Invasive Acacia Research Articles

Distribution and management of Acacia implexa (Benth.) in South Africa: A suitable target for eradication?

This study is the first detailed assessment of an invasion by Acacia implexa (Benth.) (screw-pod wattle) anywhere in the world. Approximately 30000 A. implexa individuals were found spread over about 600ha (a condensed canopy area of ~96ha) in three geographically distinct populations, all in the Western Cape, South Africa. Population structures indicate rapidly increasing populations at all sites, with vegetative suckering dominating over reproduction by seed. Populations appear capable of rapidly densifying if given the opportunity, creating monocultures and even out-competing other invasive acacias. Although seed viability is high (~60%), there is relatively low recruitment from seed, likely as a result of high seed predation (with some previous estimates of up to 100% seed loss). While high seed mortality may suggest limited rates of spread (populations are at most 1km from initial plantings at two of the sites), seeds appear to be able to disperse along roads and watercourses (in particular plants are now established about 5km down the Eerste River from the putative initial planting site in Stellenbosch). This suggests that the extent of all three populations could increase much more rapidly in the future. Formal risk assessment and bioclimatic niche modelling indicate that this species has the potential to invade large parts of South Africa, particularly in coastal regions. Given the current limited distribution, the potential threats posed and the success of control to date, we consider eradication a feasible and desirable management goal. The estimated cost of clearing established stands was ~ZAR 700000, but given the strong ability of A. implexa to resprout, proper control and follow-up would be essential to prevent re-establishment of dense stands and further spread. A systematic eradication programme over the next decade will cost an estimated ZAR 1.5million, giving a total eradication cost of ZAR 2.2million. We support the proposed listing of the species as category 1a under the National Environmental Management: Biodiversity Act, and suggest that the species should not be introduced to other countries without clear and comprehensive contingency plans.

Invasive acacias experience higher ant seed removal rates at the invasion edges

Abstract. Seed dispersal is a key process for the invasion of new areas by exotic species. Introduced plants often take advantage of native generalist dispersers. Australian acacias are primarily dispersed by ants in their native range and produce seeds bearing a protein and lipid rich reward for ant mutualists (elaiosome). Nevertheless, the role of myrmecochory in the expansion of Australian acacias in European invaded areas is still not clear. We selected one European population of Acacia dealbata and another of A. longifolia and offered elaiosome-bearing and elaiosome-removed seeds to local ant communities. For each species, seeds were offered both in high-density acacia stands and in low-density invasion edges. For both acacia species, seed removal was significantly higher at the low-density edges. For A. longifolia, manual elimination of elaiosomes reduced the chance of seed removal by 80% in the low-density edges, whereas it made no difference on the high-density stands. For A. dealbata, the absence of elaiosome reduced seed removal rate by 52%, independently of the acacia density. Our data suggests that invasive acacias have found effective ant seed dispersers in Europe and that the importance of such dispersers is higher at the invasion edges.

Wood anatomical traits as a measure of plant responses to water availability: invasive Acacia mearnsii De Wild. compared with native tree species in fynbos riparian ecotones, South Africa

Riparian ecotones in the fynbos biome of South Africa are heavily invaded by woody invasive alien species, which are known to reduce water supply to downstream environments. To explore whether variation in species-specific functional traits pertaining to drought-tolerance exist, we investigated wood anatomical traits of key native riparian species and the invasive Acacia mearnsii across different water availability proxies. Wood density, vessel resistance against implosion, vessel lumen diameter and vessel wall thickness were measured. Wood density varied significantly between species, with A. mearnsii having denser wood at sites in rivers with high discharge. As higher wood density is indicative of increased drought tolerance and typical of drier sites, this counter-intuitive finding suggests that increased wood density was more closely related to midday water stress, than streamflow quantity per se. Wood density was positively correlated with vessel resistance against implosion. Higher wood density may also be evidence that A. mearnsii is more resistant against drought-induced cavitation than the studied native species. The observed plastic response of A. mearnsii anatomical traits to variable water availability indicates the ability of this species to persist under various environmental conditions. A possible non-causal relationship between wood anatomy and drought tolerance in these riparian systems is discussed.

