Morpho-physiological growth performance and anti-oxidative capabilities of Acacia jacquemontii and Acacia nilotica upon exposure to Co3O4 Nbs in lead-contaminated soil.

Immense crowd of heavy metal in cultivated land is evolving as a global concern as a result of boosted level of soil toxicity. Amongst various metals, Lead (Pb) contamination has become alarming for plant and human heath through ingesting of polluted soils and food crops. To counterfeit this, a nanotechnological neutralizer effective in form of soiling of cobalt oxide Co3O4 Nbs to Acacia jacquemontii and Acacia nilotica with various meditations as 25, 50, 75 and 100 ppm). A Substantial result was observed on growth of plants but premium results were got by applications of cobalt oxide Nbs at 75 ppm. By this means, enhanced root length (39%), fresh weight (32%), shoot length (58%), as well as dry weight (28%) in selected Acacia species compared to control. Chlrophy contents in A. jacquemontii were estimated to be 0.23, 2.73 and 3.19 mg/L with treated with different concentrations of cobalt Nbs while in A. nilotica, the contents were 0.51, 2.93 and 3.12 mg/L respectively on same concentration. The atomic absorption (AAS), antioxidant activity and defendable positive comeback by using Co3O4 Nbs. Hence, the greenly synthesized Co3O4 Nbs counter acts lead toxicity to override and preserving the growth of plant. Such nanotechnological kits can consequently enhance the alternative system to stunned toxicity for distinguish the yield demand end to end with the progress of agronomic management approaches.

Chewing insects, pollinators, and predators on Acacia auriculiformis A. Cunn. ex Beth (Fabales: Fabaceae) plants fertilized with dehydrated sewage sludge / Insetos mastigadores, polinizadores e predadores em plantas de Acacia auriculiformis A. Cunn. ex Beth (Fabales: Fabaceae) fertilizada com lodo de esgoto desidratado

Fertilization with dehydrated sewage sludge can speed up the recovery process of degraded areas due to nutrients concentration, favoring the development of pioneer plants such as Acacia auriculiformis A. Cunn. ex Beth (Fabales: Fabaceae) and the emergence of insects. This study aimed the evaluation of chewing, pollinating insects, predators, their ecological indices and relationships on A. auriculiformis plants fertilized with dehydrated sewage sludge. The experimental design was completely randomized with two treatments (with and without dehydrated sewage sludge) and 24 repetitions. The prevalence of chewing insects Parasyphraea sp. (Coleoptera: Chrysomelidae), Nasutitermes sp. (Blattodea: Termitidae), and Tropidacris collaris (Stoll, 1813) (Orthoptera: Romaleidae), defoliation, and ecological indices of abundance of Coleoptera and Orthoptera were observed on fertilized A. auriculiformis. Acacia auriculiformis plants, with a superior number of branches/tree, revealed greater abundance of Coleoptera and Orthoptera, species richness of pollinating insects, defoliation, numbers of Parasyphraea sp. and T. collaris. The ones with larger leaves/branches displayed greater abundance of species richness of Coleoptera and Diabrotica speciosa (Germar, 1824) (Coleoptera: Chrysomelidae). Therefore, the use of A. auriculiformis plants, fertilized with dehydrated sewage sludge, is promising in the recovery of degraded areas due to the ecological indices increase of chewing and pollinators insects and spiders in the analyzed area.

A fertilização com lodo de esgoto desidratado pode acelerar o processo de recuperação de áreas degradadas devido à concentração de nutrientes, favorecendo o desenvolvimento de plantas pioneiras tais como Acacia auriculiformis A. Cunn. ex Beth (Fabales: Fabaceae) e de seus insetos. O objetivo deste trabalho foi avaliar os insetos mastigadores, polinizadores e predadores e seus índices e relações ecológicas em plantas de A. auriculiformis fertilizadas com lodo de esgoto desidratado, em área degradada, durante 24 meses. O delineamento foi inteiramente casualizados com dois tratamentos (com e sem adubação com lodo de esgoto desidratado) e 24 repetições (uma repetição = uma planta). O maior número de insetos mastigadores Parasyphraea sp. (Coleoptera: Chrysomelidae), Nasutitermes sp. (Blattodea: Termitidae) e Tropidacris collaris (Stoll, 1813) (Orthoptera: Romaleidae), de desfolha, e do índice ecológico abundância de Coleoptera e de Orthoptera foram maiores em plantas de A. auriculiformis fertilizadas do que nas não fertilizadas com lodo de esgoto desidratado. Plantas de A. auriculiformis, com maior quantidade de galhos/árvore, apresentaram maiores abundâncias de Coleoptera e Orthoptera, riqueza de espécies de insetos polinizadores, desfolha e números de Parasyphraea sp. e T. collaris, e as com maior folhas/galho os de riqueza de espécies de Coleoptera e Diabrotica speciosa (Germar, 1824) (Coleoptera: Chrysomelidae). Por tanto, a utilização de A. auriculiformis, adubada com lodo de esgoto desidratado, é promissora na recuperação de áreas degradadas devido ao aumento dos índices ecológicos de insetos mastigadores, polinizadores e aranhas na área.

