Testing for allelopathic effects in plant competition: does activated carbon disrupt plant symbioses?

Activated carbon (AC) is widely used in ecological studies to elucidate the role of allelopathic substances in interspecific plant competition. However, by adsorbing chemical signalling compounds AC may also have negative effects on plants with symbiosis partners such as arbuscular mycorrhizal fungi and rhizobia. Here we test whether addition of AC has detrimental effects on the mycorrhizal root colonization of the native forb Plantago lanceolata and the exotic legume Lupinus polyphyllus, the nodulation of L. polyphyllus, and the nutrient uptake and growth of the plants growing in intra- and interspecific competition. Allelopathic effects probably occurred in the germination and seedling establishment phase when P. lanceolata suffered from the presence of L. polyphyllus. However, this negative effect of L. polyphyllus on P. lanceolata was not ameliorated by AC addition. AC negatively affected L. polyphyllus root biomass in week 4, and root and shoot biomass of P. lanceolata in week 9 of the experiment; both effects were independent of the presence and absence of the competing plant species. Mycorrhizal root colonization of both plant species was reduced in the presence of AC, although the effect tended to be stronger for L. polyphyllus. No significant effect of AC on the nodulation of L. polyphyllus was detected. P. lanceolata was the superior competitor and led to reduced biomasses of L. polyphyllus in interspecific competition. We conclude that AC can reduce the mycorrhization and performance of plants which may lead to changes in interspecific competition without the involvement of allelopathy. Contrary to former studies the AC used in our study did not enhance the nutrient availability for the plants, but reduced plant growth and mycorrhization. We suggest that the nutrient properties of the used AC are of crucial importance for the direction and the mechanisms of the effects and should always be reported.     

Plant nitrogen acquisition and interactions under elevated carbon dioxide: impact of endophytes and mycorrhizae

Both endophytic and mycorrhizal fungi interact with plants to form symbiosis in which the fungal partners rely on, and sometimes compete for, carbon (C) sources from their hosts. Changes in photosynthesis in host plants caused by atmospheric carbon dioxide (CO₂) enrichment may, therefore, influence those mutualistic interactions, potentially modifying plant nutrient acquisition and interactions with other coexisting plant species. However, few studies have so far examined the interactive controls of endophytes and mycorrhizae over plant responses to atmospheric CO₂ enrichment. Using Festuca arundinacea Schreb and Plantago lanceolata L. as model plants, we examined the effects of elevated CO₂ on mycorrhizae and endophyte (Neotyphodium coenophialum) and plant nitrogen (N) acquisition in two microcosm experiments, and determined whether and how mycorrhizae and endophytes mediate interactions between their host plant species. Endophyte‐free and endophyte‐infected F. arundinacea varieties, P. lanceolata L., and their combination with or without mycorrhizal inocula were grown under ambient (400 μmol mol⁻¹) and elevated CO₂ (ambient + 330 μmol mol⁻¹). A ¹⁵N isotope tracer was used to quantify the mycorrhiza‐mediated plant acquisition of N from soil. Elevated CO₂ stimulated the growth of P. lanceolata greater than F. arundinacea, increasing the shoot biomass ratio of P. lanceolata to F. arundinacea in all the mixtures. Elevated CO₂ also increased mycorrhizal root colonization of P. lanceolata, but had no impact on that of F. arundinacea. Mycorrhizae increased the shoot biomass ratio of P. lanceolata to F. arundinacea under elevated CO₂. In the absence of endophytes, both elevated CO₂ and mycorrhizae enhanced ¹⁵N and total N uptake of P. lanceolata but had either no or even negative effects on N acquisition of F. arundinacea, altering N distribution between these two species in the mixture. The presence of endophytes in F. arundinacea, however, reduced the CO₂ effect on N acquisition in P. lanceolata, although it did not affect growth responses of their host plants to elevated CO₂. These results suggest that mycorrhizal fungi and endophytes might interactively affect the responses of their host plants and their coexisting species to elevated CO₂.     