Influence of soil microorganisms, allelopathy and soil origin on the establishment of the invasive Acacia dealbata

Background: The invasive success of Acacia dealbata is partially attributed to the release of allelopathic compounds and changes in soil characteristics. However, both mechanisms have been studied separately, which does not reflect field conditions. Aims: To test whether interactions of allelopathy, soil origin and soil microbiota affect plant germination and growth of A. dealbata. Methods: Soil sampling and collection of natural canopy leachates were conducted in Coimbra, Portugal. Germination and seedling growth were tested in sterilised and control soils from native forests and invasive acacia stands, watered either with acacia or native canopy leachates. Sapling growth was further studied in soils from native forests and invasive acacia stands. Results: No interaction between Acacia leachates and soil microbiota was detected. Leachates of A. dealbata had a negative effect on the growth of its own seedlings in native soils. Soil microbiota reduced seedling growth in native forests. Saplings of A. dealbata grew bigger in the invaded soil than in soil from native forests. Conclusions: Soil microorganisms have a positive effect on the initial establishment and growth of A. dealbata. The effect of Acacia leachates depends on the origin of the soil. Soil previously modified by A. dealbata seems to favour its own growth.

An exotic Australian Acacia fixes more N than a coexisting indigenous Acacia in a South African riparian zone

Acacia mearnsii is an introduced Australian acacia in South Africa and has invaded more than 2.5 million ha, primarily establishing in rangeland and riparian areas. Because acacias have the capability to fix N, A. mearnsii invasions may fundamentally change N dynamics in invaded systems. This study compares biological N2-fixation in the alien invasive A. mearnsii and the native A. caffra growing in a grassland riparian zone in the Komati Gorge Reserve, Mpumalanga, South Africa. A 15N natural abundance field survey suggested that both mature alien and native acacias fix N under current conditions in the riparian zone. Significantly depleted δ15N was observed in both acacias relative to reference species, although variation in δ15N was not correlated with N concentrations. Calculated contributions of N2-fixation (%Ndfa) suggest that alien acacias fix significantly more of their N than native acacias (~75 ± 5% SE and 53 ± 9% SE, respectively). There was a larger variation in δ15N and %Ndfa in the native acacia, suggesting relatively high plasticity in its N2-fixation contributions. This plasticity was interpreted as a facultative N2-fixation strategy for the native acacia, while the N2-fixation strategy of the alien acacia remained unclear. Our results emphasize the importance of potentially elevated N inputs through N2-fixation by invasive legumes in invaded landscapes. Furthermore, they suggest that N2-fixation by invasive acacias may not respond to fine-scale patchiness in soil N in the same manner as native acacias, making them potential contributors to N excess in Southern Africa.

Ecophysiological responses of three native herbs to phytotoxic potential of invasive Acacia melanoxylon R. Br.

Acacia melanoxylon R. Br. (Blackwood) is a native Australian species that has invaded woodlands and degraded natural habitats in the north western Iberian Peninsula (Galicia), Spain. Several phenolic (p-hydroxybenzoic, vanillic, p-coumaric, syringic, protocatequic, ferulic acids) and flavonoids (catechin, luteolin, rutin, apigenin, and quercetin) were identified from methanol extracts of flowers and phyllodes of A. melanoxylon by HPLC. Flowers and phyllodes of A. melanoxylon were soaked separately in the water in a ratio of 1:1 (w/v) for 24 h to prepare aqueous extracts (100, 75, 50 and 25%) and distilled water was used as control. The seeds of three native plants, cocksfoot (Dactylis glomerata), perennial ryegrass (Lolium perenne), common sorrel (Rumex acetosa) and a general test crop lettuce (Lactuca sativa) were grown in perlite culture and aqueous extracts of A. melanoxylon (flowers and phyllodes) were applied exogenously at various concentrations. A. melanoxylon flowers extract (100%) inhibited the shoot length of D. glomerata and L. perenne by 31 and 20% of the control, respectively. Leaf and root fresh weights of L. perenne, D. glomerata, and L. sativa were reduced after treatment with acacia flowers and phyllodes extract. Leaf relative water content of D. glomerata and L. perenne was reduced by acacia flowers and phyllodes extract at all concentrations. Both extracts reduced leaf osmotic potential in D. glomerata, L. perenne and L. sativa. Quantum efficiency of open PSII reaction centers (Fv/Fm) and quantum yield (ΦPSII) of photosystem II were decreased in L. perenne, D. glomerata; R. acetosa and L. sativa after treatment with acacia flowers/phyllodes extract at 100% concentration. Acacia flowers and phyllodes extract (100%) inhibited the qP level during all 6 days in D. glomerata, L. perenne, R. acetosa and L. sativa. A significant reduction in NPQ was observed during different days in all four plant species due to Acacia flowers and phyllodes extract (100%). The δ13C ratios were less negative in L. perenne, D. glomerata and L. sativa as compared to the control. A. melanoxylon flowers and phyllodes extract (100%) significantly reduced leaf protein contents in D. glomerata, L. perenne and in L. sativa.

