3.1. Ecological impact
It is important to establish an understanding of ecological effects of bush encroachment on rangeland ecosystems prior to embarking on any bush encroachment intervention. Thus, the degree of invasion should be quantified to help justify the need for, and determine the type of intervention. It is fundamental to characterise invasion and these could be in terms of identification of invading species (morphology, phenology, anatomy, physiology, mode of spread), plant population density, spatial localization (along the landscape, vegetation types, soil type, water distribution), seasonal distribution, their impact on the ecosystem stability (soil cover and biodiversity) and productivity (primary and secondary). The global reviews of plant invasions suggest that the most damaging species transform ecosystems by using excessive amounts of resources, notably, water, light, and oxygen. Invading species achieve these by adding resources such as nitrogen, promoting or suppressing fire, stabilising sand movement, and/or promoting erosion, accumulating litter and accumulating or redistributing salt . Such changes potentially alter the flow, availability, or quality of nutrient resources in biogeochemical cycles. They further modify tropic resources within the food web and alter physical resources such as living space or habitat, sediment, light and water. In addition, invaders are most likely to have substantial effects on ecosystems by rapidly changing the disturbance regime . Thus, dense stands of alien trees and shrubs in rangelands can rapidly reduce abundance and diversity of native plants .
Different invading species have similar or specific effects on rangeland ecosystem dynamics. Thus, invasion of black wattle (Acacia mearnsii) in South African rangeland ecosystems has negative ecological impacts . These impacts include reduction of surface stream flow, loss of biodiversity, increase in fire hazard, and increases in soil erosion, destabilisation of riverbanks, and loss of recreational opportunities, aesthetic costs, and nitrogen pollution and subsequently loss of grazing potential. An increase in the height and biomass of vegetation increase rainfall interception and transpiration, and decreases stream flow . Alien trees and shrubs increase above ground biomass and evapotranspiration and thereby decrease both surface water runoff and ground water recharge . The reduction of surface water runoff as a result of current invasions was estimated to be 3 300 mm3, which is about 7% of the national total , most of which is coming from the fynbos and grassland biomes . The increased biomass and evapotranspiration rates associated with invasive alien plants arise because of their greater height, root depth, and senescence, compared to the native species that they replace . Invasive plants may influence native ecosystems by exerting resource competition on native plants to altering fire dynamics . Thus, the increased biomass that accompanies plant invasions also result in more intense fires [8, 36, 70] due to an accumulation of fuel loads. On the other hand, the dense stands of invasive trees hamper access for fire management purposes , which makes it difficult for fire control in rangelands. The increase in fire intensity due to accumulation of sufficient fuel load subsequently damages vegetation and soil , which in turn leads to excessive soil erosion due to soil water repellency caused by fire .
Therefore, it suffices to indicate that the alien invasive plants reduce the functional capacity of rangeland ecosystems such as support for livestock and wildlife [36, 70]. This is among others due to competition between invasive plants and grasses that are important for grazing. This competition leads to reduction on performance of a number of ecosystem functions such as grass cover, which subsequently contributes to loss of grazing potential . There is also a significant loss of biodiversity due to competition , resulting from the displacement of species-rich indigenous plant communities by single-species stands, and disruption of important ecosystem processes . On the other hand, invasion of riverbanks causes deep channelling followed by slumping during floods and that result in destabilized riverbanks. Subsequently, the invasion along the riverbanks leads to loss of recreational opportunities due to reduction of access for anglers, canoeists, white-water rafters, and swimmers. Invasive plants further detract from the wilderness character of many rural landscapes and conservation areas and that imposes reduction of the aesthetic value of ecosystems. An increase in soil nitrogen levels in nutrient-poor environments can make habitats unsuitable for indigenous plants and more susceptible to invasion by other species, and, in turn, reducing biodiversity.
