Chemical and mechanical defense against herbivory in two sympatric species of desert Acacia

Abstract. Most African Acacia trees occur in semi-arid savannas, but some species grow in deserts at the periphery of their range in northern Africa and the Middle East. We studied Acacia raddiana and A. tortilis in the southern Negev desert of Israel. According to the resource availability hypothesis, plants in poor habitats invest heavily into anti-herbivore defense because losses to browsers are costly when growth rates are low. Few plants invest simultaneously in different categories of defense. This suggests that trade-offs exist, although little is known about how individuals allocate resources to defence in natural populations. In a field experiment, we studied chemical and mechanical defences as a response to differential browsing for more than 10 yr. Both Acacia species increased spinescence but there was no clear response in the production of total polyphenols, condensed tannins, and protein-precipitating tannins (only A. raddiana may increase secondary compounds under very high browsing levels). There was an age effect in spinescence but not in secondary compounds for both species, with younger trees investing more into anti-herbivore defense than older individuals. We were unable to find negative correlations between traits of chemical and mechanical defense. We conclude that inducibility and age effects suggest a cost for mechanical but not necessarily for chemical defense. Contrary to the predictions of the resource availability hypothesis, concentrations of secondary compounds were not higher in this desert environment than in savannas.     

Anthropogenic effects on population structure of Acacia tortilis subsp. raddiana along a gradient of water availability in South Sinai,Egypt

Acacia trees in Sinai desert are suffering population decline. This study aims to explore this assumption and to determine the effects of grazing and urbanization on size structure of Acacia tortilis subsp. raddiana along a gradient of water availability in Wadi Feiran basin, South Sinai, Egypt. Size structure of 289 Acacia trees in thirteen isolated populations was analysed. Catchment area and lineament density as water availability indicators were calculated. Effects of grazing and urbanization on Acacia populations were evaluated. A strong positive correlation was recognized between tree height, crown diameter and trunk circumference. Acacia tree density and crown diameter are positively correlated with water availability indicators. Population structure of A. tortilis is characterized by absence of juveniles, clear reduction in numbers of small and large categories and increasing in medium categories. This pattern indicates very low recruitment and high mortality of small and large trees. The variation in water availability may have considerable effects on the structure of A. tortilis. Overgrazing, cutting and urbanization are the main causes of population decline of Acacia in Sinai desert. In addition, drought conditions and bruchid seed beetles aggravate the anthropogenic effect on recruitment of Acacia in South Sinai.     

From snapshot information to long-term population dynamics of Acacias by a simulation model

The African Acacia species A. raddiana is believed to be endangered in the Negev desert of Israel. The ecology of this species is not well understood. The main idea of our study is to learn more about the long-term population dynamics of these trees using snapshot information in the form of size frequency distributions. These distributions are highly condensed indices of population dynamics acting over many years. In this paper, we analyse field data on recruitment, growth, and mortality and use an existing simulation model of the population dynamics of A. raddiana (SAM) to produce contrasting scenarios of these live history processes that are based on the analysed field evidence. The main properties of simulated as well as observed tree size frequency distributions are characterised with Simpson's index of dominance and a new permutation index. Finally, by running the SAM model under the different scenarios, we study the effect of these different processes on simulated size frequency distributions (pattern) and we compare them to size distributions observed in the field, in order to identify the processes acting in the field. Our study confirms rare recruitment events as a major factor shaping tree size frequency distributions and shows that the paucity of recruitment has been a normal feature of A. raddiana in the Negev over many years. Irregular growth, e.g., due to episodic rainfall, showed a moderate influence on size distributions. Finally, the size frequency distributions observed in the Negev reveal the information that, in this harsh environment, mortality of adult A. raddiana is independent of tree size (age).     