Invasiveness in introduced Australian acacias: the role of species traits and genome size

AbstractAim To assess associations between invasiveness, genome size and species traits in Australian Acacia species introduced outside their native range.Location Global.Methods Holoploid genome size was determined by flow cytometry for 92 species in Acacia subgenus Phyllodineae native to Australia. These data were used to test whether genome size was significantly different for invasive (n = 21; species known to be established and spreading in foreign environments) and non‐invasive (n = 71; species that have been introduced to foreign environments but which are not known to be spreading) species. Data for five functional traits [seed mass, specific leaf area (SLA), relative growth rate (RGR), maximum height and dispersal mode] and three characteristics of native range (size, temperature range and precipitation range) were used to test for univariate and multivariate relationships between (1) invasiveness and traits and (2) genome size and traits.Results Genome size ranged from 1.20 to 2.13 pg/2C and was not significantly smaller in invasive compared with non‐invasive acacias. However, invasive acacias were found to be taller and possess a larger native range size and a wider range of annual precipitation when compared with non‐invasive acacias. Multivariate analyses revealed significant differences between invasive and non‐invasive acacias (PERMANOVA; P < 0.01) driven largely by differences in native range characteristics. We detected a positive relationship between genome size and SLA (P = 0.02) and elaiosome dispersal mode (P < 0.01) in analyses across species, but these findings were not supported by evolutionary divergence analyses.Main conclusions Genome size variation does not underpin variation in traits associated with the invasive/non‐invasive dichotomy in introduced acacias, probably because of the very small DNA values in the Acacia species studied. Acacias introduced into new environments are most likely to become invasive if they are tall shrubs or trees and are widely distributed in their native range.

The evolution and phylogenetic placement of invasive Australian Acacia species

AbstractAim Acacia is the largest genus of plants in Australia with over 1000 species. A subset of these species is invasive in many parts of the world including Africa, the Americas, Europe, the Middle East, Asia and the Pacific region. We investigate the phylogenetic relationships of the invasive species in relation to the genus as a whole. This will provide a framework for studying the evolution of traits that make Acacia species such successful invaders and could assist in screening other species for invasive potential.Location Australia and global.Methods We sequenced four plastid and two nuclear DNA regions for 110 Australian Acacia species, including 16 species that have large invasive ranges outside Australia. A Bayesian phylogenetic tree was generated to define the major lineages of Acacia and to determine the phylogenetic placement of the invasive species.Results Invasive Acacia species do not form a monophyletic group but do form small clusters throughout the phylogeny. There are no taxonomic characters that uniquely describe the invasive Acacia species.Main conclusions The legume subfamily Mimosoideae has a high percentage of invasive species and the Australian Acacia species have the highest rate of all the legumes. There is some evidence of phylogenetic clumping of invasive species of Acacia in the limited sampling presented here. This phylogeny provides a framework for further testing of the evolution of traits associated with invasiveness in Acacia.

Jack‐of‐all‐trades and master of many? How does associated rhizobial diversity influence the colonization success of Australian Acacia species?