In order to develop the effective invasion control in rangelands, it is significant to understand the mechanisms that are employed by the invader species to survive and colonise the new ecosystems. There are a number of ways through which invasive plants survive and outcompete the indigenous species in rangelands; one of the mechanisms is their ability to grow rapidly compared to indigenous plants. Thus, invasive alien plants typically grow more rapidly, often increasing the proportion of biomass contributed by alien plants. The large biomass contributed by invasive plants is composed of leaves, bark, seed, flowers, and twigs that become ‘terrestrial litter’ after abscission . Such litter enters and is retained in water bodies where its rate of breakdown by invertebrate feeding as well as decomposition through fungal and bacterial activity differs from that of inputs from indigenous plants . The often large differences in litter inputs from invasive alien plants relative to indigenous species leads to reduced decomposition rate and dramatically alters the nutrient cycle in rangeland ecosystem . Additions in the biomass contributed by alien plants can increase the amount of metabolised nutrients, which in turn escalates natural eutrophication processes  as well as free-floating and rooted aquatic macrophyte invasions . Thus, eutrophication leads to gradual changes in the plant and animal populations and the development of potentially toxic algal blooms and, therefore, a slow decline in water and habitat quality . The level of impact that litter from invasive alien plants has on nutrient cycles is determined by vegetative spread, plant structure, phenology, plant water and nutrient uptake efficiency, photosynthesis type, presence of symbionts and nitrogen fixation, phosphorus content and tissue chemistry such as allelopathy .
|Scientific name||Common name||Impact||Weediness||Biocontrol||% Weedy relatives||Combined Score||CARA category|
|Large–large||Bromus diandrus||Ripgut brome||0||2||10||5||53|
|Pinus taeda||Loblolly pine||10||1||10||4||87||2|
|Tecoma stans||Yellow bells||5||1||10||3||69||1|
|Tipuana tipu||Tipu tree||5||1||10||10||73||3|
|Large–moderate||Celtis sinensis/||Chinese nettle tree/|
|Celtis occidentalis/||Common hackberry/|
|Celtis australis||European hackberry||0||1||10||1||45||Proposed|
|Cytisus scoparius||Scotch broom||5||5||10||4||86||1|
|Pennisetum purpureum||Elephant grass||10||3||10||2||95||Proposed|
|Toona ciliata||Toon tree||5||1||10||2||64||3|
|Ulex europaeus||European gorse||5||5||10||1||80||1|
|Large–small||Acacia paradoxa||Kangaroo thorn||5||2||10||3||69||1|
|Pueraria lobata||Kudzu vine||5||3||10||5||76||1|
|Moderate–large||Acacia elata||Peppertree wattle||5||2||10||3||69||3|
|Acacia podalyriifolia||Pearl acacia||5||1||10||3||67||3|
|Ardisia crenata||Coralberry tree||5||1||10||0||66||1|
|Cinnamomum camphora||Camphor tree||10||2||10||0||90||1/3|
|Cotoneaster franchetii||Orange cotoneaster||5||2||10||1||69||3|
|Cotoneaster pannosus||Silver-leaf cotoneaster||5||2||10||1||69||3|
|Eucalyptus cladocalyx||Sugar gum||5||1||10||2||68||2|
|Eucalyptus saligna||Saligna gum||5||1||10||2||66|
|Eugenia uniflora||Surinam cherry||5||2||10||0||68||1|
|Hedychium coronarium||White ginger lily||10||2||10||1||87||1|
|Hedychium gardnerianum||Kahili ginger lily||10||3||10||1||92||1|
|Ligustrum japonicum||Japanese wax-leaved privet||5||1||10||3||66||3|
|Ligustrum lucidum||Chinese wax-leaved privet||5||4||10||3||78||3|
|Ligustrum ovalifolium||Californian privet||5||1||10||3||68||3|
|Ligustrum sinense||Chinese privet||5||4||10||3||80||3|
|Lonicera japonica||Japanese honeysuckle||5||6||10||1||83||Proposed|