Three–way interactions between Acacia, large mammalian herbivores and bruchid beetles - a review

Large mammalian herbivores are both predators and dispersers of Acacia seeds. While some of the seeds are destroyed during passage through the herbivore's digestive tract, others are defecated unharmed. Ingestion by large herbivores facilitates germination by scarification of the seed coat. The extent of the influence of herbivores on seed dispersal and germination depends on seed retention time and tooth size, which are both positively correlated with body size. Infestation by bruchid beetles (Bruchidae) reduces Acacia germination. Herbivores may reduce bruchid infestation in several ways. Larvae in recently infested seeds are killed by stomach acids penetrating the seed through the larval entry hole. Seeds that are partly excavated by burrowing larvae in more advanced stages may be crushed by the herbivore's teeth. Lastly, but probably most crucially, herbivores simply remove seeds from the natal tree prior to infestation or at least prior to reinfestation. The timing and magnitude of herbivory is crucial for both the reduction of bruchid infestation and Acacia seedling establishment. Although it is widely agreed that a three–way interaction exists between bruchid beetles, Acacia trees and large mammalian herbivores, it is also apparent that the relationship is highly complex and is not yet completely understood.     

Host-associated differences in fitness within and between populations of a seed beetle (Bruchidae): effects of plant variability

Summary Experiments were conducted with the sexually reproducing seed beetle Stator limbatus and its hosts in north-central Arizona to determine if it was substructured into units, each specialized for higher fitness on a specific host species. Unlike many studies, we incorporated scale, i.e., conducting experiments between and within beetle populations on seeds from within and between plant species. Of particular interest was whether intraspecific plant variability prevented beetle specialization within beetle populations. Results suggest that S. limbatus is specialized to certain hosts. On the palo verde Cercidium floridum, beetles originally reared from this host had significantly higher emergence compared to beetles transferred from other hosts. We did not test directly for a genetic basis for this. Alternative hypotheses of variation in symbiotic microorganisms in larval guts and maternal effects were assessed. Essentially no bacteria, yeast or protozoa were found, and maternal effects as expressed by varying egg weights were not detected; however, other microorganisms might have been present and maternal effects through inducible enzymes was possible. Caution, then, is needed in any genetic interpretations of our results. The differences on C. floridum were detected from tests between and within beetle populations. Evidence for specialization was not detected on the other hosts, Cercidium microphyllum and Acacia greggii. On the other hosts, beetles performed well regardless of their source. Significant differences were detected among individual plants of C. floridum as to the suitability of their seeds for deveoopment of S. limbatus. No such differences were detected among the other host plants. These patterns of conspecific plant variability are opposite of what is expected if plant variability prevents specialization of beetles to particular species of hosts. Thus, the data suggest seed variability among plants does not prevent specialization to host species in this system. We discuss how the patterns of host use in this study relate to the hypothesis of sympatric host race formation.     

Do spatial effects play a role in the spatial distribution of desert‐dwelling Acacia raddiana?

Abstract. We investigated the spatial pattern of A. raddiana in the Negev desert of Israel in order to gain insights into the factors and processes driving the dynamics of this species. Using a scale‐dependent measure, the ring statistic, we analysed both patterns observed in the field and time series of spatial tree distributions produced by a simulation model. In the field, random spacing was the predominant pattern observed. However seedlings were clumped on small scales. We ran the model under two contrasting scenarios representing hypotheses that explain the clumping of seedlings and the random distribution of trees. One hypothesis is that there is spatial heterogeneity in seed distribution, germination and seedling mortality, but that these heterogeneities are not correlated with each other in space. The second hypothesis assumes a correlation between these heterogeneities leading to areas suitable for establishment. However, the suitability of the sites is temporally variable. Furthermore, the second hypothesis assumes density‐dependent tree mortality due to competition. Both hypotheses lead to spatial distributions that are in qualitative agreement with the patterns observed in the field. Therefore, the classical view that a clumped seedling distribution and a random pattern of older trees is due to clumped regeneration and density‐dependent mortality may not hold for Acacia trees in the Negev.     

Seed protection through dispersal by African savannah elephants (Loxodonta africana africana) in northern Tanzania.