AbstractAim To evaluate the role of rhizobial diversity, and symbiotic promiscuity, on the invasive ability of Australian acacias (Acacia species in subgenus Phyllodineae native to Australia).Location Global.Methods A bibliographic review of the rhizobial diversity associated with Australian Acacia species was performed to assess symbiotic promiscuity for invasive and non‐invasive species. The rhizobial diversity associated with Acacia dealbata and A. saligna in Australia and Portugal and with A. pycnantha in Australia and South Africa was assessed by 16S rDNA and intergenic spacer sequencing of bacteria isolated from field‐collected nodules.Results All studied Australian acacias are nodulated by strains in the genus Bradyrhizobium, which appears to be the dominant group of acacia symbionts in native and non‐native soils. Both literature and experimental data from this study suggest that Australian bradyrhizobia might have been co‐introduced with acacias to new geographical regions. The studied Acacia species can also harbour other root‐nodulating alpha and betaproteobacteria genera, although these are less abundant than Bradyrhizobium.Main conclusions There is no clear difference in the diversity of rhizobial species associated with invasive and non‐invasive Australian acacias. All studied invasive acacias nodulate in both native and non‐native regions, harbouring predominantly Bradyrhizobium strains but showing some degree of symbiotic promiscuity. The co‐introduction of compatible root‐nodulating bacteria from Australia might explain the establishment of invasive populations, but novel associations with rhizobia from the invaded soils are also possible. Invasive legumes might use both strategies but species with low symbiotic promiscuity would become invasive only if compatible bacteria are co‐introduced in the new regions. The progress of invasion and the impacts on the invaded ecosystems might also differ depending on the nodulation strategy.

Adoption, use and perception of Australian acacias around the world

AbstractAim To examine the different uses and perceptions of introduced Australian acacias (wattles; Acacia subgenus Phyllodineae) by rural households and communities.Location Eighteen landscape‐scale case studies around the world, in Vietnam, India, Réunion, Madagascar, South Africa, Congo, Niger, Ethiopia, Israel, France, Portugal, Brazil, Chile, Dominican Republic and Hawai‘i.Methods Qualitative comparison of case studies, based on questionnaire sent to network of acacia researchers. Information based on individual knowledge of local experts, published and unpublished sources.Results We propose a conceptual model to explain current uses and perceptions of introduced acacias. It highlights historically and geographically contingent processes, including economic development, environmental discourses, political context, and local or regional needs. Four main groupings of case studies were united by similar patterns: (1) poor communities benefiting from targeted agroforestry projects; (2) places where residents, generally poor, take advantage of a valuable resource already present in their landscape via plantation and/or invasion; (3) regions of small and mid‐scale tree farmers participating in the forestry industry; and (4) a number of high‐income communities dealing with the legacies of former or niche use of introduced acacia in a context of increased concern over biodiversity and ecosystem services.Main conclusions Economic conditions play a key role shaping acacia use. Poorer communities rely strongly on acacias (often in, or escaped from, formal plantations) for household needs and, sometimes, for income. Middle‐income regions more typically host private farm investments in acacia woodlots for commercialization. Efforts at control of invasive acacias must take care to not adversely impact poor dependent communities.

Post‐clearing recovery of coastal dunes invaded by Acacia longifolia: is duration of invasion relevant for management success?

