The influence of Tamarix on soil chemical, biological, physical properties and nutrient availability is largely unknown. In South Africa, Tamarix usneoides is indigenous specie and it is unclear on how and when the alien Tamarix ramosissima and Tamarix chinensis species were introduced (Marlin et al., 2017). Weiersbye et al., (2006) reported that the native T. usneoides species are of importance in the gold mines in South Africa due to their ability to hyper-accumulate sulphate and metals from Acid Mine Drainage and in the process excrete gypsum. Invasive T. ramosissima and T. chinensis morphological similarities, spreading capacity and their ability to hybridize make them the most dangerous invaders in South Africa. Their morphologically similarities makes it difficult to separate these two Tamarix species (Crins, 1989). The soils in areas covered by this invasive species are altered mainly through nitrogen fixation, litter production (Musil & Midgley 1990; Witkowski 1991; Musil 1993; Yelenik et al. 2004) and altered the soil microbial abundance and functioning (Cris”stomo et al. 2013).These shrubs are documented to tolerate an extreme range of environmental conditions, including from drought to inundation, and highly saline soils making these species to out compete other species growing in the same vicinity.
Their high resilience of Tamarix species to soluble and saline soils, their ability to translocate them into their shoot systems where they are excreted through their characteristic leaf salt glands which eventually get deposited into the soil below the crown either as dripping excretes or as annual defoliations are believed to hinder the germination and growth of other salt sensitive local plants species in the vicinity (Decker, 1961; Glenn et al., 1998). Vegetative growth of Tamarix is species is accelerated under arid and semi-arid climatic conditions (Yong et al., 2012) Assumptions including early recovery of soil from negative impacts of non-native species after management procedures are implemented are often expected (Wittenberg & Cock 2005). However, is it not always as easy as such when to restore the invasive species footprints in the soil such as salinity issues that might persist longer even after invasion is brought under control (Corbin & D’Antonio 2004; Marchante et al. 2009) hence making the clearance of invasive species to be unlike that of plants (Magadlela & Mdzeke, 2004).
The wide spread of invasive species and their economic impact is well documented (Mack et al., 2000; Pimentel, 2011; Jackson, 2015).they are in fact considered as the second major threat of the ecosystem and biodiversity after a direct destruction (Schei, 1996). This is facilitated by the increased global trade and tourism that involves unprecedented movements of cargoes and people, respectively with relatively easier and fasters mode of transportations than ever (Richardson et al., 2011). Invasive species are disease tolerant and have the ability to resist environmental uncertainties such as veld fires and drought and have improved ability of using limited resources effectively (Keane, 2002; Funk, 2013). Among this is the exotic Tamarix species with physiological traits adaptations that makes it well suited to survive under natural or modified ecosystems such as the salt glands that excrete excess salts from saline ground water taken up by the roots (DeLoach et al., 2000). Baum (1978) described Tamarix species as deciduous trees or shrubs, normally deep rooted with a tap root of 30m deep, lateral roots 50m long and with a pink attractive colour.
Tamarix species are widely spread all over the world, in USA (Baum, 1967) listed eight naturalised Tamarix species (T. africana, T. aphylla, T. aralensis, T. canariensis, T. chinensis, T. gallica, T. parviflora and T. ramosissima). South Africa has a native Tamarix species (T. usneoides) (Heywood et al., 2007). However, Henderson, (2001) reported that T. ramosissima has been documented as the key invaders in South Africa, and Mayonde et al., (2016) concluded that in South Africa both exotic and indigenous Tamarix species hybrides according to the information given by molecular phylogenetic analysis. Molecular phylogenetic has established that among the Tamarix species T. aphylla and T. usneoides are more similar to each other as compared to T. ramosissima (Gaskin et al., 2004).
Invasive species do not only alter soil biological, physical and chemical properties, but also pose a significant reduction in South Africa’s freshwater and wetland ecosystems (Richardson et al., 1997). Gibbons et al. (2017) reported that an increase in soil pH and nitrate levels under invasive species. There is much evidence that invasive plant species can modify physical or chemical attributes of soil, including inputs and cycling of nitrogen and other elements (Ehrenfeld, 2003; Haubensak et al., 2004; Hawkes et al., 2005), pH (Kourtev, 2002), and soil organic matter and aggregation (Saggar et al., 1999). Moreover, there is a direct alteration of soil chemical properties through degradation of leaf litter deposited into the soil high in nutrients (Weidenhamer and Callaway, 2010) especially sodium and chlorine which can increase soil salinity. According to (Berry,1970) wide range of salts and nutrients were observed on Tamarix leaf exudates, which includes: Al, B, Ca,Cl,Na,Mg,HCO3, NO3 and SO4. Direct alteration of soil chemistry is also via exudation drip, rinsing and mobilization of water soluble salts off the Tamarix foliage by rainwater.