Interactive effects of mycorrhizal fungi,salt stress,and competition on the herbivores of Baccharis halimifolia

1. Plant stress and association with mycorrhizal fungi have been shown to significantly influence plant quality, yet their roles in influencing plant–insect interactions remain unclear. Even less is known about how these factors might interact with or be modified by within‐trophic level interactions. 2. In the present study, the results of a factorial field experiment are reported in which the effects of within‐trophic‐level interactions, plant stress, and mycorrhizae on three herbivores of Baccharis halimifolia were examined. 3. Plant stress was increased by adding salt to the soil, and availability of mycorrhizal fungi was increased by inoculating plant roots. These treatments were applied to plants with either low or high densities of a competitor (Trirhabda baccharidis). 4. For the two leaf miners, Amauromyza maculosa and Liriomyza trifolii, increased soil salinity and high densities of the competitor Trirhabdabaccharidis resulted in significant decreases in density. Neither of these treatments affected the gall maker Neolasioptera lathami. 5. Mycorrhizal fungi increased the densities of all three herbivores, possibly by increasing foliar nitrogen levels. For the two leaf miners, there was also evidence that mycorrhizae ameliorated the negative effects of salt stress. There was also evidence that high levels of competition dampened the positive effects of mycorrhizae on the two leaf miners.     

Sexual differentiation and seasonal variation in response to conspecific and heterospecific acoustic signals

Interspecific territoriality is frequently reported between closely related species; however, few studies have demonstrated interspecific territoriality between distantly related species living in sympatry. We conducted playback experiments to investigate territorial behaviour in male and female White‐bellied Wrens (Uropsila leucogastra) in response to simulated conspecific and heterospecific intruders during the breeding and non‐breeding seasons. We explored whether heterospecific songs of the Happy Wren (Pheugopedius felix), a distantly related species and ecological competitor, elicited antagonistic responses from focal White‐bellied Wrens, and whether such responses differed between the sexes. We also examined whether male and female responses to conspecific and heterospecific rivals varied with season. We found that male White‐bellied Wrens always responded to conspecific song, and responded significantly more to heterospecific song compared to a control stimulus (Tropical Parula, Setophaga pitiayumi). In contrast, although female White‐bellied Wrens responded strongly to conspecific song, their response to heterospecific song did not differ significantly from the control stimulus. The proportion of males that responded to heterospecific songs and the proportion of females that responded to conspecific songs varied seasonally, showing significantly lower responses during the breeding season. The intense responses of male White‐bellied Wrens to playback of heterospecific songs suggest that they recognise ecological competitors based on their vocal signals. Furthermore, the decrease in agonistic interactions during the breeding season is in line with the hypothesis that aggressive behaviour may be detrimental to reproductive and parental activity, and the hypothesis that heterospecific animals pose less of a threat during the breeding season.     

Differential Response to Interspecific and Intraspecific Signals Amongst Chickadees

Black‐capped chickadees (Poecile atricapillus) and mountain chickadees (P. gambeli) have a similar vocal repertoire and share many other life history traits; yet, black‐capped chickadees are socially dominant to mountain chickadees where populations overlap. Previous research suggested that in contact zones, both species respond weakly to heterospecific songs during the breeding season, and have suggested minimal interspecific competition. However, both black‐capped and mountain chickadees discriminate between conspecific and heterospecific chick‐a‐dee calls, suggesting attention is paid to interspecific signals. We compared the responses of both black‐capped and mountain chickadees to conspecific and heterospecific chick‐a‐dee calls during the winter, when both species compete for the same food resources. We conducted an aviary playback experiment exposing both species to playback composed of heterospecific and conspecific chick‐a‐dee calls, which had been recorded in the context of finding food sources. Responses from the tested birds were measured by recording vocalizations and behaviour. Black‐capped chickadees responded significantly more to conspecific than to heterospecific stimuli, whereas the subordinate mountain chickadees responded to both mountain and black‐capped chickadee calls. Based upon the reactions to playbacks, our results suggest these two closely related species may differ in their perception of the relative threat associated with intra‐ versus interspecific competitors.     