First-year Acacia seedlings are anisohydric "water-spenders" but differ in their rates of water use.

PREMISE: First-year seedlings (FYS) of tree species may be a critical demographic bottleneck in semi-arid, seasonally dry ecosystems such as savannas. Given the highly variable water availability and potentially strong FYS-grass competition for water, FYS water-use strategies may play a crucial role in FYS establishment in savannas and, ultimately, in tree-grass competition and coexistence. METHODS: We examined drought responses in FYS of two tree species that are dominant on opposite ends of an aridity gradient in Serengeti, Acacia (=Vachellia) tortilis and A. robusta. In a glasshouse experiment, gas exchange and whole-plant hydraulic conductance (Kplant ) were measured as soil water potential (Ψsoil ) declined. Trajectory of the Ψleaf /Ψsoil relationship during drought elucidated the degree of iso/anisohydry. RESULTS: Both species were strongly anisohydric "water-spenders," allowing rapid wet-season C gain after pulses of moisture availability. Despite being equally vulnerable to declines in Kplant under severe drought, they differed in their rates of water use. Acacia tortilis, which occurs in the more arid regions, initially had greater Kmax , transpiration (E), and photosynthesis (Anet ) than A. robusta. CONCLUSIONS: This work demonstrates an important mechanism of FYS establishment in savannas: Rather than investing in drought tolerance, savanna FYS maximize gas exchange during wet periods at the expense of desiccation during dry seasons. FYS establishment appears dependent on high C uptake during the pulses of water availability that characterize habitats dominated by these species. This study increases our understanding of species-scale plant ecophysiology and ecosystem-scale patterns of tree-grass coexistence.

Ecological indices of phytophagous Hemiptera and their natural enemies on Acacia auriculiformis (Fabales: Fabaceae) plants with or without dehydrated sewage sludge application in a degraded area.

Soil fertilization with dehydrated sewage sludge (DSS) accelerates the recovery process of degraded areas by improving nutrient concentration, and favors the development of trophic webs with pioneer plants such as Acacia auriculiformis A. Cunn. ex Beth (Fabales: Fabaceae), phytophagous Hemiptera, predators, and protocooperanting ants. This study aimed to evaluate the development and production of A. auriculiformis litter with or without dehydrated sewage sludge application and the ecological indices of sucking insects (Hemiptera), their predators and protocooperating ants, as bioindicators, in a degraded area for 24 months. Complete randomization was applied for two treatments (with or without application of dehydrated sewage sludge) in 24 replications (one repetition = one plant). We evaluated the number of leaves/branch and branches/plant, percentage of soil cover (litter), ecological indices of phytophagous Hemiptera, their predators, and protocooperating ants. The plants of A. auriculiformis, that were applied with dehydrated sewage sludge, had superior development when compared to plants where DSS were not applied. The highest abundance and richness of phytophagous Hemiptera species and Sternorrhyncha predators occurred on A. auriculiformis plants that were applied with dehydrated sewage sludge. The increase in richness of species of protocooperanting ants that established mutualistic relationships positively influenced the phytophagous Hemiptera. The use of A. auriculiformis, with application of dehydrated sewage sludge, can increase recovery of degraded areas due to its higher soil cover (e.g., litter) and results in higher ecological indices of phytophagous Hemiptera and their predators.

Rhizobial inoculation in black wattle plantation (Acacia mearnsii De Wild.) in production systems of southern Brazil.

