Fuel moisture content enhances nonadditive effects of plant mixtures on flammability and fire behavior

Fire behavior of plant mixtures includes a complex set of processes for which the interactive contributions of its drivers, such as plant identity and moisture, have not yet been unraveled fully. Plant flammability parameters of species mixtures can show substantial deviations of fire properties from those expected based on the component species when burnt alone; that is, there are nonadditive mixture effects. Here, we investigated how fuel moisture content affects nonadditive effects in fire behavior. We hypothesized that both the magnitude and variance of nonadditivity in flammability parameters are greater in moist than in dry fuel beds. We conducted a series of experimental burns in monocultures and 2‐species mixtures with two ericaceous dwarf shrubs and two bryophyte species from temperate fire‐prone heathlands. For a set of fire behavior parameters, we found that magnitude and variability of nonadditive effects are, on average, respectively 5.8 and 1.8 times larger in moist (30% MC) species mixtures compared to dry (10% MC) mixed fuel beds. In general, the moist mixtures caused negative nonadditive effects, but due to the larger variability these mixtures occasionally caused large positive nonadditive effects, while this did not occur in dry mixtures. Thus, at moister conditions, mixtures occasionally pass the moisture threshold for ignition and fire spread, which the monospecific fuel beds are unable to pass. We also show that the magnitude of nonadditivity is highly species dependent. Thus, contrary to common belief, the strong nonadditive effects in mixtures can cause higher fire occurrence at moister conditions. This new integration of surface fuel moisture and species interactions will help us to better understand fire behavior in the complexity of natural ecosystems.     

Nitrogen enrichment contributes to positive responses to soil microbial communities in three invasive plant species

Increased resource availability and feedbacks with soil biota have both been invoked as potential mechanisms of plant invasion. Nitrogen (N) deposition can enhance invasion in some ecosystems, and this could be the result of increased soil N availability as well as shifts in soil biota. In a two-phase, full-factorial greenhouse experiment, we tested effects of N availability and N-impacted soil communities on growth responses of three Mediterranean plant species invasive in California: Bromus diandrus, Centaurea melitensis, and Hirschfeldia incana. In the first phase, plants were grown individually in pots and inoculated with sterile soil, soil from control field plots or soil from high N addition plots, and with or without supplemental N. In the second phase, we grew the same species in soils conditioned in the first phase. We hypothesized growth responses would differ across species due to species-specific relationships with soil biota, but overall increased N availability and N-impacted soil communities would enhance plant growth. In the first phase, Centaurea had the greatest growth response when inoculated with N-impacted soil, while Bromus and Hirschfeldia performed best in low N soil communities. However, in phase two all species exhibited positive growth responses in N-impacted soil communities under high N availability. While species may differ in responses to soil biota and N, growth responses to soils conditioned by conspecifics appear to be most positive in all species under high N availability and/or in soil communities previously impacted by simulated N deposition. Our results suggest N deposition could facilitate invasion due to direct impacts of soil N enrichment on plant growth, as well as through feedbacks with the soil microbial community.     

Contrasting trait responses to latitudinal climate variation in two lineages of an invasive grass

Plants are expected to respond to global environmental change through shifts in functional traits and in their ranges. These shifts could alter productivity and interactions among species or genetic lineages, ultimately leading to changes in distributions and abundance. In particular, cosmopolitan species are predicted to increase growth with decreasing latitude due to differences in climate and temperature. The pattern of changes in growth may vary among genotypes within species, leading to different responses with latitude. To evaluate whether climate can affect geographically distinct genotypes of cosmopolitan invasive species differently, we evaluated the trait responses of two lineages of the common reed, Phragmites australis, to variation in environmental conditions spanning North America’s Atlantic coast. Using three reciprocal transplant common gardens, we tested for the effects of garden location and plant lineage on traits related to biomass production, flowering frequency, leaf morphology, and leaf-level physiology. We found that aboveground biomass, stem density, and flowering frequency responded non-linearly to increasing latitude in one or both lineages. These results suggest that measures of plant traits over narrow latitudinal ranges may not accurately reflect organismal-level responses to global change at broad spatial scales. Given the responses to latitude that we observed in P. australis, we propose that feedbacks between growth and reproductive rate will influence range shifts in these two lineages. Such range shifts could lead to genetic admixtures, subsequently yielding more productive, locally-adapted genotypes.     