We examined effectiveness of African savannah elephant dung as a protective barrier for seeds of three tree species, Acacia tortilis Hayne, Tamarindus indica L. and Ximenia aegyptiaca L. Seeds were collected from dung and underneath fruiting trees in Tarangire National Park, Tanzania. Experimental treatments were established to test: (i) the efficacy of dung in protecting seeds of A. tortilis from bruchid beetle infestation and the role of animals larger than insects in removing seeds; (ii) the same tenets as in Experiment 1, using seeds of T. indica; and (iii) the effect of distance on survival of seeds of X. aegyptiaca. Sites were established during two field seasons underneath conspecific trees, where seed predation was likely highest. Repeated‐measures two‐way ANOVA indicated that there was no treatment effect for Experiment 1. For Experiments 2 and 3 in October 2013, seeds in dung experienced less beetle infestation than fresh seeds. Repeated‐measures two‐way ANOVA and Tukey's HSD indicated that treatment effect differed among all treatments. Passed seeds at distances ≥5 m experienced less beetle infestation than fresh seeds underneath conspecifics. African savannah elephants appear to be important seed dispersers of these three tree species.     

Large African herbivores,bruchid beetles and their interactions with Acacia seeds

This study investigated the interactions of large African herbivores and bruchid seed beetles with Acacia seeds. The germination of bruchid-infested and uninfested seeds was compaed. The effects of pod consumption by large herbivores on bruchid infestation and seed germination was also assessed. Bruchid-infested seeds did germinate, and the germination of bruchid-infested and uninfested A. tortilis, A. nilotica and A. hebeclade seeds did not differ. Pod ingestion by large herbivores lowered the bruchid infestation of consumed and defaecated seeds compared to uningested seeds. Uninfested, ingested and voided A. tortilis seeds germinated seeds. Furthermore, infested A. tortilis seeds egested by giraffe, kudu and ostrich germinated better than infested, uningested seeds. Pod ingestion by large herbivores may reduce bruchid infestation, increase Acacia seed germination and therefore increase potential Acacia seedling recruitment.     

Directed dispersal of mistletoe (Plicosepalus acaciae) by Yellow-vented Bulbuls (Pycnonotus xanthopygos)

We studied avian dispersal of seeds from the hemiparasitic mistletoe Plicosepalus acaciae (Loranthaceae) to its tree hosts Acacia raddiana and A. tortilis in the Syrian–African Rift (Arava) valley, Israel. The Yellow-vented Bulbul (Pycnonotus xanthopygos) was the sole avian visitor observed feeding on mistletoe fruits. Bulbuls consumed mistletoe fruits whenever they were available, but the fruits only constituted a significant portion of the diet (71% of foraging attempts) when they were most abundant. These birds are potentially good dispersal vectors of P. acaciae because they swallowed the fruit whole and defecated viable seeds that were covered in a viscid pulp, which allowed the seeds to adhere to substrates when voided. In addition, bulbuls spent a large proportion (66–93%) of total observation time perched in Acacia trees, allowing for directed dispersal. Ephemeral river valleys (wadis) with high mistletoe infection were adjacent to those containing no infections, demonstrating that mistletoe dispersal is common within, but not among wadis. This is consistent with the flight behaviour in bulbuls, which do not typically move among wadis. We combined data on bulbul movements between Acacia trees with transit times of mistletoe seeds to create a hypothetical seed shadow as a function of distance from the parent mistletoe plant. Because they are directed dispersers, the movement patterns of bulbuls may explain the current distribution of P. acaciae in the Arava valley.     

Water status of isolated Negev desert populations of Acacia raddiana with different mortality levels

Many populations of native Acacia raddiana trees in ephemeral riverbeds in the Negev desert are suffering from high mortality. Road-building techniques that cut off water to trees in downstream populations and the pumping of aquifers for agriculture have been mooted as causes of this mortality. We studied the water relations of nine isolated populations of these trees using a pressure chamber and stable carbon isotopes in order to determine whether current water stress could be the cause of this mortality. High mortality populations had more negative water potentials, lower relative water contents, and lower cell water volumes at full turgor than low mortality populations. Thus, mortality is correlated with current inter-population differences in water stress. However, there was no difference in water potentials between trees upstream and downstream of roads. Furthermore, greater water stress in trees after a dry winter than in the previous summer indicates that these trees are largely drawing on surface floods for water. Thus, road-building practices and aquifer pumping are unlikely causes of current water stress. Hence, water stress is linked to mortality in these trees, although our results indicate that irregular water stress (perhaps in drought years only) is probably the cause of this mortality.     