Forest fire may disrupt plant–microbial feedbacks

Plant–microbial feedbacks are important drivers of plant community structure and dynamics. These feedbacks are driven by the variable modification of soil microbial communities by different plant species. However, other factors besides plant species can influence soil communities and potentially interact with plant–microbial feedbacks. We tested for plant–microbial feedbacks in two Eucalyptus species, E. globulus and E. obliqua, and the influence of forest fire on these feedbacks. We collected soils from beneath mature trees of both species within native forest stands on the Forestier Peninsula, Tasmania, Australia, that had or had not been burnt by a recent forest fire. These soils were subsequently used to inoculate seedlings of both species in a glasshouse experiment. We hypothesized that (i) eucalypt seedlings would respond differently to inoculation with conspecific versus heterospecific soils (i.e., exhibit plant–microbial feedbacks) and (ii) these feedbacks would be removed by forest fire. For each species, linear mixed effects models tested for differences in seedling survival and biomass in response to inoculation with conspecific versus heterospecific soils that had been collected from either unburnt or burnt stands. Eucalyptus globulus displayed a response consistent with a positive plant–microbial feedback, where seedlings performed better when inoculated with conspecific versus heterospecific soils. However, this effect was only present when seedlings were inoculated with unburnt soils, suggesting that fire removed the positive effect of E. globulus inoculum. These findings show that external environmental factors can interact with plant–microbial feedbacks, with possible implications for plant community structure and dynamics.     

Intra versus interspecific interactions of ladybeetles (Coleoptera: Coccinellidae) attacking aphids

Summary The nature and relative strengths of intra versus interspecific interactions among foraging ladybeetle larvae were studied experimentally by measuring short-term growth rates of predators and reductions in population sizes of prey in laboratory microcosms. In these microcosms, ladybeetle larvae foraged singly or as conspecific or heterospecific pairs, for pea aphids on bean plants over a two-day period. Similarly sized third instar larvae ofHippodamia convergens andH. tredecimpunctata, H. convergens andH. sinuata, andH. convergens andCoccinella septempunctata, were tested in experiments designed to ensure that paired larvae experienced moderate competition. Interspecific competition in these experiments did not differ significantly from intraspecific competition, in that an individual's weight gain did not depend on whether its competitor was heterospecific or conspecific. Furthermore, aphid populations were reduced equally by heterospecific and conspecific pairs. These results suggest that there is little or no difference between intra and interspecific interactions among larvae of these ladybeetles when two similarly sized individuals co-occur on a host plant. Thus, the species diversityper se of assemblages of ladybeetle larvae may have little influence over the short term on the reduction of aphid populations by ladybeetle predation.     

Pollen competition between two sympatric Orchis species (Orchidaceae): the overtaking of conspecific of heterospecific pollen as a reproductive barrier

The frequency of hybrid formation in angiosperms depends on how and when heterospecific pollen is transferred to the stigma, and on the success of that heterospecific pollen at fertilising ovules. We applied pollen mixtures to stigmas to determine how pollen interactions affect siring success and the frequency of hybrid formation between two species of Mediterranean deceptive orchid. Plants of Orchis italica and O. anthropophora were pollinated with conspecific and heterospecific pollen (first conspecific pollen then heterospecific pollen and vice versa) and molecular analysis was used to check the paternity of the seeds produced. In this pair of Mediterranean orchids, competition between conspecific and heterospecific pollen functions as a post‐pollination pre‐zygotic barrier limiting the frequency of the formation of hybrids in nature. Flowers pollinated with heterospecific pollen can remain receptive for the arrival of conspecific pollen for a long time. There is always an advantage of conspecific pollen for fruit formation, whether it comes before or after heterospecific pollen, because it overtakes the heterospecific pollen. The conspecific pollen advantage exhibited in O. italica and O. anthropophora is likely to result from the reduced germination of heterospecific pollen or retarded growth of heterospecific pollen tubes in the stigma and ovary. Overall, the results indicate that our hybrid zone represents a phenomenon of little evolutionary consequence, and the conspecific pollen advantage maintains the genetic integrity of the parental species.     