Positive Feedbacks to Fire-Driven Deforestation Following Human Colonization of the South Island of New Zealand

Altered fire regimes in the face of climatic and land-use change could potentially transform large areas from forest to shorter-statured or open-canopy vegetation. There is growing concern that once initiated, these nonforested landscapes could be perpetuated almost indefinitely through a suite of positive feedbacks with fire. The rapid deforestation of much of New Zealand following human settlement (ca. 750 years ago) provides a rare opportunity to evaluate the feedback mechanisms that facilitated such extensive transformation and thereby help us to identify factors that confer vulnerability or resilience to similar changes in other regions. Here we evaluate the structure of living and dead vegetation (fuel loading) and microclimate (fuel moisture) in beech (Nothofagaceae) forests and adjacent stands that burned within the last 60–140 years and are dominated by mānuka (Leptospermum scoparium) or kānuka (Kunzea spp.). We show that the burning of beech forests initiates a positive feedback cycle whereby the loss of microclimatic amelioration under the dense forest canopy and the abundant fine fuels that dry readily beneath the sparse mānuka/kānuka canopy enables perpetuation of these stands by facilitating repeated burning. Beech regeneration was limited to a narrow zone along the margin of unburned stands. The high flammability of vegetation that develops after fire and the long time to forest recovery were the primary factors that facilitated extensive deforestation with the introduction of human-ignited fire. Evaluating these two characteristics may be key to determining which regions may be near a tipping point where relatively small land-use- or climatically driven changes to fire regimes could bring about extensive deforestation.     

Species mixture effects on flammability across plant phylogeny: the importance of litter particle size and the special role for non‐Pinus Pinaceae

Fire affects and is affected by plants. Vegetation varies in flammability, that is, its general ability to burn, at different levels of ecological organization. To scale from individual plant traits to community flammability states, understanding trait effects on species flammability variation and their interaction is important. Plant traits are the cumulative result of evolution and they show, to differing extents, phylogenetic conservatism. We asked whether phylogenetic distance between species predicts species mixture effects on litterbed flammability. We conducted controlled laboratory burns for 34 phylogenetically wide‐ranging species and 34 random two‐species mixtures from them. Generally, phylogenetic distance did not predict species mixture effects on flammability. Across the plant phylogeny, most species were flammable except those in the non‐Pinus Pinaceae, which shed small needles producing dense, poorly ventilated litterbeds above the packing threshold and therefore nonflammable. Consistently, either positive or negative dominance effects on flammability of certain flammable or those non‐flammable species were found in mixtures involving the non‐Pinus Pinaceae. We demonstrate litter particle size is key to explaining species nonadditivity in fuelbed flammability. The potential of certain species to influence fire disproportionately to their abundance might increase the positive feedback effects of plant flammability on community flammability state if flammable species are favored by fire.     

Competition overwhelms the positive plant–soil feedback generated by an invasive plant

Invasive plant species can modify soils in a way that benefits their fitness more than the fitness of native species. However, it is unclear how competition among plant species alters the strength and direction of plant–soil feedbacks. We tested how community context altered plant–soil feedback between the non-native invasive forb Lespedeza cuneata and nine co-occurring native prairie species. In a series of greenhouse experiments, we grew plants individually and in communities with soils that differed in soil origin (invaded or uninvaded by L. cuneata) and in soils that were live vs. sterilized. In the absence of competition, L. cuneata produced over 60% more biomass in invaded than uninvaded soils, while native species performance was unaffected. The absence of a soil origin effect in sterile soil suggests that the positive plant–soil feedback was caused by differences in the soil biota. However, in the presence of competition, the positive effect of soil origin on L. cuneata growth disappeared. These results suggest that L. cuneata may benefit from positive plant–soil feedback when establishing populations in disturbed landscapes with few interspecific competitors, but does not support the hypothesis that plant–soil feedbacks influence competitive outcomes between L. cuneata and native plant species. These results highlight the importance of considering whether competition influences the outcome of interactions between plants and soils.     

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.     

Native ecosystem engineer facilitates recruitment of invasive crab and native invertebrates

Native ecosystem engineers that add physical structure to ecosystems can facilitate invasive species. In this study we determined the effects of the native tube-forming serpulid worm, Galeolaria caespitose on the recruitment of the invasive New Zealand porcelain crab, Petrolisthes elongatus, and whether invasive crab recruitment was related to the recruitment of native species. P. elongatus is abundant beneath intertidal rocks around Tasmania, southern Australia, and the underside of these rocks is usually covered with a calcareous matrix formed by the serpulid. We used an experimental approach to investigate whether rocks, serpulids on the underside of rocks and adult P. elongatus influenced the recruitment P. elongatus and native communities. P. elongatus and native invertebrates only recruited in the presence of rocks indicating the importance of rock as primary recruitment habitat. Moreover, the presence of serpulids on the underside of rocks significantly increased the recruitment of P. elongatus and native invertebrates compared to rocks without serpulids. Rocks with higher densities of adult P. elongatus at the end of the experiment also had higher densities of P. elongatus recruits. The density of P. elongatus recruits did not influence native species richness and abundance although there was some evidence that high P. elongatus recruitment was correlated with shifts in native community structure. We have shown that a native ecosystem engineer facilitates recruitment of an invasive crab but this does not appear to influence the recruitment of native species.     