Summary1. Invasive plants destroy the structure and function of many ecosystems but removal is expensive, so the likelihood of success should be assessed before a major control programme commences. Trial clearings can clarify how best to design and perform broad‐scale clearing programmes. Such trials need to consider the range of conditions that might affect the outcome of control operations, not least the duration of invasion before clearing commences, a parameter that has rarely been considered previously.2. The recovery of plant communities was monitored over 6 years in a Portuguese coastal dune system after the removal of invasive Acacia longifolia plants, together with the underlying litter in half of the cleared areas but not in the remainder. Areas that had been invaded for >20 years (long‐invaded) as opposed to <7 years (recently invaded) were compared.3. After the removal of A. longifolia, recently invaded areas had more exotic plant species but also higher native species richness, plant cover, initial diversity and species turnover rates than long‐invaded areas. Generalist native species were initially very abundant in cleared areas but progressively gave way to species more typical of dunes. Six years after clearing, many species that usually occur on dunes were still missing. Therophytes were the most abundant life form immediately after clearing, but nanophanerophytes, chamaephytes and A. longifolia increased with time. Seedlings of A. longifolia were most abundant in long‐invaded areas, but litter removal along with plants promoted increased plant species richness and cover and decreased susceptibility to reinvasion.4. Synthesis and applications. A number of variables can affect the outcome of clearing invasive plant species. For A. longifolia on Portuguese dunes, recently invaded areas should be prioritized and thick litter layers should be removed along with the invader. Even so, recovery of native flora was not complete in this study, and other management actions are needed to supplement clearing operations, for example propagation of native species or prescribed fires to deplete the invasive species seed bank. Although complete restoration of the ecosystem will almost always be impossible, it should be possible in the long term to create an ecosystem with a structure and function that resembles the original habitat.

Attraction of ants by an invasive Acacia

Abstract. 1. Invasive plants are often released from the herbivores of their native range, but may also be deprived of their co-evolved mutualists. In southern Portugal Acacia dealbata has become naturalised in secondary woodland habitats and is apparently not damaged by local herbivores. It possesses inactive extra-floral nectaries (EFNs). 2. Artificial damage to leaves, mimicking herbivore attack, induced extra-floral nectar production on both adult trees and seedlings. This response was restricted to individual leaves rather than systemic. 3. Following EFN activation, trees were tended by the invasive Argentine ant Linepithema humile. Seedlings received a tenfold greater visitation rate from either L. humile or the native ant Plagiolepis pygmaea, which appears to displace the former. Eight days after the damage treatment the ants and nectar had largely gone. 4. There was no indication that either species of ant would defend the plant against potential herbivores. 5. This is the first recorded ant-plant interaction among two invasive species. Given the lack of natural herbivore damage and the absence of suitable ants in this novel community, nectar induction by A. dealbata is likely to be a dysfunctional response to damage in its invasive range, with little possibility of developing into a mutualism.

Rhizobial hitchhikers from Down Under: invasional meltdown in a plant–bacteria mutualism?

AbstractAim This study analysed the diversity and identity of the rhizobial symbionts of co‐existing exotic and native legumes in a coastal dune ecosystem invaded by Acacia longifolia.Location An invaded coastal dune ecosystem in Portugal and reference bradyrhizobial strains from the Iberian Peninsula and other locations.Methods Symbiotic nitrogen‐fixing bacteria were isolated from root nodules of plants of the Australian invasive Acacia longifolia and the European natives Cytisus grandiflorus, Cytisus scoparius and Ulex europaeus. Total DNA of each isolate was amplified by polymerase chain reaction (PCR) with the primer BOX A1R. Subsequent PCR‐sequencing and phylogenetic analyses of the internal transcribed spacer region and the nifD and nodA genes were performed for all different strains.Results The four plant species analysed were nodulated by bacteria from three different Bradyrhizobium lineages, although most of the isolates belonged to the Bradyrhizobium japonicum lineage sensu lato. Ninety‐five per cent of the bradyrhizobia isolated from A. longifolia, C. grandiflorus and U. europaeus in the invaded ecosystem had nifD and nodA genes of Australian origin. Seven isolates obtained in this study define a new distinctive nifD group of Bradyrhizobium from western and Mediterranean Europe.Main conclusions These results reveal the introduction of exotic bacteria with the invasive plant species, their persistence in the new geographical area and the nodulation of native legumes by rhizobia containing exotic symbiotic genes. The disruption of native mutualisms and the mutual facilitation of the invasive spread of the introduced plant and bradyrhizobia could constitute the first report of an invasional meltdown documented for a plant–bacteria mutualism.