Busch & Smith (1995) demonstrated that Tamarix species normally dominates along the river banks, with saline soils and groundwater. Invasive species are favoured by soil with high Ca2+/Mg2+ (Armstrong & Huenneke, 1992), invasive grass species dominates where these cations ratios were increased. There is contradiction in literature about the increase of salts in the soil by invasive Tamarix species, as to whether Tamarix species increases salts in the soil or Tamarix species are favoured by saline soils. Lesica and DeLuca (2004) noted that there was a significant difference between soils beneath mature Tamarix canopies and soil in areas covered by native grassland in Montana, USA for example soils beneath matured Tamarix canopies had significantly high electrical conductivity, organic N and P, and a decrease in K concentration and soil pH.
Other studies concluded that high salinity in Tamarix habitats is derived from cultural practices or geologic factors whereas (Vandersande et al., 2001) stated that Tamarix species are highly tolerant to high salinity. However, physiological response of T.ramosissima to salinity was tested using salinity treatment of 40 000 ppm which is regarded as the endpoint, T.ramosissima managed to acclimate to this extreme salinity for approximately over 35 days, but eventually died between 60 to 75 days after induction of the treatment.
Studies conducted (Brotherson and Field, 1987; Brotherson and Winkel, 1986; Everitt, 1980) have shown that saturated soils and sunlight are the major requirement for successful Tamarix seedling emergency and can grow well in soils with high organic matter such as clay and loamy soils , intermediate moisture and high water tables (Brotherson and Field, 1987).
Tamarix species are widely distributed, their distribution and abundance of riparian vegetation is driven by high flows and fluvial disturbance regimes; low flows, alluvial groundwater conditions, and water availability; soil texture; soil and aquifer salinity; and fire regimes. T.ramosissima and T. chinensis invasive species in South Africa (Marlin et al., 2017) are native to temperate Asia distributed from Turkey to Korea, and China, Korea and Japan (Baum, 1978) respectively. Among all the nine provinces in South Africa, Tamarix invasive species is highly distributed in the Western, Northern and Eastern Cape Provinces.
Current studies have shown that (Scout et al., 2001; Ehrenfeld, 2004; Wolfe and Klironomos, 2005; Eppstein and Molofsky, 2007) the interaction between invasive plants and soil can make soil more conducive for alien plants and less conducive for indigenous plants. Controlling Tamarix species would be helpful, as their continuous spread would hinder the growth of crops and exploit resources that are of importance to human and livestock such as water and native riparian plant more palatable to livestock. Their impact on the alteration of soil properties can have a negative impact on soil food web (Duda et al., 2003) hence affecting soil decomposition processes and also leading to soil destruction. The spread of invaders should be controlled, (van der Putten et al., 2013) reported that even without direct competition from invaders, restoration of native plants is difficult from a soil that once supported the growth of invaders.
The footprints left by invasive Tamarix species, even after management interventions such as control or eradication are implemented can results in secondary invasion (Yelenik et al. 2004; Malcolm et al. 2008; Gonzalez-Munoz et al. 2012) meaning that the soil will be left highly susceptible to invasion; secondly the soil will prevent native species restoration and re-establishment (Maron & Jefferies 1999) and thirdly will make the soil more favourable to an alternative (e.g other happhytes) specie, which is difficult to completely restore to native species conditions (Marchante et al. 2008; Suding et al. 2013; Gaertner et al. 2014). Implementations of management procedures for specific invasive species does not solve plant invasion problems nor completely restores the natural ecosystem conditions, but optimise invasion.
Tamarisk (Tamarix species), also known as salt cedar is a tall multi-branched riparian, feathery tree or shrub that can grow to a height >12 m (Brotherson and Field 1987) and their common name saltcedar comes from their tolerance to saline soils. Tamarix species can tolerate high salt concentrations, ranging from 650 to 36000 ppm (Brotherson and Winkel, 1986).They have a broad and bushy shape with numerous large basal branches and a deep and extensive root system. The primary root can grow to a depth >30 m (Baum 1978), and the roots can spread up to 50 m horizontally after reaching the water table and also have an ability to produce adventitious buds (DiTomaso,1996). The leaves are scale-like, 1.5 to 3.5 mm long, with salt-secreting glands (Baum, 1978). The hermaphrodite flowers which are pink or white in colour can be pollinated by insects and wind (Brotherson and Field, 1987). Saltcedar generally flowers in its third year of development or later, however they may flower during their first year (FEIS,1996). The flowers are abundant between April and August, but may be found any time of the year in desert areas. Their petals may be persistent or fall soon after blossoming.
Tamarix, a genus that is native in Europe and Asia (Baum, 1978), is an invasive species in America and Oceania (Williams and West 2000, Lesica and Miles 2004, Natale et al. 2010, Pattison et al. 2011) and is represented by 55 species around the world (Heywood et al., 2007). Several species of Tamarix have spread to other continents and regions by anthropogenic means for shade, erosion control, and as an ornamental in the 1800s (Pearce and Smith 2003). New world countries where it has invaded and become naturalized include Argentina (Gaskin and Schaal 2003), Australia (Griffin et al. 1989), Canada (Alberta Agricultural and Rural Development [AARD] 2008), Mexico (Glenn and Nagler 2005), South Africa (CARA 2001; Henderson 2001 ), and the United States (Robinson 1965; Gaskin and Schaal,2003). There are three Tamarix species that are known to occur in South Africa; T. usneoides which is the only species indigenous to southern Africa (Obermeyer 1976). Tamarix ramosissima and T. chinensis which are both exotic and have been reportedly invasive (Henderson 2001; Bredenkamp 2003). Tamarix ramosissima and T. chinensis are introduced shrubs that have established along North American waterways from Mexico to Montana (Robinson, 1965). Tamarix invasions have contributed to the reduction of native riparian habitats in the southwestern United States by replacing native species and desiccating essential wetlands (Horton, 1977; Howe and Knopf, 1991; Busch and Smith, 1995).