Black wattle (Acacia mearnsii De Wild.) is a tree legume native to southeast Australia, but present in all continents. Today it covers about 142,400 ha in Brazil, with plantations concentrated in the southern region of the country. Black wattle may form nodules and establish rhizobial symbiosis capable of fixing N2, but rhizobial inoculation is not done in commercial plantations. About 40 kg ha-1 of urea is applied during seedling transplantation. In this review, evidences by which rhizobial inoculation affects monoculture, mixed cultivation, and agroforestry black wattle production systems were searched in literature. Previous measurements in cultivated forests have indicated that biological nitrogen fixation in black wattle may provide up to 200 kg of N ha-1 year-1 to the soil. Therefore, rhizobia inoculation may bring several opportunities to improve black wattle production systems. Black wattle is not a very selective partner in the rhizobial symbiosis, but the genus Bradyrhizobium dominates the rhizobial diversity of black wattle nodules. Investigation on rhizobial diversity in soils where the crop is cultivated may represent an opportunity to find more effective rhizobia strains for inoculants. The successful history of biological nitrogen fixation in grain legumes must inspire the history of tree legumes. Microbiology applied to forestry must overcome challenges on the lack of trained professionals and the development of new application technologies.

Development and evolution of age-dependent defenses in ant-acacias.

Age-dependent changes in plant defense against herbivores are widespread, but why these changes exist remains a mystery. We explored this question by examining a suite of traits required for the interaction between swollen thorn acacias (genus Vachellia) and ants of the genus Pseudomyrmex In this system, plants provide ants with refuge and food in the form of swollen stipular spines, protein-lipid-rich "Beltian" bodies, and sugar-secreting extrafloral nectaries-the "swollen thorn syndrome." We show that this syndrome develops at a predictable time in shoot development and is tightly associated with the temporal decline in the microRNAs miR156 and miR157 and a corresponding increase in their targets-the SPL transcription factors. Growth under reduced light intensity delays both the decline in miR156/157 and the development of the swollen thorn syndrome, supporting the conclusion that these traits are controlled by the miR156-SPL pathway. Production of extrafloral nectaries by Vachellia sp. that do not house ants is also correlated with a decline in miR156/157, suggesting that this syndrome evolved by co-opting a preexisting age-dependent program. Along with genetic evidence from other model systems, these findings support the hypothesis that the age-dependent development of the swollen thorn syndrome is a consequence of genetic regulation rather than a passive developmental pattern arising from developmental constraints on when these traits can develop.

Changes in cambial activity are related to precipitation patterns in four tropical hardwood species grown in Indonesia.

PREMISE: Cambial activity in some tropical trees varies intra-annually, with the formation of xylem rings. Identification of the climatic factors that regulate cambial activity is important for understanding the growth of such species. We analyzed the relationship between climatic factors and cambial activity in four tropical hardwoods, Acacia mangium, Tectona grandis, Eucalyptus urophylla, and Neolamarckia cadamba in Yogyakarta, Java Island, Indonesia, which has a rainy season (November-June) and a dry season (July-October). METHODS: Small blocks containing phloem, cambium, and xylem were collected from main stems in January 2014, October 2015 and October 2016, and examined with light microscopy for cambial cell division, fusiform cambial cells, and expanding xylem cells as evidence of cambial activity. RESULTS: During the rainy season, when precipitation was high, cambium was active. By contrast, during the dry season in 2015, when there was no precipitation, cambium was dormant. However, in October 2016, during the so-called dry season, cambium was active, cell division was conspicuous, and a new xylem ring formation was initiated. The difference in cambial activity appeared to be related to an unusual pattern of precipitation during the typically dry months, from July to October, in 2016. CONCLUSIONS: Our results indicate that low or absent precipitation for 3 to 4 months induces cessation of cambial activity and temporal periodicity of wood formation in the four species studied. By contrast, in the event of continuing precipitation, cambial activity in the same trees may continue throughout the year. The frequency pattern of precipitation appears to be an important determinant of wood formation in tropical trees.

Global predictors of alien plant establishment success: combining niche and trait proxies.