|Myoporum tenuifolium ssp. montanum||Manatoka||5||0||10||2||69|
|Nephrolepis exaltata||Sword fern||10||0||10||3||82||1|
|Pyracantha coccinea||Red firethorn||5||0||10||8||61|
|Spartium junceum||Spanish broom||5||3||10||10||82||1|
|Syzygium paniculatum||Australian water pear||5||0||10||0||61|
|Moderate–moderate||Albizia procera||False lebbeck||5||1||10||2||64||1|
|Alhagi maurorum||Camelthorn bush||5||2||10||10||79||11|
|Anacardium occidentale||Cashew nut||5||1||10||1||63|
|Catharanthus roseus||Madagascar periwinkle||0||2||10||3||51|
|Cestrum parqui||Chilean cestrum||10||3||10||1||91||1|
|Cynodon nlemfuensis||East African couch||5||2||10||10||76|
|Cytisus monspessulanus||Montpellier broom||5||0||10||4||66||1|
|Gleditsia triacanthos||Honey locust||5||2||10||1||68||2|
|Montanoa hibiscifolia||Tree daisy||0||1||10||1||44|
|Passiflora edulis||Passion fruit||0||2||10||1||50||1|
|Physalis peruviana||Cape gooseberry||0||2||10||5||54|
|Phytolacca octandra||Forest inkberry||0||2||10||6||55|
|Pyracantha crenulata||Himalayan firethorn||5||1||10||8||73||3|
|Senna bicapsularis||Rambling cassia||5||0||10||1||62||3|
|Senna pendula var. glabrata||Rambling cassia||5||2||10||1||68||3|
|Sesbania bispinosa var. bispinosa||Spiny sesbania||0||0||10||4||45|
|Sophora japonica||Japanese pagoda tree||0||0||10||2||42|
|Syzygium jambos||Rose apple||5||1||10||0||66||3|
|Tithonia diversifolia||Mexican sunflower||0||1||10||3||48||1|
|Ulmus parvifolia||Chinese elm||0||0||10||5||46|
|Verbena brasiliensis||Slender wild verbena||0||1||10||2||45|
|Riparian–large||Canna indica||Indian shot||5||2||10||10||79||1|
|Canna x generalis||Garden canna||5||1||10||10||72|
|Cortaderia jubata||Purple Pampas||5||3||10||2||75||1|
|Cortaderia selloana||Pampas grass||5||5||10||2||81||1|
|Oenothera biennis||Evening primrose||5||1||10||4||67|
|Populus deltoides||Match poplar||Proposed|
|Mimosa pigra||Giant sensitive plant||5||4||10||1||76||3|
|Myriophyllum spicatum||Spiked water-milfoil||5||4||10||3||80||1|
|Oenothera glazioviana||Evening primrose||5||2||10||4||72|
|Oenothera indecora||Evening primrose||5||1||10||4||68|
|Oenothera jamesii||Giant evening primrose||5||0||10||4||64|
|Oenothera laciniata||Cutleaf evening primrose||5||1||10||4||67|
|Oenothera tetraptera||White evening primrose||5||0||10||4||66|
|Parkinsonia aculeata||Jerusalem thorn||5||1||10||0||66|
|Small–large||Alpinia zerumbet||Shell ginger||5||0||10||0||62|
|Grevillea robusta||Australian silky oak||5||2||10||0||67||3|
|Quercus robur||English oak||5||1||10||1||67|
Emerging invaders grouped according to categories (Source: )
N. B: Scores for ‘Impact’, ‘Weediness’, Biocontrol’ and ‘Weedy relatives’ are standardized by dividing the maximum score for that criterion and multiplying by 10. Scores for these four criteria were weighted, with ‘Impact’, ‘Weediness’ and Biocontrol’ receiving an equal weighting of four, and ‘Weedy relatives’ receiving a lower weighting of one. The weighted criteria were summed to obtain the ‘Combined score’ for each species. ‘CARA category’ lists the species regulated by the Conservation of Agricultural Resources Act (Act 43 of 1983), where 1 refers to Category 1 prohibited weeds that must be controlled in all situations; 2 includes Category 2 plants with commercial value that may be planted in demarcated areas subject to a permit, provided that steps are taken to control spread; 3 includes Category 3 ornamental plants that may no longer be planted or traded, but may remain in place provided a permit is obtained and steps taken to control their spread; and ‘proposed’ includes those species that were proposed for listing under the Conservation of Agricultural Resources Act, but require further investigation before they can be included.