Nagler et al., (2011) noted that there has been a great deal of ecological research conducted on saltcedar over the past decade. However early reviews regarded these plants as uniformly invasive, and remarks on their distribution emphasized the speed with which they spread, with regards to the observation that they outcompeted and replaced native vegetation. Saltcedar increased in area from 4,000 ha in the 1920s (Neil, 1985) to 360,000 ha in 1965 (Robinson,1965) and to 600,000 ha in 1987 (Brotherson and Field, 1987), representing a range expansion of 3’4% per year, at rates up to 20 km per river reach per year (Graf, 1978).
Many areas invaded by T. ramosissima species such as rivers in western U.S, the native riparian trees in that vicinity such as cottonwood (Populus spp.), willow (Salix spp.) and mesquite (Prosopis spp.) on floodplains (DiTomosa, 1998; Glenn and Nagler, 2005; Friedman et al., 2005) has been partially replaced. The decrease in number of native riparian phreatophytes due to high invasion of tamarisk has contributed to the improvement of water management that affect natural waterways, change channel morphology and pumping of ground water for urban, agricultural, and industrial uses (DiTomaso 1998; Shafroth et al. 1998; Lite and Stromberg 2005).
Tamarix trees has a major effect on the alteration and degradation of many riparian ecosystems by consuming large quantities of water, reducing the width of river channels, salinizing soil, and modifying wildlife habitats (Zavaleta et al., 2001). In South Africa, invasive Tamarix use up to twice as much water as other reference species (Le Maitre et al. 2013). Tamarix trees are referred to as phreatophytic species as they are deep-rooted plant that can obtain a significant portion of the water from the water table, however even its dense and copious fine roots can draw moisture from the entire soil profile (Nippert et al. 2010).
Tamarix species are associated with exploitation of water resources and high evapotranspiration rates. Evapotranspiration (ET) is regarded as a major source of water depletion from riverine systems in arid and semiarid climates. Evapotranspiration (ET) by riparian vegetation (Tamarix species) is a significant factor of the water budget of arid and semiarid watersheds (Goodrich et al., 2000; Scott et al., 2000; Dahm et al., 2002). There are uncertainties on the amount of water used by riparian vegetation (Drexler et al., 2004; Unland et al., 1998), including the saltcedar. However, several studies engaging eddy covariance methods show that Tamarix ET is not greater than riparian stands dominated by native woody taxa (Weeks et al. 1987, Cleverly et al. 2002, Dahm et al. 2002). This means that Tamarix species are not the riparian vegetation with highest ET. Total annual ET of Tamarix stands ranges from 74 cm to 122 cm in an unflooded and flooded stand respectively (Cleverly et al. 2002).
Soil properties usually exhibit spatial heterogeneity in many ecosystems (Abril et al., 2009, Allington and Valone 2013) especially in arid and semi-arid regions because soil nutrient resources are more abundant under the canopies of shrubs or trees than in soils of open land as a result of patchy distribution of vegetation (Gallardo et al., 2000). Spatial heterogeneity of soil resources due to patchy distribution of shrubs or trees has been described as the creation of resource islands or fertile islands (Daryanto et al. 2012). Resource islands have been shown to exert significant effects on species diversity, abundance, vegetation dynamics, and community structure (Anderson et al. 2004) by changing the soil physio-chemical properties under canopies (Wezel et al., 2000, Canton et al., 2004, Yang et al., 2011).
The conceivable impacts of Tamarix on soil physical and chemical properties and the way it interacts with different plants is still debateable (Liu et al., 2017). A few specialists speculates that Tamarix discharges salt, resulting in an increase in salt content, electrical conductivity, and pH of the soil under the leaf conopy (Ladenburger et al. 2006, Yin et al. 2010, Su et al. 2012). These alterations encourage soil hardening and salinization (Zhang et al. 2002), hinder the development of non-halophytes, cause a decrease in biodiversity, and alter plant community (Zhang et al. 2002, Bateman et al. 2015). Other studies contend that Tamarix does not cause an increase in soil salt content under the canopy (Bagstad et al. 2006, Li et al. 2007), rather it may bring about an increase soil nutrients levels and by doing so exert a fertile islands effect (Lesica and DeLuca 2004, Bagstad et al. 2006, Yin et al. 2010), which may or may not influence local biodiversity (Riper et al. 2008, Johnson et al. 2010, Brand and Noon 2011, Lehnhoff et al. 2012).