Mycorrhizae transfer carbon from a native grass to an invasive weed: evidence from stable isotopes and physiology

Invasive exotic weeds pose one of the earth's most pressing environmental problems. Although many invaders completely eliminate native plant species from some communities, ecologists know little about the mechanisms by which these exotics competitively exclude other species. Mycorrhizal fungi radically alter competitive interactions between plants within natural communities, and a recent study has shown that arbuscular mycorrhizal (AM) fungi provide a substantial competitive advantage to spotted knapweed, Centaurea maculosa, a noxious perennial plant that has spread throughout much of the native prairie in the northwestern U.S. Here we present evidence that this advantage is potentially due to mycorrhizally mediated transfer of carbon from a native bunchgrass, Festuca idahoensis, to Centaurea. Centaurea maculosa, Festuca idahoensis (Idaho fescue, C₃), and Bouteloua gracilis (blue gramma, C₄) were grown in the greenhouse either alone or with Centaurea in an incomplete factorial design with and without AM fungi. Centaurea biomass was 87–168% greater in all treatments when mycorrhizae were present in the soil (P < 0.0001). However, Centaurea biomass was significantly higher in the treatment with both mycorrhizae and Festuca present together than in any other treatment combination (P < 0.0001). This high biomass was attained even though Centaurea photosynthetic rates were 14% lower when grown with Festuca and mycorrhizae together than when grown with Festuca without mycorrhizae. Neither biomass nor photosynthetic rates of Centaurea were affected by competition with the C₄ grass Bouteloua either with or without mycorrhizae. The stable isotope signature of Centaurea leaves grown with Festuca and mycorrhizae was more similar to that of Festuca, than when Centaurea was grown alone with mycorrhizae (P = 0.06), or with Festuca but without mycorrhizae (P = 0.09). This suggests that carbon was transferred from Festuca to the invasive weed. We estimated that carbon transferred from Festuca by mycorrhizae contributed up to 15% of the aboveground carbon in Centaurea plants. Our results indicate that carbon parasitism via AM soil fungi may be an important mechanism by which invasive plants out compete their neighbors, but that this interaction is highly species-specific.     

Nutrient enrichment enhances hidden differences in phenotype to drive a cryptic plant invasion

Many mechanisms of invasive species success have been elucidated, but those driving cryptic invasions of non‐native genotypes remain least understood. In one of the most successful cryptic plant invasions in North America, we investigate the mechanisms underlying the displacement of native Phragmites australis by its Eurasian counterpart. Since invasive Phragmites’ populations have been especially prolific along eutrophic shorelines, we conducted a two‐year field experiment involving native and invasive genotypes that manipulated nutrient level and competitor identity (inter‐ and intra‐genotypic competition) to assess their relative importance in driving the loss of native Phragmites. Inter‐genotypic competition suppressed aboveground biomass of both native and invasive plants regardless of nutrient treatment (～ 27%), while nutrient addition disproportionately enhanced the aboveground biomass (by 67%) and lateral expansion (by > 3 × farther) of invasive Phragmites. Excavation of experimental plots indicated that nutrient addition generates these differences in aboveground growth by differentially affecting rhizome production in invasive vs native plants; invasive rhizome biomass and rhizome length increased by 595% and 32% with nutrient addition, respectively, while natives increased by only 278% and 15%. Regardless of nutrient level, native rhizomes produced twice as many roots compared to invasives, which field surveys revealed are heavily infected with mycorrhizal symbionts. These results suggest that native Phragmites competes well under nutrient‐limited conditions because its rhizomes are laden with nutrient‐harvesting roots and mycorrhizae. Invasive Phragmites’ vigorous aboveground response to nutrients and scarcity of lateral roots, in contrast, may reflect its historic distribution in eutrophic Eurasian wetlands and correspond to its prevalence in New England marshes characterized by elevated nutrient availability and relaxed nutrient competition. These findings reveal that discrete differences in phenotype can interact with anthropogenic modification of environmental conditions to help explain the success of cryptic invaders.     