The majority of invasive and/or encroaching species in rangelands is dominated by the genus Acacia, which is the second largest with over 900 species . Australian acacias are important invaders of South African rangeland areas . In the fynbos ecosystems where soil nutrients are generally poor, the invasion by nitrogen-fixing acacias increases nitrogen inputs, and subsequently leads to an increase in soil fertility. Therefore, the massive increase in soil fertility permits acacia species to propagate and outcompete indigenous species . There are a number of acacia species found in rangelands and their ability to fix nitrogen has been widely reported; these include Acacia cyclops, A. dealbata, A. mearnsii and A. saligna [90, 95]. The groundwater on places that were invaded by A. saligna has shown elevated NO3- and NO2- concentrations compared to groundwater in natural ecosystems . The presence of A. saligna, as well as the nutrient leaching that occurred after its removal, result in seasonal nitrogen concentrations that are higher than the water quality targets for domestic use (NOx < 6 mg/l) [94, 96]. Therefore, the removal of alien plants would be beneficial from both a water quantity as well as water quality perspective .
In natural communities, plants compete in different ways; one of these ways is chemical interactions in the form of allelopathy [87, 97]. Invasive plants interfere with other plants by releasing allelochemicals into the environment and that negatively affects surrounding plants, thus giving the producer a competitive advantage. Invasive plants possess physiological traits that enable them to exploit ecological opportunities. The word allelopathy comes from the Latin words allelon, which means of each other and pathos, which means to suffer, which is commonly associated with the chemical inhibition of one species of plants by another . Allelopathy is the process through which invasive plants such as eucalyptus, Pinus, Chromolaena and Lantana produce biochemicals that influence the growth, survival, and reproduction of indigenous species. However, it is important to note that most of the plant species naturally produce number of allelopathic substances such as monoterpenes and phenols . Phenolics and volatile compounds can be released from eucalyptus foliage. These biochemicals can act as antibiotics in certain soils, possibly affecting nitrogen cycles.
Although it has not been evaluated, the impacts of allelochemicals may subsequently influence water quality through soil erosion or surface runoff processes . Allelochemicals are believed to be present in almost all plant tissues such as leaves, flowers, fruits, stems, roots, rhizomes, seeds, and pollen where they may be released from plants into the environment by means of volatilization, leaching, root exudation, and decomposition of plant residues [99, 100]. Invasive plants use the mechanism of allelopathy to outcompete other plants . Allelochemicals can be found present in litter and on the soil surface where plants grow. Rain assists with the leaching of allelopathic substances into the soil, where they may affect the germination and growth of other plants [97, 101]. Allelopathic substances might play a role in shaping plant community structure in semi-arid and arid environments . Thus, allelopathic substances inhibit plant growth depending on the concentration, leachability, season, and age of the plants . Phytotoxins can persist in the soil and litter layer for long after allelopathic plants senesce, thereby reducing the establishment potential of an area. Allelopathic substances can be present in the soil and often determined by a number of important factors . These factors include the density at which the leaves fall, the rate at which this material decomposes, the distance from other plants and, finally, rainfall [101, 102, 103]. Phenolics signify the main allelopathic compounds that inhibit seed germination, plant growth and other physiological processes that result in changes of floristic composition within a plant community.
Competition between plants can lead to the allelopathic inhibition of germination or growth via phytotoxic chemical releases, which are caused by competing species. However, allelopathy can be extremely difficult to demonstrate in the field due to difficulties in differentiating allelopathic effects from resource competition [87, 99]. Allelochemical compounds are in fact released into the soil and accumulate to levels of toxicity, which leads to inhibition of germination . Allelochemicals released by invasive plants may affect native plant survival and production in a number of ways. These include the modification of the soil microbiota [74, 104], and enhancement of growth of beneficial microbes in their rhizosphere leading to an establishment of positive feedbacks that can contribute to the decrease of native biodiversity . Allelochemicals are further known to inhibit absorption of ions . Other than allelopathic effects, invasive plants exert competition of resource especially through light . Therefore, allelopathy and resource competition operate simultaneously influencing each other and, in the meantime, they are influencing plant community structure .