Biological invasions are on the rise globally. To reduce future invasions, it is imperative to determine the naturalization potential of species. Until now, screening approaches have relied largely on species-specific functional feature data. Such information is, however, time-consuming and expensive to collect, thwarting the screening of large numbers of potential invaders. We propose to resolve such data limitations by developing indicators of establishment success of alien species that can be readily derived from open-access databases. These indicators describe key features of successfully established aliens, including estimates of potential range size, niche overlap with human-disturbed environments, and proxies of species traits related to their palaeoinvasions and local dominance capacities. We demonstrate the utility of this new approach by applying it to two large and highly invasive plant groups: Australian acacias and eucalypts. Our results show that these indicators robustly predict establishment successes and failures in each clade independently, and that they can cross-predict establishment in these two clades. Interestingly, the indicator identified as most important was species potential range size on Earth, a variable too rarely considered as a predictor. By successfully identifying key features that predispose Australian plants to naturalize, we provide an objective and cost-effective protocol for flagging high-risk introductions.

Acacia Ants Respond to Plant-Borne Vibrations Caused by Mammalian Browsers.

Living in the African savanna is dangerous, especially for plants. Many plants therefore engage in mutualism with ants, in which plants provide food and shelter in exchange for protection against herbivores. Ants become alarmed when the plant takes on some sort of damage. They immediately emerge from their plant shelter and aggressively defend the plant. Mammalian herbivores can have devastating effects on trees by browsing, breaking tree branches, stripping bark, and pushing over entire trees. However, mutualistic ants substantially reduce the amount of damage. To efficiently protect the tree, ants need to rapidly react together when the tree is under attack. Here, we show that the acacia ant Crematogaster mimosae defends its host tree by exploiting plant-borne vibrations caused by browsers feeding on the tree. Experiments with controlled vibrations show that ants discriminate browser-induced vibrations from those induced by wind, become alarmed, and patrol on the branches. Browser-induced vibrations serve as a long-distance alarm cue. The vibrations propagate through the whole acacia tree and trigger ants' defensive behavior, even on the other side of the tree. Furthermore, the ants make use of tropotactic directional vibration sensing to orient to the attacked part of the tree and fight back the attacker.

A spatio-temporal, landscape perspective on Acacia dealbata invasions and broader land use and cover changes in the northern Eastern Cape, South Africa.

Biological invasions and human land use both have the potential to drastically alter the patterns and processes of landscapes, driving habitat fragmentation and altering natural disturbance regimes. The proliferation of an invasive species depends on composition and configuration of the landscape, as well as the invasiveness of the species. To effectively manage a highly invasive species, such as Acacia dealbata, it is crucial to understand the historical progression of the invasion within the landscape. This study sought to examine the landscape dynamics of biological invasions by tracking the historical spread of A. dealbata and broader land use/land cover (LULC) changes at different spatio-temporal scales in the northern Eastern Cape. A time-series of aerial photographs were systematically classified according to designated A. dealbata and LULC categories in ArcGIS to track the changes in the extent and rate of spread of A. dealbata. Markedly dynamic, multi-directional, and spatio-temporally variable LULC transitions were observed across the northern Eastern Cape over the last six decades. A. dealbata frequently retained a high proportion of cover over time, and despite the loss of cover to other LULC classes, a net increase in A. dealbata cover occurred as it spread at an overall annual rate of 0.11-0.21%, occupying approximately 8-18% of land cover across all sampled sites by 2013. Any management interventions to limit or control A. dealbata should therefore consider the spatio-temporal and LULC nuances of landscapes.

Rain-based soil solarization for reducing the persistent seed banks of invasive plants in natural ecosystems - Acacia saligna as a model.

BACKGROUND: A large persistent seed bank of invasive plants is a significant obstacle to restoration programs. Soil solarization was demonstrated to be an effective method for reducing the seed bank of Australian acacias. However, use of this method in natural habitats might be limited due to the requirement to moisten the soil by irrigation. This study examined the possibility of replacing irrigation by trapping the soil moisture caused by the most recent rainfall, i.e. rain-based soil solarization (RBS). RESULTS: Exposure of Acacia saligna seeds to 57 °C at 20% soil moisture for 68 h resulted in almost complete loss of seed viability. Similarly, RBS treatment significantly reduced the viability of A. saligna seeds buried at a soil depth of 1-19 cm as well as seed density in the natural seed bank, and almost completely eliminated seedling emergence from natural seed banks of A. saligna and other environmental weeds. CONCLUSION: Our results indicate that RBS is an effective method for reducing the seed bank of invasive plants in natural habitats located in various climate regions characterized by different soil types. This is the first demonstration of a successful application of RBS for soil disinfestation. © 2018 Society of Chemical Industry.

Dietary specialization in mutualistic acacia-ants affects relative abundance but not identity of host-associated bacteria.