Invasive alien plants of Russia: insights from regional inventories

Recent research on plant invasions indicates that some parts of the world are understudied with temperate Asia among them. To contribute towards closing this gap, we provide a standardized list of invasive alien plant species with their distributions in 45 Russian regions, and relate the variation in their richness to climate, socioeconomic parameters and human influence. In total, we report 354 invasive alien species. There are, on average, 27 ± 17 (mean ± SD) invasive plants per region, and the invasive species richness varies from zero in Karelia to 71 in Kaluga. In the European part of Russia, there are 277 invasive species in total, in Siberia 70, and in the Far East 79. The most widespread invaders are, in terms of the number of regions from which they are reported, Acer negundo, Echinocystis lobata (recorded in 34 regions), Erigeron canadensis and Elodea canadensis (recorded in 30 regions). Most invasive species in Russia originate from other parts of temperate Asia and Europe. There were significant differences in the representation of life forms between the European, Siberian and Far East biogeographical regions, with perennials being over-represented in the Far East, and shrubs in the European part of Russia. The richness of invasive species can be explained by climatic factors, human population density and the percentage of urban population in a region. This publication and the associated dataset is the first comprehensive treatment of the invasive flora of Russia using standardized criteria and covering 83% of the territory of this country.     

Effect of plant–soil feedbacks on the growth and competition of <Emphasis Type="Italic">Lactuca</Emphasis> species

Plant species generate specific soil communities that feedback on plant growth and competition. These feedbacks have been implicated in plant community composition and dispersion. We used Lactuca sativa and its wild progenitor Lactuca serriola to test the hypotheses that separate Lactuca species generate unique soil communities and that these soil communities differentially influence host, and neighboring, plant growth and competition. We grew each Lactuca in competition with the other, in sterile and non-sterile soils. We then examined the growth of each Lactuca species in sterile, non-sterile, and preconditioned soil. Finally, we used TRFLP techniques to explore whether the two Lactuca species generate significantly different bacterial communities in their rhizosphere soils. L. sativa proved to be the stronger competitor of the two species. However, sterilization increased the competitive effect of L. serriola background competitors. The growth experiment showed a significant effect on plant species, soil treatment, and the interaction of the two. Preconditioning soil caused reduced growth in both Lactuca species. Only L. serriola showed significantly increased growth in sterile soils. Our TRFLP analysis showed that the L. sativa soil community was significantly less diverse and that soil preconditioning had the largest impact on the community composition. These results show that Lactuca serriola’s rhizosphere communities generate a stronger negative feedback for plant growth than do the communities associated with L. sativa. Our study suggests that selection for plants that are able to grow in dense monoculture may have released Lactuca from species-specific negative soil feedbacks. This has important implications for both agriculture and the evolution of invasive plant species.     

A Model for the Spread of an Invasive Weed, <Emphasis Type="Italic">Tradescantia fluminensis</Emphasis>

Tradescantia fluminensis is an invasive weed and a serious threat to native forests in eastern Australia and New Zealand. Current methods of eradication are often ineffective, so understanding the growth mechanisms of Tradescantia is important in formulating better control strategies. We present a partial differential equation (PDE) model for Tradescantia growth and spatial proliferation that accounts for Tradescantia’s particular creeping and branching morphology, and the impact of self-shading on plant growth. This is the first PDE model to represent a weed that spreads via a creeping growth habit rather than by seed dispersal. We use a travelling wave analysis to investigate how Tradescantia extends to colonise new territory. Numerical simulations and analysis show that the model provides a good qualitative representation of the behaviour of this plant. This model provides a foundation for assessing different control and eradication strategies for Tradescantia.     