Allelochemicals, as soon as released into the soil, may inhibit germination, shoot, and root growth of other plants, which will affect nutrient uptake thereby destroying the plant’s usable source of nutrients . Allelopathy of invasive plants delays the germination and growth of seedlings of other species and eventually hinders their growth completely. Therefore, degree of inhibition due to allelopathy is largely dependent on the concentration of the extracts and, to a lesser extent, on the species from which they were derived [101, 108]. The effects of allelopathy on germination and growth of plants occur through a variety of mechanisms including reduced mitotic activity in roots and hypocotyls, suppressed hormone activity, reduced rate of ion uptake, inhibited photosynthesis, and respiration, inhibit protein formation, decreased permeability of cell membranes and/or inhibition of enzyme action . Plants that germinate at slower rates are often smaller; thereby, this may seriously influence their chances of competing with neighbouring plants for resources such as water . Indirectly, allelopathic effects of invasive species on germination and growth of native species determine their competitive ability against them . The roots of Aloe ferox have allelopathic inhibition on tomato seed germination . Accumulation of allelochemicals in the rhizosphere because of root and microbial exudates and/or metabolism may affect the germination. However, under arid conditions germination will be less affected since microbial activities are very low due to low availability of soil moisture . The effects of allelochemicals on the root growth are due to cell division destruction . L. maackii also exudes allelopathic compounds from its leaves or roots that inhibit germination and growth of species that grow on the same site . Allelochemicals could be found on any part of the plant; however, the concentration varies with plant parts. The leaf extracts of L. maackii appeared to have a more negative effect on seed germination than root extracts . Generally, leaf extract concentrations have a stronger effect on germination of seeds of other plants . However, it is important to note that allelopathic chemicals from one plant can hinder germination of seeds of the same plant. For example, chenopod seed germination can also be inhibited by extracts generated from its leaves . However, all extracts, except the one obtained from the leaves of E. tomentosa significantly inhibited the germination of lettuce seed and appeared to stunt the growth of roots and shoots of germinants .
There are different allelochemicals exuded by invasive plants; these may have direct and indirect effects on germination and establishment of native species. However, phenolics are widely recognized for their allelopathic potential in plants, and can be found in a variety of tissues. Phytotoxic activity of allelochemicals in soil has been considered as plant-to-plant interaction, which is mediated by chemicals released from the plants . Indirect effects of allelochemicals include its influence on the availability of nutrients in the soil, which may cause changes in soil chemical characteristics . Allelochemicals might inhibit the growth of nitrifying bacteria, which would decrease N-availability at the plant level . Additionally, chemical compounds produced in the process of litter decomposition are inhibitory for both heterotrophic and autotrophic bacteria and fungi [110,111] and, thus, rates of mineralization may be reduced. Allelochemicals such as phenolic acids are considered to have an important influence on nutrient cycling in terrestrial ecosystems . The allelochemicals can produce some changes in the resource exploitation competition in such way that allelochemicals affect the mycorrhizae that allow the plant to absorb the nutrients, which leads to decrease in the soil productivity [106, 112]. Soil microorganisms are affected by root exudates that eventually affect other plant roots. Some chaparral species produce substances, which accumulate on the soil surface and make the soil less wettable . The allelochemicals affect availability and accumulation of inorganic ions, although their activities are influenced by ecological factors such as nutrient limitation, light regime and soil moisture deficiency .
Allelochemicals, such as phenolics and terpenoids, play an important role in the inhibition of nitrification and, thus, influence soil productivity of a plant community . Thus, any influence on nutrient dynamics may ultimately affect the growth of plants in the community, which will lead to the increase of invasive plants. Reduced soil fertility may enhance the production of allelochemicals from invasive plants . The addition of plant litter to soil may influence nutrient mobilization and soil pH, which can further influence nutrient immobilization and microbial activity . Therefore, litter can alter the chemistry of the soil in such a way that it inhibits germination of other plants . Chemicals released into the environment by a plant may not necessarily have direct effects on community structure but abiotic soil factors can influence these chemicals. Many phenolic acids have potential to influence microbial population, cause a shift in the microbial community, and eventually affect soil productivity of the area . The soil microflora is directly responsible for decomposition and mineralisation processes and soil fauna is of considerable importance in regulating these processes through influencing the growth and activity of soil microbes . Allelochemicals exuded from roots of invasive plants and residue decomposition play an important role in inhibiting plant pathogens particularly those borne in soil . However, amended soils with allelopathic residues tend to be rich in organic matter . Electrical conductivity (EC) of the amended soils increased as compared to the control and all nutrients were significantly more . Although, earlier reports show that inclusion of plant litter, in addition to releasing putative phytotoxins into the soil medium, alters the soil nutrient dynamics and, thus, affects the plant growth [106, 112, 116]. A similar increase in electrical conductivity of the soil incorporated with residues of allelopathic plants was reported . In fact, the behaviour of the allelopathic compounds present in soil remains unclear .