Acacia-ant mutualists in the genus Pseudomyrmex nest obligately in acacia plants and, as we show through stable isotope analysis, feed at a remarkably low trophic level. Insects with diets such as these sometimes depend on bacterial symbionts for nutritional enrichment. We, therefore, examine the bacterial communities associated with acacia-ants in order to determine whether they host bacterial partners likely to contribute to their nutrition. Despite large differences in trophic position, acacia-ants and related species with generalized diets do not host distinct bacterial taxa. However, we find that a small number of previously undescribed bacterial taxa do differ in relative abundance between acacia-ants and generalists, including several Acetobacteraceae and Nocardiaceae lineages related to common insect associates. Comparisons with an herbivorous generalist, a parasite that feeds on acacias and a mutualistic species with a generalized diet show that trophic level is likely responsible for these small differences in bacterial community structure. While we did not experimentally test for a nutritional benefit to hosts of these bacterial lineages, metagenomic analysis reveals a Bartonella relative with an intact nitrogen-recycling pathway widespread across Pseudomyrmex mutualists and generalists. This taxon may be contributing to nitrogen enrichment of its ant hosts through urease activity and, concordant with an obligately host-associated lifestyle, appears to be experiencing genomewide relaxed selection. The lack of distinctiveness in bacterial communities across trophic level in this group of ants shows a remarkable ability to adjust to varied diets, possibly with assistance from these diverse ant-specific bacterial lineages.

Coordination between leaf, stem, and root hydraulics and gas exchange in three arid-zone angiosperms during severe drought and recovery.

The ability to resist hydraulic dysfunction in leaves, stems, and roots strongly influences whether plants survive and recover from drought. However, the coordination of hydraulic function among different organs within species and their links to gas exchange during drought and recovery remains understudied. Here, we examine the interaction between gas exchange and hydraulic function in the leaves, stems, and roots of three semiarid evergreen species exposed to a cycle of severe water stress (associated with substantial cavitation) and recovery. In all species, stomatal closure occurred at water potentials well before 50% loss of stem hydraulic conductance, while in two species, leaves and/or roots were more vulnerable than stems. Following soil rewetting, leaf-level photosynthesis (Anet ) returned to prestress levels within 2-4 weeks, whereas stomatal conductance and canopy transpiration were slower to recover. The recovery of Anet was decoupled from the recovery of leaf, stem, and root hydraulics, which remained impaired throughout the recovery period. Our results suggest that in addition to high embolism resistance, early stomatal closure and hydraulic vulnerability segmentation confers drought tolerance in these arid zone species. The lack of substantial embolism refilling within all major organs suggests that vulnerability of the vascular system to drought-induced dysfunction is a defining trait for predicting postdrought recovery.

Stem gravitropism and tension wood formation in Acacia mangium seedlings inclined at various angles.

Background and Aims: In response to a gravitational stimulus, angiosperm trees generally form tension wood on the upper sides of leaning stems in order to reorientate the stems in the vertical direction. It is unclear whether the angle of inclination from the vertical affects tension wood formation. This study was designed to investigate negative gravitropism, tension wood formation and growth eccentricity in Acacia mangium seedlings inclined at different angles. Methods: Uniform seedlings of A. mangium were artificially inclined at 30°, 45°, 60° and 90° from the vertical and harvested, with non-inclined controls, 3 months later. We analysed the effects of the angle of inclination on the stem recovery angle, the anatomical features of tension wood and radial growth. Key Results: Smaller inclination angles were associated with earlier stem recovery while stems subjected to greater inclination returned to the vertical direction after a longer delay. However, in terms of the speed of negative gravitopism towards the vertical, stems subjected to greater inclination moved more rapidly toward the vertical. There was no significant difference in terms of growth eccentricity among seedlings inclined at different angles. The 30°-inclined seedlings formed the narrowest region of tension wood but there were no significant differences among seedlings inclined at 45°, 60° and 90°. The 90°-inclined seedlings formed thicker gelatinous layers than those in 30°-, 45°- and 60°-inclined seedlings. Conclusion: Our results suggest that the angle of inclination of the stem influences negative gravitropism, the width of the tension wood region and the thickness of gelatinous layers. Larger amounts of gelatinous fibres and thicker gelatinous layers might generate the higher tensile stress required for the higher speed of stem-recovery movement in A. mangium seedlings.