Acacia trees as keystone species in Negev desert ecosystems

Abstract. The only trees in most of the Negev desert are 3 native Acacia species. We tested the hypothesis that they act as keystone species as a result of the improved soil conditions under their canopies. Furthermore, because many Acacia populations suffer high levels of mortality due to water stress, we tested whether trees in high mortality populations had diminished effects on plant species and soil quality under their canopies. We show that plant species diversity beneath the tree canopies is higher than in the surrounding areas. There was also a clearly identifiable suite of species with higher occurrence under the trees. Plant species composition differed significantly between high and low mortality sites. However, there was higher species diversity in high mortality sites and under trees with higher water stress. Soil nutrient content was higher under the trees than in the open areas, especially under larger trees and trees with higher water status. The results indicate that there is a combination of positive and negative effects of Acacia trees on the under‐canopy environment, which may include positive effects of higher soil nutrients and a negative influence of higher soil salinity.     

Interactions of Acacia raddiana with herbaceous vegetation change with intensity of abiotic stress

Theoretical models predict that the relative importance of facilitation and competition may vary inversely across gradients of abiotic stress. However, these predictions have not been thoroughly tested in the field, especially in arid and semi-arid environments. In this study, we evaluated how the net effect of Acacia tortilis subsp. raddiana (syn. A. raddiana) trees on the herbaceous species varies across a gradient of water stress in arid Tunisian ecosystems. Our results show that the influence of trees on the herbaceous stratum is beneficial. An Acacia tree improves the richness of herbaceous species around it by two to three times. The positive effect of Acacia raddiana on species composition is characterised in particular by a better development of some species of high pastoral value such as Cenchrusciliaris, Cynodon dactylon, Eragrostis papposa, Sisymbrium irio and Chenopodium album. We fitted the relationship between seasonal rainfall and the relative neighbour effect index, which varied across this gradient, to a linear model. Our results show that herbaceous plant cover, plant biomass, plant density and diversity are higher under a tree canopy and this positive interaction still increase with higher abiotic stress conditions.     

Spatio‐temporal dynamics of an Ips acuminatus outbreak and implications for management

• 1 Understanding spatio‐temporal processes of bark beetle infestations is crucial for predicting beetle behaviour and aiding management decisions aiming to prevent or mitigate tree mortality. We recorded the spatial and temporal distribution of killed trees during the 5‐year period of an Ips acuminatus outbreak.
• 2 Killed trees were always grouped in well‐defined patches (infestation spots). In years of high population density, infestation spots were large and aggregated, whereas, in years of low density, infestation spots were small and weakly aggregated or randomly distributed within the study area.
• 3 Most trees were killed in the spring by beetles that had hibernated but, in some years, trees were also killed in the summer by new‐generation beetles originating from spring attacks. Spring‐killed trees always formed new infestation spots at new locations (i.e. spot proliferation). By contrast, summer‐killed trees always occurred at the edge of active spots established in the spring, thus resulting in spot growth.
• 4 With regard to management strategies, the results obtained in the present study suggest that areas located in close proximity to infestations of the previous year should be prioritized for risk assessment. Because large spots account for most of the observed tree mortality, the cut‐and‐remove method should be focused on these spots as soon as crown discoloration appears in the summer. If applied timely, this strategy will remove the new‐generation beetles originating from the spring attacks before they emerge and also reduce the risk of spot growth.