Effects of mycorrhizae on growth and demography of tallgrass prairie forbs

The effects of arbuscular mycorrhizal (AM) symbiosis on ramet and genet densities, vegetative growth rates, and flowering of three forb species were studied in native tallgrass prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed for six growing seasons on replicate plots in an annually burned and an infrequently burned watershed with the fungicide benomyl. Benomyl reduced mycorrhizal root colonization to an average of 4.2%, approximately a two-thirds reduction relative to controls (13.7% colonization). Mycorrhizae influenced the population structure of these forbs. Although mycorrhizal suppression had no long-term effect on genet densities and no effect on ramet survivorship throughout the growing season, the number of ramets per individual was significantly increased such that ramet densities of all three species were approximately doubled in response to long-term mycorrhizal suppression. Effects of mycorrhizae on ramet growth and reproduction varied among species. Ramet growth rates, biomass, and flowering of Salvia azurea were greater in plots with active mycorrhizal symbiosis, whereas mycorrhizae reduced ramet growth rates and biomass of Artemesia ludoviciana. Aster sericeus ramet growth rates and biomass were unaffected by the fungicide applications, but its flowering was reduced.The pattern of responses of these three species to mycorrhizae differed considerably between the two sites of contrasting fire regime, indicating that the interaction of fire-induced shifts in resource availability and mycorrhizal symbiosis together modulates plant responses and the intensity and patterns of interspecific competition between and among tallgrass prairie grass and forb species. Further, the results indicate that effects of mycorrhizae on community structure are a result of interspecific differences in the balance between direct positive effects of the symbiosis on host plant performance and indirect negative effects mediated through altered competitive interactions.     

Mixed colonies of two species of congeneric stingless bees (Hymenoptera: Apinae,Meliponini) display environmentally‐acquired and endogenously‐produced recognition signals

Nestmate recognition is fundamental for the maintenance of social organization in insect nests. It is becoming well recognized that cuticle hydrocarbons mediate the recognition process, although the origin of recognition cues in stingless bees remains poorly explored. The present study investigates the effects of endogenously‐produced and environmentally‐acquired components in cuticular hydrocarbons in stingless bees. The tests are conducted using colonies of Plebeia droryana Friese and Plebeia remota Holmberg. Recognition tests are performed with four different groups: conspecific nestmates, conspecific non‐nestmates, heterospecifics and conspecific, genetically‐related individuals that emerge in a heterospecific nest. This last group is produced by introducing brood cells of P. droryana into a P. remota colony, and the resulting adult bees are tested for acceptance 10 days after emergence. For all groups, 15 individuals are sampled for chemical analysis. The results show the acceptance of all conspecific nestmates, and the rejection of almost every conspecific non‐nestmate and every heterospecific bee. Genetically‐related individuals emerging from heterospecific nests present intermediate rejection (66.7% rejection). Chemical analysis shows that P. droryana individuals emerging in a P. remota nest have small amounts of alkene and diene isomers found in P. remota cuticle that are not found in workers from the natal nest. The data clearly show that the majority of the compounds present in P. droryana cuticle are endogenously produced, although a few unsaturated compounds are acquired from the environment, increasing the chemical differences and, consequently, the rejection percentages.     

Temporal carry‐over effects in sequential plant–soil feedbacks

Plant–soil feedbacks (PSF) strongly influence plant performance. However, to what extent these PSF effects are persistent in the soil and how they are altered by species that subsequently condition the soil is unclear. Here we test how conspecific and heterospecific soil‐conditioning effects interact across different soil‐conditioning phases. We conducted a fully factorial glasshouse experiment where six plant species conditioned soils in two consecutive phases and measured the performance of Jacobaea vulgaris. The species that conditioned the soil during the second conditioning phase strongly determined the performance of J. vulgaris, but also the order and combination of species that conditioned the soil in the two phases accounted for a large part of the variance. For shoot biomass this interaction was the dominant variance component. We show that soil conditioning legacies carry‐over and interact with the conditioning effects of succeeding plants. In the field, species replacements at the patch level often appear to be unpredictable and we suggest that sequential feedbacks may explain these apparently unpredictable transitions.     

Mycorrhizal inoculant alleviates salt stress in<Emphasis Type="Italic"> Sesbania aegyptiaca</Emphasis> and<Emphasis Type="Italic"> Sesbania grandiflora</Emphasis> under field conditions: evidence for reduced sodium and improved magnesium uptake

A field experiment was conducted to examine the effect of the arbuscular mycorrhizal fungus Glomus macrocarpum and salinity on growth of Sesbania aegyptiaca and S. grandiflora. In the salt-stressed soil, mycorrhizal root colonisation and sporulation was significantly higher in AM-inoculated than in uninoculated plants. Mycorrhizal seedlings had significantly higher root and shoot dry biomass production than non-mycorrhizal seedlings grown in saline soil. The content of chlorophyll was greater in the leaves of mycorrhiza-inoculated as compared to uninoculated seedlings. The number of nodules was significantly higher in mycorrhizal than non-mycorrhizal plants. Mycorrhizal seedling tissue had significantly increased concentrations of P, N and Mg but lower Na concentration than non-mycorrhizal seedlings. Under salinity stress conditions both Sesbania sp. showed a high degree of dependence on mycorrhizae, increasing with the age of the plants. The reduction in Na uptake together with a concomitant increase in P, N and Mg absorption and high chlorophyll content in mycorrhizal plants may be important salt-alleviating mechanisms for plants growing in saline soil.     