The modes of release of the allelopathic compounds are not specific because they vary from plant to plant . Thus, allelochemicals are released into the environment by root exudation, leaching from aboveground parts, volatilisation, and decomposition of plant material and ultimately enter into the soil [99, 110, 121]. Therefore, allelochemicals may reach other plants through transport such as root exudates into the soil and may induce the inhibitory activity on the other plants. The behaviour of allelochemicals in soil is run by the physicochemical properties including soil organic matter and organisms . The model that has assumptions such as “allelochemicals are released into the soil from living plants and degraded into non-allelopathic substances was developed. Therefore, rate of the release is proportional to the amount of allelochemicals in living plants and rate of allelochemicals degradation is proportional to the amount of allelochemicals released . However, the soil microorganisms were also reported to produce and release allelochemicals . The release of allelochemicals by mature shrubs may inhibit plant germination, survival or growth . Allelopathic content of a plant varies according to its maturity . Allelopathic compounds released from different plant parts can be either released continuously within specific periods such as specific developmental stages or influenced by external factors such as precipitation . The synthesis and exudation of allelochemicals via roots is usually enhanced by stress conditions that the plant encounters such as extreme temperature, drought, and ultraviolet exposure .
The visible effects of allelopathy frequently observed are inhibited or delayed seed germination or reduced seedling growth. The diversity of structure among allelochemicals suggests that they have no common mode of action . Plant exudates can also have an indirect effect on the surrounding environment and reduce neighbouring plant germination or growth, independent of toxicity . Allelopathic activities are more pronounced when allelopathic potential species grow under water stress . Phenolic acids that were tested had a similar mode of action such as inhibition of nutrient uptake by roots of plants . In most cases, various allelochemicals take action as growth regulators by inhibiting growth and changing development . The common mode of action of allelochemicals is quite related to the membrane destruction . It was discovered that allelochemicals affect plants on cell division, cell elongation, cell structure, cell wall, ultra-structure of the cell [112, 127]. Phenolic allelochemicals can also lead to increased cell membrane permeability; cell contents spill which lead to the increase of lipid peroxidation, and eventually, slow growth or death of plant tissue occurs [112, 126, 127]. Furthermore, nutrient uptake can be affected negatively by allelochemicals. This occurs when these allelochemicals inhibit nutrient absorption of the plant . The mode of action of benzoic acid involved the inhibition of nutrient uptake by plant roots, which resulted in growth inhibition . The radicle elongation was significantly reduced by the extract of leaves, and leaves and stem at the three concentrations of Acacia mearnsii, which signifies that A. mearnsii has allelopathic potential . The impact of allelochemicals also have been observed on the respiration of the plants which affect oxygen absorption capacity , eventually inhibit photosynthesis by reducing the chlorophyll content which affect photosynthesis rate [98, 112, 126]. There is an inhibition of the activity of hydroxyphenylpyruvate dioxygenase (HPPD) enzyme due to isoxaflutole, which results in the inhibition of meristmatic tissue, which leads to inhibition of shoot growth . Therefore, the modes of action of most allelochemicals and phytotoxins are complex and are not clearly understood .
The active compound or compounds must be isolated in an amount adequate for identification and for further characterisation in bioassays . Screening of fractions of plant extracts or leachates for their effects on seed germination of various plant species are frequently used to identify phytotoxic compounds . The identification of an active phytotoxic compound from a suspected allelopathic plant does not establish that this is the only compound involved in allelopathy. The release of allelochemicals of different chemical classes from allelopathic plant species has been documented including tannins, cyanogenic glycosides, several flavonoids and phenolic acids . The most clearly identified compounds can be divided into four groups: phenolic acids, hydroxamic acids, alkaloids, and quinones. In the study of allelopathy, plants are identified based on the allelochemical release . Most studies utilized some parts of the plants such as roots, leaves and leaves plus stem to establish the existence of allelochemicals on the identified plants [107, 128].