Germination, physiological and biochemical responses of acacia seedlings (Acacia raddiana and Acacia tortilis) to petroleum contaminated soils.

Along the arid Arava, southern Israel, acacia trees (Acacia raddiana and Acacia tortilis) are considered keystone species. Yet they are threatened by the ongoing aquifer depletion for agriculture, the conversion of natural land to agricultural land, seed infestation by bruchid beetles, and the reduction in precipitation level in the region. In the acacia dominated Evrona reserve (southern Arava), adding to these threats are recurrent oil spills from an underground pipeline. We report here a study of the effects of contaminated soils, from a recent (December 2014) and a much older (1975) oil spills. The effects of local petroleum oil-contaminated soils on germination and early growing stages of the two acacia species were studied by comparisons with uncontaminated (control) soils from the same sites. For both acacia species, germination was significantly reduced in the 2014 oil-contaminated soils, whereas delayed in the 1975 oil-contaminated soil. There was no significant effect of oil volatile compounds on seed germination. At 105 days post transplanting (DPT), height, leaf number, stem diameter, and root growth were significantly smaller in the oil-contaminated soils. While photosynthetic performance (quantum yield of photosystem II) did not differ considerably between treatments, reductions of chlorophylls content and protein content were found in seedlings growing in the contaminated soils. Significant increases in superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities were found in roots of seedlings growing in oil-contaminated soils. These results demonstrate that seed germination and seedling growth of both acacia species were strongly restricted by oil contamination in soils, from both recent (2014) and a 40-year old (1975) oil spills. Such long-term effects of oil spills on local acacia seedlings could shift the structure of local acacia communities. These results should be taken into account by local authorities aiming to clean up and restore such polluted areas.

Differences in osmotic adjustment, foliar abscisic acid dynamics, and stomatal regulation between an isohydric and anisohydric woody angiosperm during drought.

Species are often classified along a continuum from isohydric to anisohydric, with isohydric species exhibiting tighter regulation of leaf water potential through stomatal closure in response to drought. We investigated plasticity in stomatal regulation in an isohydric (Eucalyptus camaldulensis) and an anisohydric (Acacia aptaneura) angiosperm species subject to repeated drying cycles. We also assessed foliar abscisic acid (ABA) content dynamics, aboveground/belowground biomass allocation and nonstructural carbohydrates. The anisohydric species exhibited large plasticity in the turgor loss point (ΨTLP ), with plants subject to repeated drying exhibiting lower ΨTLP and correspondingly larger stomatal conductance at low water potential, compared to plants not previously exposed to drought. The anisohydric species exhibited a switch from ABA to water potential-driven stomatal closure during drought, a response previously only reported for anisohydric gymnosperms. The isohydric species showed little osmotic adjustment, with no evidence of switching to water potential-driven stomatal closure, but did exhibit increased root:shoot ratios. There were no differences in carbohydrate depletion between species. We conclude that a large range in ΨTLP and biphasic ABA dynamics are indicative of anisohydric species, and these traits are associated with exposure to low minimum foliar water potential, dense sapwood and large resistance to xylem embolism.

Influence of neighboring plants on the dynamics of an ant-acacia protection mutualism.

Ant-plant protection symbioses, in which plants provide food and/or shelter for ants in exchange for protection from herbivory, are model systems for understanding the ecology of mutualism. While interactions between ants, host plants, and herbivores have been intensively studied, we know little about how plant-plant interactions influence the dynamics of these mutualisms, despite strong evidence that plants compete for resources, that hosting ants can be costly, and that host-plant provisioning to ants can therefore be constrained by resource availability. We used field experiments in a semiarid Kenyan savanna to examine interactions between the ant-plant Acacia drepanolobium, neighboring grasses, and two species of symbiotic acacia-ants with divergent behaviors: Crematogaster mimosae, an aggressive symbiont that imposes high costs to host trees via consumption of extrafloral nectar, and Tetraponera penzigi, a less-protective symbiont that imposes lower costs because it does not consume nectar. We hypothesized that by competing with acacias for resources, neighboring grasses (1) reduce hosts' ability to support costly C. mimosae, while having little or no effect on the ability of hosts to support low-cost T. penzigi, and (2) reduce sapling growth rates irrespective of ant occupant. We factorially manipulated the presence/absence of grasses and the identity of ant occupants on saplings and evaluated effects on colony survivorship and sapling growth rates over 40 weeks. Contrary to prediction, the high-cost/high-reward nectar-dependent mutualist C. mimosae had higher colony-survival rates on saplings with grass neighbors present. Grasses appear to have indirectly facilitated the survival of C. mimosae by reducing water stress on host plants; soils under saplings shaded by grasses had higher moisture content, and these saplings produced more active nectaries than grass-removal saplings. Consistent with prediction, survival of low-cost/low-reward T. penzigi did not differ significantly between grass-removal treatments. Saplings occupied by low-cost/low-reward T. penzigi grew 100% more on average than saplings occupied by high-cost/high-reward C. mimosae, demonstrating that mutualist-partner identity strongly and differentially influences demographic rates of young plants. In contrast, contrary to prediction, grass neighbors had no significant net impact on sapling growth rates. Our results suggest that neighboring plants can exert strong and counterintuitive effects on ant-plant protection symbioses, highlighting the need to integrate plant-plant interactions into our understanding of these mutualisms.