Effects of leaf litter on inter-specific competitive ability of the invasive plant <Emphasis Type="Italic">Wedelia trilobata</Emphasis>

Allelochemicals released by invasive plants contribute to the successful invasion of new habitats. However, the relationship between allelopathic effects and competitive ability of invasive plants has not been characterized. We quantified the neighbor effects of Wedelia trilobata (family: Asteraceae) and the allelopathic effects of its leaf litter on two Asteraceae competitor species (invasive Eupatorium catarium and non-invasive Lactuca sativa) and on its own ramet growth. The seed germination rate and seedling biomass of the two competitor species decreased following treatment with W. trilobata leaf extracts. When co-cultured with W. trilobata, the total biomass of the two competitor species significantly decreased regardless of whether leaf extracts were present. Under low plant density co-culture conditions, W. trilobata leaf extracts enhanced the inhibitory effects on E. catarium. In contrast, W. trilobata leaf extracts promoted the growth of W. trilobata adventitious roots, resulting in increased competitive ability. Therefore, W. trilobata growth was promoted by its own allelochemicals in leaf extracts, whereas the growth of the invasive and non-invasive competitors was inhibited by the same chemicals. These responses facilitated the invasion by W. trilobata. Our study demonstrates that leaf litter of invasive plants may inhibit the growth of neighboring species to enhance the competitive ability of the invasive plants during the early stages of invasion.     

The New Zealand Kumara or sweet potato

The varieties of kumara (sweet potato,Ipomoea batatas (Linn.) Poir.) being grown by the Maoris of the North Island of New Zealand are described and classed as Maori (pre-European) or European introductions. While the number of varieties is small compared with that claimed for the early Maori, it appears from the historical evidence that the variation in pre-European stocks was limited compared with that found elsewhere in the species. The New Zealand varieties have not been induced to flower, but varieties from the Pacific Islands, southeast Asia and South America have flowered in New Zealand, and some Peruvian varieties have set seed. The interaction of climate and variety in the manifestation of sexual reproduction in the species is considered in relation to an alternative explanation for the large number of varieties attributed to the pre-European Maori. Economic characters exhibited in the New Zealand varieties could form a useful basis for a plant improvement programme.     

<Emphasis Type="Italic">Drosophila</Emphasis> as models to understand the adaptive process during invasion

The last few decades have seen a growing number of species invasions globally, including many insect species. In drosophilids, there are several examples of successful invasions, i.e. Zaprionus indianus and Drosophila subobscura some decades ago, but the most recent and prominent example is the invasion of Europe and North America by the pest species, Drosophila suzukii. During the invasive process, species often encounter diverse environmental conditions that they must respond to, either through rapid genetic adaptive shifts or phenotypic plasticity, or by some combination of both. Consequently, invasive species constitute powerful models for investigating various questions related to the adaptive processes that underpin successful invasions. In this paper, we highlight how Drosophila have been and remain a valuable model group for understanding these underlying adaptive processes, and how they enable insight into key questions in invasion biology, including how quickly adaptive responses can occur when species are faced with new environmental conditions.     

Establishment and impact of insect agents deployed for the biological control of invasive Asteraceae: prospects for the control of <Emphasis Type="Italic">Senecio madagascariensis</Emphasis>

Several invasive Asteraceae have been targeted for biological control worldwide, with variable success. Senecio madagascariensis Poiret, which invades agricultural lands in Australia and Hawaii, is a recent target. Since several potential insect agents were recorded in the plant’s native range in South Africa, we assessed biocontrol efforts against asteraceous weeds to determine those most likely to deliver success. Some 108 insect species, from five orders and 23 families, were deployed against 38 weed taxa, mostly in the mainland USA, Canada, Australia and New Zealand. Coleoptera (mainly Curculionidae and Chrysomelidae), Diptera (Tephritidae) and Lepidoptera (Tortricidae) featured the most. Despite high establishment success (73% of releases across countries), only 37% of successful releases achieved meaningful impact. Although root-feeding and stem-feeding insects appeared to be the best candidates, neither insect family nor feeding guild significantly influenced the probability of success. This synthesis of the global contribution of different guilds of specialist herbivores to the management of invasive Asteraceae is guiding the selection of candidate agents for the biocontrol of S. madagascariensis in Australia.     

A new <Emphasis Type="Italic">Gephyraulus</Emphasis> species (Diptera: Cecidomyiidae) inducing flower bud galls on the European sea rocket <Emphasis Type="Italic">Cakile maritima</Emphasis> Scop. (Brassicaceae)

The European sea rocket Cakile maritima Scop. (Brassicaceae) is a common herb growing on sandy coastlines worldwide and is considered a useful plant because of its medicinal importance, its edibility, and potential as an oilseed crop. However, C. maritima is an invasive plant over a wide range, e.g., eastern South America, North America, northern Iran, Australia and New Zealand, and has a limited number of associated herbivorous insects. During investigations on gall midges (Diptera: Cecidomyiidae) in Egypt, we found a gall midge inducing flower bud galls on C. maritima and preventing fruit production, which suggested that this gall midge is a potential pest of this plant. In this paper, we describe this gall midge species, Gephyraulus zewaili Elsayed and Tokuda sp. nov., as new to science by comparing its morphology with that of close congeners. Partial sequence data of the mitochondrial DNA cytochrome oxidase subunit I gene are also provided.     