Disturbance influences the outcome of plant-soil biota interactions in the invasive Acacia longifolia and in native species

Interrelated causes of plant invasion have been gaining increasing recognition. However, research on this subject has mainly focused around conceptual models. Here we explore whether plant–soil biota feedbacks and disturbance, two major factors capable of facilitating invasive plants in introduced ranges, interact to preferentially benefit exotics compared to native plants. We investigated the influence of fire disturbance on plant–soil biota interactions for the invasive Acacia longifolia and two dominant natives (Cytisus striatus and Pinus pinaster) in Portuguese dune systems. In the first experiment, we grew exotic and native plants in soil inoculated with soil biota from unburned or recently burned soils collected in an area with small invasion intensity by A. longifolia. Soil biota effects on the exotic legume A. longifolia changed from neutral to positive after fire, whereas the opposite outcome was observed in the native legume C. striatus, and a change from negative to neutral effects after fire occurred in the native P. pinaster. Fire reduced mycorrhizal colonization in all species and rhizobial colonization in C. striatus but not in A. longifolia. In the second experiment, we grew the exotic and native plants with conspecific and heterospecific soil biota from undisturbed soils (area with low invasion intensity by A. longifolia), and from post-fire soils (area affected by a fire ∼12 years ago and currently heavily invaded by A. longifolia). The exotic benefited more from post-fire than from undisturbed soil biota, particularly from those associated with natives. Natives did not experience detrimental effects with invasive-associated soil biota. Our results show that fire disturbance affected the functional interactions between soil biota and plants that may benefit more the exotic than some native species. Disturbance may open a window of opportunity that promotes invader success by altering soil enemy and mutualistic impacts.

Effect of invasive Acacia dealbata Link on soil microorganisms as determined by PCR-DGGE

Acacia dealbata Link is an Australian woody legume that has become a serious environmental problem in Northwest Spain where it forms dense monospecific patches modifying the structure of different native ecosystems and threatening native aboveground biodiversity. In spite of the dramatic changes observed in the vegetation of invaded sites little is known about the consequences of invasion for soil microorganisms. To investigate the effect of A. dealbata invasion on the structure of soil fungi and bacteria communities, samples were taken from invaded and non-invaded areas from three different ecosystems in Northwest Spain: pine forest, shrubland and grassland. In each ecosystem type, soil samples were taken in areas of native vegetation, areas invaded by A. dealbata and in the transition zone between native and invaded vegetation. Soil microorganisms were analyzed in the different samples by PCR-DGGE using general primers for eubacteria and fungi. Soil analyses were also performed to evaluate the effect of A. dealbata invasion on soil fertility. The invasion by A. dealbata consistently increased soil N, C, organic matter and exchangeable P content in the three studied ecosystems. A clear effect of the invasion on the overall structure of microorganism communities was only observed in the shrubland where soil fungal communities in the invaded and transition areas clustered together and apart from the native soil. Significant differences in soil microorganisms richness and diversity between invaded and not invaded soils were only found in the grassland. Grassland invasion by A. dealbata lead to a significant increase of bacterial richness and to a significant reduction in fungal richness and diversity. Our results show that although the changes on soil chemistry due to A. dealbata invasion are consistent among the studied ecosystems, the effect on soil microorganisms depends on the ecosystem type affected by the invasion.

Differential responses to allelopathic compounds released by the invasive Acacia dealbata Link (Mimosaceae) indicate stimulation of its own seed

Invasion by exotic species constitutes a major threat to natural ecosystems. One of the most invasive species of current concern in Europe is Acacia dealbata Link (Mimosaceae), the expansion of which is reducing populations of native species, and hence biodiversity. To investigate the potential involvement of allelopathic mechanisms in this process, we evaluated the germination and seedling growth performance of Lactuca sativa L. (Compositae), Zea mays L. (Gramineae), Dactylis glomerata L. (Gramineae), Arabidopsis thaliana (L.) Heynh. (Cruciferae) and A. dealbata, in the presence of mixtures of throughfall and litter leachate and of aqueous soil extracts collected under A. dealbata at different times of the year. Each test species responded differently to the solutions being assayed suggesting that A. dealbata allelochemicals may affect a variety of physiological growth parameters. In most cases the observed effects were stimulatory rather than inhibitory, which may have long-term deleterious effects on native plant populations since premature growth during periods of poor resources can be detrimental. Interestingly, increased radical growth of A. dealbata was detected with either extract suggesting a positive autoalleloapthy effect.