Shifts in the bacterial community composition along deep soil profiles in monospecific and mixed stands of Eucalyptus grandis and Acacia mangium.

Our knowledge of the rhizosphere bacterial communities in deep soils and the role of Eucalyptus and Acacia on the structure of these communities remains very limited. In this study, we targeted the bacterial community along a depth profile (0 to 800 cm) and compared community structure in monospecific or mixed plantations of Acacia mangium and Eucalyptus grandis. We applied quantitative PCR (qPCR) and sequence the V6 region of the 16S rRNA gene to characterize composition of bacterial communities. We identified a decrease in bacterial abundance with soil depth, and differences in community patterns between monospecific and mixed cultivations. Sequence analysis indicated a prevalent effect of soil depth on bacterial communities in the mixed plant cultivation system, and a remarkable differentiation of bacterial communities in areas solely cultivated with Eucalyptus. The groups most influenced by soil depth were Proteobacteria and Acidobacteria (more frequent in samples between 0 and 300 cm). The predominant bacterial groups differentially displayed in the monospecific stands of Eucalyptus were Firmicutes and Proteobacteria. Our results suggest that the addition of an N2-fixing tree in a monospecific cultivation system modulates bacterial community composition even at a great depth. We conclude that co-cultivation systems may represent a key strategy to improve soil resources and to establish more sustainable cultivation of Eucalyptus in Brazil.

Fire disturbance disrupts an acacia ant-plant mutualism in favor of a subordinate ant species.

Although disturbance theory has been recognized as a useful framework in examining the stability of ant-plant mutualisms, very few studies have examined the effects of fire disturbance on these mutualisms. In myrmecophyte-dominated savannas, fire and herbivory are key drivers that could influence ant-plant mutualisms by causing complete colony mortality and/or decreasing colony size, which potentially could alter dominance hierarchies if subordinate species are more fire resilient. We used a large-scale, replicated fire experiment to examine long-term effects of fire on acacia-ant community composition. To determine if fire shifted ant occupancy from a competitive dominant to a subordinate ant species, we surveyed the acacia-ant community in 6-7 yr old burn sites and examined how the spatial scale of these burns influenced ant community responses. We then used two short-term fire experiments to explore possible mechanisms for the shifts in community patterns observed. Because survival of ant colonies is largely dependent on their ability to detect and escape an approaching fire, we first tested the evacuation response of all four ant species when exposed to smoke (fire signal). Then to better understand how fire and its interaction with large mammal herbivory affect the density of ants per tree, we quantified ant worker density in small prescribed burns within herbivore exclusion plots. We found clear evidence suggesting that fire disturbance favored the subordinate ant Crematogaster nigriceps more than the dominant and strong mutualist ant C. mimosae, whereby C. nigriceps (1) was the only species to occupy a greater proportion of trees in 6-7 yr old burn sites compared to unburned sites, (2) had higher burn/unburn tree ratios with increasing burn size, and (3) evacuated significantly faster than C. mimosae in the presence of smoke. Fire and herbivory had opposite effects on ant density per meter of branch for both C. nigriceps and C. mimosae, with fire decreasing ant densities per meter of branch and the presence of large herbivores increasing ant density. Taken together, these experiments suggest that major ecosystem disturbances like fire can disrupt mutualistic associations and maintain diversity in partner quality and identity despite a clear dominance hierarchy.