and Alberta (Martine et al., 2008; USDA NRCS, 2010). According to Rice (2010), however, C. arborescens is not presently considered nox-ious anywhere within North America.
Physical control
Physical removal can involve: mowing, hand-pulling, stabbing, soil solarisation, burning, bull-dozing surface material to remove root crowns from the soil, cutting, gird­ling, flooding or mulching (Hobbs and Humphries, 1995; Heiligmann, 1997; Tu et al., 2001; Shafroth et al., 2005; Meloche and Murphy, 2006; Delanoy and Archibold, 2007; White, 2007). Physical removal is likely the most environmentally friendly method, but it is labour intensive (Hobbs and Humphries, 1995). Burning is effective for killing seedlings and can also top-kill adult shrubs. How­ever, it must be repeated annually or biennially over sever­al years (Delanoy and Archibold, 2007). There have been combinations of hand pulling and mulching, which results
In what appears to be a unique study of invasiveness potential, Henderson and Chapman (2006) studied the ef­fects of a non-native C. arborescens population on native shrub species in Elk Island National Park, Canada. Native shrub diversity, but not species richness, was affected by C. arborescens density. The diversity of native shrubs was highest at intermediate C. arborescens density, which was correlated with a displacement of Corylus cornuta, a dom-
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in a reduced juvenile population because light penetration to the seedlings is reduced (Meloche and Murphy, 2006, Tu et al., 2001). This control technique is only effective over a one-year span because the mulch eventually gets dispersed (Meloche and Murphy, 2006). If implemented, hand-pulling and mulching need to be repeated each year to be effective (Meloche and Murphy, 2006). In addition, there are problems with the effect of mulching stunting the growth of native plant species (Tu et al., 2001). The least effective physical control method is the cut-stump method with no application of herbicide (Meloche and Murphy, 2006). On its own, this method leaves open canopy areas, which may promote regrowth and seed germination, increasing the overall number of juveniles present and worsening the problem (Meloche and Murphy, 2006; Del-anoy and Archibold, 2007).
the success of only one for every six biological treatments worldwide.
CONCLUSIONS
Caragana arborescens is a prolific seed disperser with a long life history strategy (Martine et al., 2008). Further­more, C. arborescens is protected by tolerance to drought, cold, salinity and infertile soils (Zolotukhin, 1980; Dietz et al., 2008; USDA NRCS, 2010). Together, these character­istics result in the potential for high recruitment rates and spread across landscapes, including habitats marginal or inhospitable to native plant species. Therefore, the poten­tial exists for various forests of Canada and the USA to be rapidly invaded by C. arborescens once small populations have established (e.g., Henderson and Chapman, 2006).
Chemical control
Our literature review reveals that C. arborescens has various positive characteristics and applications, including some directly related to human welfare, although the in­tentional farming of this species for such purposes should likely be restricted to its native range. Caragana abore-scens may have the ability to severely impact forests; how­ever, very little research addressing these issues has been conducted in North America. Future studies should focus on (i) the potential of C. arborescens to invade and/or alter forests of North America, (ii) whether their phenolic com­pounds affect native flora, and (iii) how to effectively con­trol populations of C. arborescens in North America if it becomes invasive. Thus far, herbicides appear to be more effective and carry a lower financial cost than physical removal. However, many native species are susceptible to herbicides; thus, their environmental costs should be taken into account (Henderson and Chapman, 2006). Overall, much remains to be learned about the effects, both positive and negative, of this exotic member of North America's flora.
Chemical control treatments involve herbicide ap­plication and are the most common form of invasive and noxious plant control (Hobbs and Humphries, 1995). Her­bicides vary by effectiveness and application methods, but can typically be applied more easily to larger areas than can physical control treatments (Hobbs and Humphries, 1995). A variety of commercial (e.g., GarlonTM 4) and traditional (e.g., vinegar- and clove oil-based) herbicides have been applied in shrub control programs (e.g., Sha-froth et al., 2005; Meloche and Murphy, 2006; Tyler et al., 2006; Delanoy and Archibold, 2007; Heiligmann, 1997; Chirillo, 2008). Although herbicides have largely been applied to other invasive plant species to date (e.g., buckthorn, Rhamnus cathartica L.), one potentially effec-tive strategy is a combination of stump cutting or girdling followed by the immediate application of herbicides. Un-fortunately, there are challenges to using environmentally friendly traditional methods as alternatives to commercial herbicides because of safety concerns for people working with vinegar and clove oil (Chirillo, 2008). We do note that Captan and Thiram should not be used on C. arbore-scens because they have been found to increase germina-tion by inhibiting seed-borne diseases (Dietz et al., 2008).
Acknowledgements. We thank Jill Luka and the City of Calgary for the opportunity to review the biology of C. arborescens and to be involved with the Caragana Control Trials Project. Research in the laboratory of SMV is sup­ported by grants from the Natural Sciences and Engineer­ing Research Council of Canada (NSERC).
Biological control
Biological control treatments involve identifying her­bivores, seed predators and/or pathogens that appear to control the exotic plant population in its native range, and introducing them to the invaded range the exotic plant now occupies (Shafroth et al., 2005; Hobbs and Humphries, 1995). Biological control treatments are a lengthy and complicated process requiring extensive research on the impact the predator or pathogen will have on the environ­ment it is introduced to (Hobbs and Humphries, 1995). We are unaware of any effective biological control methods for C. arborescens. Although they may sound preferable to other control methods, especially chemical control, they can be risky in terms of becoming harmful to native species. Additionally, they have historically had a low suc­cess rate, with Hobbs and Humphries (1995) estimating
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雜草型樹錦雞兒(Caragana arborescens)之生物學綜論，特別
著重其在北美洲的潛在影響
Katelyn B. SHORTT and Steven M. VAMOSI
Department of Biological Sciences, University of Calgary, Calgary, Canada T2N 1N4
引進外來物種及其定居咸認爲是包括被子植物在內的原生生物多樣性的一大威脅。原產歐亞大陸的
豆科植物「樹錦雞兒」於1700年代中葉引進北美洲，歷經250年這種植物幾乎已定居於加拿大全國和
美國半數的州；但有關它對於原生生態系潛在影響的文獻卻不多見。爲呼應加拿大卡加利市2009年倡
導的控制樹錦雞兒試驗計劃，我們就樹錦雞兒的生物學、民族植物學、對生態系之影響、與可能的控制
的方法進行綜論，並提供證據表明樹錦雞兒對於生態的正面及負面影響及其利用。値得注意的是，持續
的棲地返化與氣候變遷將促進樹錦雞兒在更多的地區成爲入侵性的惡劣雜草。我們必需特別關注樹錦雞
兒的全部分布區域，尤其是受到人爲活動干擾的破碎棲地、新近毀林地區以及濕地。
關鍵詞：生物控制；樹錦雞兒；生態系；入侵種。