High competitiveness of a resource demanding invasive acacia under low resource supply

Mechanisms controlling the successful invasion of resource demanding species into low-resource environments are still poorly understood. Well-adapted native species are often considered superior competitors under stressful conditions. Here we investigate the competitive ability of the resource demanding alien Acacia longifolia, which invades nutrient-poor Mediterranean sand dunes such as in coastal areas of Portugal. We explore the hypothesis that drought may limit invasion in a factorial competition experiment of the alien invasive versus two native species of different functional groups (Halimium halimifolium, Pinus pinea), under well-watered and drought conditions. Changes in biomass, allocation pattern, and N-uptake-efficiency (via 15N-labeling) indicated a marked drought sensitivity of the invader. However, highly efficient drought adaptations of the native species did not provide a competitive advantage under water limiting conditions. The competitive strength of H. halimifolium towards the alien invader under well-watered conditions turned into a positive interaction between both species under drought. Further, low resource utilization by native species benefited A. longifolia by permitting continued high nitrogen uptake under drought. Hence, the N-fixing invader expresses low plasticity by continuous high resource utilization, even under low resource conditions. The introduction of novel traits into a community like N-fixation and high resource use may promote A. longifolia invasiveness through changes in the physical environment, i.e., the water and nutrient cycle of the invaded sand dune system, thereby potentially disrupting the co-evolved interactions within the native plant community.

Fungal associations in Asphondylia (Diptera: Cecidomyiidae) galls from Australia and South Africa: implications for biological control of invasive acacias

Gall-forming Asphondylia are well represented on Australian Acacia and have potential for biological control where Australian acacias cause ecological or economic harm, particularly South Africa. Asphondylia in Australia and South Africa are associated with communities of fungi in their galls. In Australia, Botryosphaeria dothidea (as its Dichomera synanamorph) is the most abundant and sometimes the only fungus present and is implicated as the primary species forming a mutualistic relationship with Asphondylia. In the combined analysis of ITS and elongation factor 1-α sequence data, isolates of B. dothidea from Australia and South Africa form distinct sub-clades. Female Asphondylia carry B. dothidea (as Dichomera conidia) in mycangia located posterior to sternite 7. While conidia are always present on field-collected specimens, laboratory-reared females rarely carry conidia. The mechanism and location of spore collection remains unresolved, but needs to be understood if Asphondylia species are to be utilised for biological control of invasive Australian acacias. As B. dothidea is a polyphagous plant pathogen capable of infecting crops of economic importance, including Acacia plantations, the introduction of novel strains of B. dothidea associated with biological control of acacia is undesirable, however endemic forms of the fungus could possibly be exploited by introduced Asphondylia.

Paddock-scale patterns of seed production and dispersal in the invasive shrub Acacia nilotica (Mimosaceae) in northern Australian rangelands

Paddock-scale patterns of seed production and dispersal in the invasive shrub Acacia nilotica (Mimosaceae) in northern Australian rangelands

Soil recovery after removal of the N2-fixing invasive Acacia longifolia: consequences for ecosystem restoration

Invasion by Acacia longifolia alters soil characteristics and processes. The present study was conducted to determine if the changes in soil C and N pools and processes induced by A. longifolia persist after its removal, at the Sao Jacinto Dunes Nature Reserve (Portugal). Some areas had been invaded for a long time (>20 years) and others more recently ( 30%, β-glucosaminidase activity (N mineralization index) >60% and potential nitrification >95%. Removal of plants and litter resulted in a >35% decrease in C and N content after four and half years. In recently invaded areas, β-glucosaminidase activity and potential nitrification showed a marked decrease (>54% and >95%, respectively) after removal of both A. longifolia and litter. Our results suggest that after removal of an N2-fixing invasive tree that changes ecosystem-level processes, it takes several years before soil nutrients and processes return to pre-invasion levels, but this legacy slowly diminish, suggesting that the susceptibility of native areas to (re)invasion is a function of the time elapsed since removal. Removal of the N-rich litter layer facilitates ecosystem recovery.