Echoes of a flood pulse: short-term effects of record flooding of the Illinois River on floodplain lakes under ecological restoration

Until recently, only one native and three apparently introduced Daphnia species were known from New Zealand. We demonstrate that (1) Daphnia in subalpine habitats in southern New Zealand differ morphologically and genetically from the native taxon previously labelled Daphnia carinata to merit species nova status and (2) the name of the latter should revert to D. thomsoni, used by Sars (1894) for Daphnia described from New Zealand mud. We compare some key characteristics and cytochrome c oxidase subunit 1 (CO1) sequences of the New Zealand native and other morphologically similar species. Distinctive characteristics of subalpine populations, described as Daphnia tewaipounamu sp. nov., are a wide cephalic shield with lateral flanges curving dorsally via rounded fornices, dorsal cervical depression variably expressed as a ‘step’ in the cephalic shield exuviae and retention of ephippia within shed carapace exoskeletons long after ecdysis. CO1 sequences revealed that D. tewaipounamu sp. nov. belongs to the D. carinata complex but is highly divergent (>14%) from other known members of this complex. New Zealand D. thomsoni is divergent (>15%) from D. carinata s.s. However, it is not endemic to New Zealand, as we confirmed its presence in Tasmania, and some Australian populations are closely related to it.     

Physiological integration increases the survival and growth of the clonal invader <Emphasis Type="Italic">Carpobrotus edulis</Emphasis>

Clonal growth seems to be a common trait for many of the most aggressive invasive plant species. However, little research has been conducted to determine the role of clonality in the successful invasion of new areas by exotic species. Carpobrotus edulis (L.) N.E. Br. is a mat-forming succulent plant, native to South Africa that is invasive in coastal dunes of Australia, New Zealand, USA and Southern Europe. Although Carpobrotus edulis is a clonal plant, there is no information on the role of clonality for the invasion by this species, therefore the objective of this study was to test whether or not physiological integration improves the performance of C. edulis invading coastal sand dunes. To do that, a 6-month field experiment was designed in which the stolon connections between the apical ramets and the C. edulis mats were severed to prevent physiological integration. This treatment was applied to ramets growing under high and low competition with the native species. Apical ramets with intact stolon connections were used as control. Integration improved the survivorship and growth of apical ramets, both in high and low competition. Connected ramets showed a more pronounced increase of clonal growth (estimated as stolon length) during the experimental period and a higher total biomass and number of ramets at the completion of the experiment. In terms of survivorship, the benefit of integration was greater under high competition. Physiological integration can therefore be considered an important factor in the invasiveness of C. edulis, both in open space and in direct competition with the native plants.     

Non-native plants affect generalist pollinator diet overlap and foraging behavior indirectly,via impacts on native plant abundance

Flowering invasive plants can have dramatic effects on the resource landscape available to pollinators. Because many pollinators exhibit behavioral plasticity in response to competitor or resource density, this in turn can result in impacts on ecological processes such as pollination and plant reproduction. We examine how interactions between five common generalist eusocial bees change across an invasion gradient by examining how bee abundance and diet overlap changed with variation in both invasive plant abundance and competitor abundance in a temperate oak-savannah ecosystem. Specifically we focus on the bumblebees Bombus bifarius, B. mixtus, B. melanopygus and B. vosnesenskii, as well as the non-native honeybee Apis mellifera, and their interactions with the native flowering plants Camassia quamash, Camassia liechtlinii, and the invasive shrub Cytisus scoparius. We further examine whether changes in pollinator visits to the invasive and two common native plants can explain changes in diet overlap. Abundance of the invasive plant and other common floral resources had strong impacts on focal bee abundance, with certain species more likely to be present at highly invaded sites. This may be because highly invaded sites tended to be embedded in forested landscapes where those bees are common. Diet overlap was most affected by abundance of a common native plant, rather than the invasive plant, with diet overlap increasing non-linearly with abundance of the native plant. Furthermore, Apis mellifera, did not appear to have direct competitive effects on native bumblebees in this habitat. However, visit patterns suggest that bees most abundant at highly invaded sites may compete for access to native resources. Thus the impacts of this invasive plant on our focal bee species may be primarily indirect, via its’ competitive effects on native plants.