     

Population differences in host use by a seed-beetle: local adaptation, phenotypic plasticity and maternal effects

For insects that develop inside discrete hosts, both host size and host quality constrain offspring growth, influencing the evolution of body size and life history traits. Using a two-generation common garden experiment, we quantified the contribution of maternal and rearing hosts to differences in growth and life history traits between populations of the seed-feeding beetle Stator limbatus that use a large-seeded host, Acacia greggii, and a small-seeded host, Pseudosamanea guachapele. Populations differed genetically for all traits when beetles were raised in a common garden. Contrary to expectations from the local adaptation hypothesis, beetles from all populations were larger, developed faster and had higher survivorship when reared on seeds of A. greggii (the larger host), irrespective of their native host. We observed two host plant-mediated maternal effects: offspring matured sooner, regardless of their rearing host, when their mothers were reared on P. guachapele (this was not caused by an effect of rearing host on egg size), and females laid larger eggs on P. guachapele. This is the first study to document plasticity by S. limbatus in response to P. guachapele, suggesting that plasticity is an ancestral trait in S. limbatus that likely plays an important role in diet expansion. Although differences between populations in growth and life history traits are likely adaptations to their host plants, host-associated maternal effects, partly mediated by maternal egg size plasticity, influence growth and life history traits and likely play an important role in the evolution of the breadth of S. limbatus’ diet. More generally, phenotypic plasticity mediates the fitness consequences of using novel hosts, likely facilitating colonization of new hosts, but also buffering herbivores from selection post-colonization. Plasticity in response to novel versus normal hosts varied among our study populations such that disentangling the historical role of plasticity in mediating diet evolution requires the consideration of evolutionary history.     

Micropropagation of the Acacia species—A review

Summary The genus Acacia comprises many species which are important for firewood. fodder, tannin, pulpwood, shelterbelts, and soil improvement. Species of Acacia are dispersed widely in tropical and subtropical regions of Australia. South America, Asia, and Africa. In the past, the majority of forest trees have been propagated through the traditional family forestry method, where trees are grown from seeds and propagated sexually. Clonal propagation through tissue culture offers an alternative to vegetative practices and has the potential to provide high multiplication rates of uniform genotypes, resulting in short-term gains. Extensive in vitro research and success have been achieved for a number of forest trees, including the Pinus, Eucalyptus, and Acacia species. Acacia mearnsii is fast becoming one of the most important trees in South African forestry. For this reason it is important to employ biotechnology to enable the industry to compete in the international market. However, for these procedures to be commercially viable, the post-culture performance in the field must be improved because the initial financial outlay is high. This review provides an overview on in vitro work done on the Acacia species.     

Evolution and ecology meet molecular genetics: adaptive phenotypic plasticity in two isolated Negev desert populations of Acacia raddiana at either end of a rainfall gradient

Background and Aims

The ecological, evolutionary and genetic bases of population differentiation in a variable environment are often related to the selection pressures that plants experience. We compared differences in several growth- and defence-related traits in two isolated populations of Acacia raddiana trees from sites at either end of an extreme environmental gradient in the Negev desert.

Methods

We used random amplified polymorphic DNA (RAPD) to determine the molecular differences between populations. We grew plants under two levels of water, three levels of nutrients and three levels of herbivory to test for phenotypic plasticity and adaptive phenotypic plasticity.

Key Results

The RAPD analyses showed that these populations are highly genetically differentiated. Phenotypic plasticity in various morphological traits in A. raddiana was related to patterns of population genetic differentiation between the two study sites. Although we did not test for maternal effects in these long-lived trees, significant genotype × environment (G × E) interactions in some of these traits indicated that such plasticity may be adaptive.

Conclusions

The main selection pressure in this desert environment, perhaps unsurprisingly, is water. Increased water availability resulted in greater growth in the southern population, which normally receives far less rain than the northern population. Even under the conditions that we defined as low water and/or nutrients, the performance of the seedlings from the southern population was significantly better, perhaps reflecting selection for these traits. Consistent with previous studies of this genus, there was no evidence of trade-offs between physical and chemical defences and plant growth parameters in this study. Rather, there appeared to be positive correlations between plant size and defence parameters. The great variation in several traits in both populations may result in a diverse potential for responding to selection pressures in different environments.     