Pollen competition and environmental effects on hybridization dynamics between Phlox drummondii and Phlox cuspidata

Pollen competition between species strongly influences hybridization dynamics in plants. By performing single- and mixed-donor pollinations, we show that soil Ca alters the outcome of interspecific pollen competition in the annual Phlox hybrid system of Phlox cuspidata and P. drummondii. In the absence of interspecific pollen competition, heterospecific pollen siring success of both species was influenced most strongly by the maternal growth environment, such that hybridization was facilitated when heterospecific pollen was deposited on stigmas of maternal plants growing in high Ca soils. When heterospecific pollen was forced to compete against conspecific pollen, however, the maternal growth environment did not influence hybridization, but the environmental origin of heterospecific pollen did, and this effect depended on the maternal species. Pollen of P. drummondii was more effective at outcompeting P. cuspidata pollen and preventing hybridization in P. drummondii dams when P. cuspidata pollen was derived from low Ca. Pollen competition within pistils of P. cuspidata was unaffected by pollen Ca environment. In situations in which P. cuspidata grows in lower soil Ca than P. drummondii, as has been documented in one population, these results suggest that the competitive ability of heterospecific pollen will be diminished by environmental effects of soil Ca. Thus, the environment in which pollen develops can influence interspecific pollen competition and hybridization frequency.     

接种AMF对菌根植物和非菌根植物竞争的影响

为了研究丛枝菌根真菌(arbuscular mycorrhizal fungus, AMF)对菌根植物与非菌根植物种间竞争的影响,以玉米(菌根植物)和油菜(非菌根植物)作为供试植物,分别进行间作、尼龙网分隔和单作,模拟这两种植物之间不同的竞争状态,接种丛枝菌根真菌Glomus intraradices和Glomus mosseae,比较菌根植物和非菌根植物的生长和磷营养状况,分析AMF侵染对植物种间竞争作用的影响。结果显示,与单作相比,间作模式下玉米的生物量及磷营养状况有所降低,但其菌根依赖性却有所提高。与不接种相比,接种处理显著降低了间作体系油菜根系的磷含量和磷吸收量,但趋于改善菌根植物玉米的磷营养状况。因此,接种AMF可以降低非菌根植物的磷营养状况及生物量,使得菌根植物的相对竞争能力明显提高,说明AMF在维持物种多样性方面有着重要的作用。     

Within‐species reproductive costs affect the asymmetry of satyrization in Drosophila

Understanding how species interactions influence their distribution and evolution is a fundamental question in evolutionary biology. Theory suggests that asymmetric reproductive interference, in which one species induces higher reproductive costs on another species, may be more important in delimiting species boundaries than interspecific competition over resources. However, the underlying mechanisms of such asymmetry remain unclear. Here, we test whether differences in within‐species reproductive costs determine the between‐species asymmetry of costs using three allopatric Drosophila species belonging to the melanogaster subgroup. Our results support this hypothesis, especially in a pair of insular species. Males of one species that induce costs to their conspecific females led to a 5‐fold increase of heterospecific females mortality with dead flies bearing spectacular large melanized wounds on their genitalia. Males of the other species were harmful neither to their conspecific nor heterospecific females. Comparative studies of within‐species reproductive costs may therefore be a valuable tool for predicting between‐species interactions and community structures.     