Convergence Between Dung Beetle Assemblages of a Post-Mining Vegetational Chronosequence and Unmined Dune Forest

Abstract In Maputaland, South Africa vegetative and microclimatic changes on mined dunes drive the composition of the dung beetle fauna toward convergence with that in natural dune forest on unmined dunes. We assessed the pattern of these changes using a 23‐year vegetational chronosequence on mined dunes, which passes from grassland (approximately 1 year) to open Acacia shrubland thicket to Acacia karroo‐dominated woodland (approximately 9 years). Across this sequence, which represents successional stages in the restoration of dune forest, there was a sequential trend toward convergence in dung beetle species composition in both the entire species complement and, particularly, in shade specialist species. However, species abundance patterns showed a trend toward convergence only in early chronosequence Acacia woodland, followed by a decline in similarity between dung beetle assemblages of older Acacia woodland and unmined natural forest. This trend toward divergence was common both to the entire species complement, which includes widespread taxa, and to species endemic to Maputaland or the east coast. These trends in similarity and dissimilarity between dung beetle assemblages closely parallel the greater physiognomic and microclimatic similarity between early Acacia woodland and natural forest and the relative dissimilarity of older Acacia woodland. In conclusion, although percentage similarities between dung beetle assemblages of approximately 12‐year woodland and natural forests were comparable with those between each natural forest stand, decline in similarity in older woodland stands suggests that lasting convergence in dung beetle species abundance will only be attained once the Acacia woodland is replaced by secondary natural forest.     

Renato Perotti

Summary

The author studies two sets of sub-fossil woods belonging to the roman buildings of Ghirza (North Africa). The woods were not well preserved and two small to allow an easy identification. They belong undoubtly to the genus Acacia. The species probably are: Acacia raddiana Savi and Acacia scorpioides (L.) Chev. Both these species grow today in the Saharian desert and are cultivated in the North African Oasis.     

The production, storage and viability of seeds of Acacia karroo and A. nilotica in a grassy savanna in KwaZulu-Natal, South Africa

African Acacias are often major contributors to the progressive increase in the woody component of savannas, a phenomenon commonly referred to as bush encroachment. They produce large quantities of seed and may have large soil‐stored seed banks. In Hluhluwe–Umfolozi Park, the number of adult Acacia nilotica trees per hectare far exceed that of A. karroo adults. The relative dominance is reversed in the juvenile stage with A. karroo outnumbering A. nilotica threefold outside closed woodlands. Acacia karroo trees were smaller than A. nilotica trees on average, but produced more seeds for a given basal diameter size class. Acacia karroo showed less bruchid infestation than A. nilotica at all stages of pod development. Unlike A. nilotica, a proportion of A. karroo seeds was able to germinate after bruchid attack. We detected no difference between the two species in the soil‐stored seed bank or in the viability of seeds found in the seed bank.     

Evaluation of traps used to monitor southern pine beetle aerial populations and sex ratios

1 Various kinds of traps have been employed to monitor and forecast population trends of the southern pine beetle (Dendroctonus frontalis Zimmermann; Coleoptera: Scolytidae), but their accuracy in assessing pine‐beetle abundance and sex ratio in the field has not been evaluated directly. 2 In this study, we used fluorescent powder to mark pine beetles emerging from six isolated infestations. We then compared estimates of total abundances and proportions of males emerging from within each infestation to the estimates from three types of traps: passive sticky traps (2, 5, 10 and 20 m away from the source of beetles), multi‐funnel traps baited with pine beetle attractants (100 m away) and pine trees baited with attractants (also 100 m away). 3 We found that the proportion of males captured in traps was significantly affected by the type of trap used. 4 Within an infestation, equal proportions of males and females were marked (0.53 ± 0.02 males; mean ± SE), but the proportions captured in trap trees and passive traps were more female biased (0.42 ± 0.03 and 0.46 ± 0.01 males, respectively). On the other hand, funnel traps provided an estimate of the proportion of males that was nearly identical to the proportion from within infestations (0.51 ± 0.03). 5 Numbers of marked beetles captured in traps were uncorrelated with the numbers of marked beetles emerging from the focal infestations. This suggests that traps positioned around an infestation may not be effective at estimating relative abundances of beetles within the infestation.