Effects of extreme weather events and legume presence on mycorrhization of Plantago lanceolata and Holcus lanatus in the field

Little is known about direct and indirect effects of extreme weather events on arbuscular mycorrhizal fungi (AMF) under field conditions. In a field experiment, we investigated the response of mycorrhization to drought and heavy rain in grassland communities. We quantified AMF biomass in soil, mycorrhization of roots of the grass Holcus lanatus and the forb Plantago lanceolata, as well as plant performance. Plants were grown in four‐species communities with or without a legume. We hypothesised that drought increases and heavy rain decreases mycorrhization, and that higher mycorrhization will be linked to improved stress resistance and higher biomass production. Soil AMF biomass increased under both weather extremes. Heavy rain generally benefitted plants and increased arbuscules in P. lanceolata. Drought neither reduced plant performance nor root mycorrhization. Arbuscules increased in H. lanatus several weeks after drought, and in P. lanceolata several weeks after heavy rain spells. These long‐lasting effects of weather events on mycorrhization highlight the indirect influence of climate on AMF via their host plant. Legume presence increased plant community biomass, but had only minor effects on mycorrhization. Arbuscule colonisation was negatively correlated with senescence during the dry summer. Mycorrhization and biomass production in P. lanceolata were positively related. However, increased mycorrhization was related to less biomass in the grass. AMF mycelium in soil might generally increase under extreme events, root colonisation, however, is host species specific. This might amplify community shifts in grassland under climate change by further increasing stress resistance of species that already benefit from changed precipitation.     

Influence of Mycorrhizal Inoculation, Inundation Period, Salinity, and Phosphorus Availability on the Growth of Two Salt Marsh Grasses, Spartina alterniflora Lois. and Spartina cynosuroides (L.) Roth., in Nursery Systems

Restoration of salt marsh ecosystems is an important concern in the eastern United States to mitigate damage caused by industrial development. Little attention has been directed to the mycorrhizal influence on plantings of salt marsh species to stabilize estuarine sediments and establish cover. In our study, seedlings of two salt marsh grasses, Spartina alterniflora and Spartina cynosuroides, were grown in soil with a commercial, mixed species inoculum of arbuscular mycorrhizal fungi. Plants were grown in experimental “ebb and flow” boxes, simulating three levels of tidal inundation, to which two levels of applied phosphorus (P) and two levels of salinity were imposed. After 2.5 months, S. alterniflora was poorly colonized by arbuscular mycorrhizae, developing only fungal hyphae and no arbuscules, but S. cynosuroides became moderately colonized. Mycorrhizal inoculation marginally improved growth and P and nitrogen (N) content of both plant species at low levels of P supply but significantly increased tillering in both plant species. This factor could be beneficial in enhancing ground cover during restoration procedures. Greater P availability increased the mycorrhizal status of S. cynosuroides and improved P nutrition of both plant species, despite a reduction in the root‐to‐shoot ratio. Increasing salinity reduced mycorrhizal colonization of S. alterniflora but not of S. cynosuroides. Growth and nutrient content of S. alterniflora was improved at higher levels of salinity, but only increased nutrient content in S. cynosuroides. Increased duration of tidal inundation decreased plant growth in both species, but tissue P and N concentrations were highest with the longest time of inundation in both species.     

Early singers attend to conspecific but not heterospecific behavioural cues at dawn

During the dawn chorus songbirds initiate singing activities just prior to sunrise. Environmental factors affect the timing of the dawn chorus, but relatively little is known about how behavioural cues influence chorus timing. We assessed whether early playback of conspecific and heterospecific song influenced chorus start times in two early‐singing temperate species, hermit thrush Catharus guttatus and veery C. fuscescens. We used arrays of GPS time‐synchronized recorders to record the dawn chorus at 30 locations. In each location, thirty minutes prior to natural chorus initiation, we broadcast: 1) an early singing species (hermit thrush or veery), 2) a late singing species (black‐throated green warbler Setophaga virens or Nashville warbler Oreothlypis ruficapilla), 3) a noise control, and 4) also recorded without playback (silent control). Playback treatments were separated by two‐three days at each location. Both hermit thrush and veery sang significantly earlier in response to conspecific playback compared to noise or silent controls. Both species also sang earlier in response to conspecific playback compared to the day before or the day after playback. Neither species sang earlier in response to heterospecific playback (regardless of whether the broadcast species was a naturally early or late song initiator) compared to noise or silent controls. Our results support the theory that the dawn chorus is a communication network where individuals attend primarily to conspecific cues; heterospecific song appears to have minimal impacts on the chorus start times of the two early‐singing species we investigated.