Are some communities of the coastal dune zonation more susceptible to alien plant invasion?

Aims Coastal areas, and in particular coastal dunes, are ecosystems strongly affected by the invasion of alien plants. However, few attempts have ever been made to quantify alien species incidence in different communities along the coastal zonation. This work aims to analyze the distribution of alien plants along the coastal zonation of sandy shores on the Tyrrhenian coast, addressing specifically differences among plant communities in abundance of alien plants.Methods The study was performed on recent dunes (Holocene) of the central western coast of Italy. We selected dune landscapes where invasion processes were particularly evident. Vegetation plots were randomly sampled and through cluster analysis, we identified six plant communities corresponding to the typical zonation described for the Tyrrhenian sandy coast of Central Italy. We evaluated and compared frequency and abundance of invasion in these different communities. Further, we investigated how propagule pressure (measured using as proxy human structures) contributed to the observed invasion patterns.Important findings We found a relatively low total number of aliens but also a differential distribution pattern and strong abundance of some of the aliens in specific sectors of the vegetation zonation. The perennial community of transition dunes appears most affected by invasion processes, related almost exclusively to the frequent and widespread Carpobrotus aff. acinaciformis. This alien species reaches high cover values, apparently lowering cover of native species of transition dune plant communities. Higher levels of invasion in the transition dune can be partially explained because of greater propagule pressure in this section of the dune profile. Our findings thus have important conservation and management implications since transition dune communities with Crucianella maritima are rare and protected (sensu Habitat 92/43/EEC Directive) along the entire Italian coast.     

Facilitation vs. competition: Does interspecific interaction affect drought responses in Sphagnum?

The stress-gradient hypothesis (SGH) predicts that the relative importance of competition decreases and facilitation increases with an increase in abiotic stress. In peatlands, Sphagnum faces the threat of drought and differentiates into hummock species (drought-tolerant) and hollow species. Whether interspecific interaction affects the influence of drought on bryophyte composition in peatlands is unknown. We established an experiment by simulating drought and building bryophyte communities with two hummock species (S. palustre and S. capillifolium) and one hollow species (S. fallax). In all three species, drought decreased biomass production, height increment and side-shoot production. Sphagnum stores water in the hyaline cells, and leaf hyaline cell percentage (HCP) in the two hummock species increased with drought while no effect was found in S. fallax, suggesting that adjusting HCP is not an effective response to drought for the hollow species. Morphological traits and carbon and nitrogen contents in hummock species responded more to drought than in the hollow species, indicating a rapid response in phenotypic plasticity is an important strategy to resist drought in the hummock species. The presence of neighboring Sphagnum species, rather than drought, decreased carbon content for all three species. All three bryophytes showed interaction between drought and neighbor in two or more plant traits. Our study, however, did not support SGH, and there were no changes from competition under wet to facilitation under dry treatments in any of the six species combinations. On the contrary, when S. fallax was the target species, a change from facilitation under wet to competition under dry treatments was observed. The results suggest that hummock species can facilitate hollow species in wet environments but they could suppress hollow species under drought conditions by competing for water resources. Both drought and strong competition are the probable reasons why hollow species rarely grow in hummocks.     

Impacts of alien plants and man‐made disturbance on soil‐growing bryophyte and lichen diversity in coastal areas of Sardinia (Italy)

Abstract

Seventy phytosociological relevés were performed in 1 m × 1 m plots at 14 study sites spread along sandy shores in northern and southern Sardinia (Italy). The plots were selected in different habitat types (open dunes, native Juniperus woodlands, maquis, and plantations with Acacia, Eucalyptus and Pinus) according to a stratified sampling method in order to investigate impacts deriving from different levels of Carpobrotus spp. cover, dry litter from exotic trees, and other disturbance types. The quantile regression and logistic regression analyses revealed that the reduction in the amount of bryophyte and lichen cover on sand dunes of the study area is caused either by a high cover of Carpobrotus spp. mats or by a high cover of dry exotic litter in dense, unmanaged or poorly managed forest plantations. Additional detrimental effects are often driven by other kinds of man‐made disturbances. Forest management in the coastal areas of Sardinia should be gradually modified to take into account the conservation of bryophytes and lichens. Some of the biological indicators used are quite widespread in the Mediterranean coastal habitats or are exclusively associated with sand dunes; therefore, they can also be conveniently used as indicators of biological impacts in other countries or islands of the same biogeographical region.     

Spatio‐temporal variation in the effect of herbaceous layer on woody seedling survival in a Chilean mediterranean ecosystem

Questions: What is the effect of herbaceous layer on seedling establishment of three woody pioneer species in open areas of central Chile under a semi‐arid mediterranean climate? How do inter‐annual and habitat conditions (slope aspect) modulate this effect? Under high stress conditions such as the drier year and habitat (north‐facing slope) do herbs reach low abundance and have neutral effects on woody seedlings? Under medium stress conditions for these woody species, such as the wetter year and south‐facing slope, does the herbaceous layer reach greater abundance and have positive effects on woody seedlings due to increasing soil water content? Location: A watershed on the outskirts of Santiago, Chile, subjected to clearing of woody vegetation through firewood extraction and human‐set fires. Methods: In spring 2007, we set up 20 plots (3 m × 2 m). Half of each plot had herbs removed manually and by application of herbicide. In both halves of each plot, one seedling (8 months old) of each of the three native woody species (Colliguaya odorifera, Schinus polygamus and Quillaja saponaria) was planted and survival monitored subsequently. The experiment was repeated in two consecutive growing seasons (2007–2008 and 2008–2009) that differed significantly in total precipitation (152 and 256.5 mm, respectively), and replicated in two sites that differed in aspect and abiotic conditions: a moister south‐ and a drier north‐facing slope. Results: In the first and drier year, the herbaceous layer had low cover and no significant effect on seedling survival of woody species. During the second year, herbs had greater cover and a significant positive effect on spring survival of C. odorifera in the north‐facing slope, which was lost after summer. During this wetter year on the south‐facing slope, herb cover had a positive effect on survival of S. polygamus (mainly during summer). Conclusions: The role of mostly ruderal herbs on woody seedling establishment depended on the species, rainfall of the current year and slope aspect, and may be explained by soil moisture patterns. This suggests that the effect of ruderal herbs on woody seedlings shifts from neutral under high stress conditions produced by drought to positive under moderate stress conditions. Our results contribute to understand interactions between ruderal herbs and woody species under contrasting abiotic conditions. Therefore, control of the herbaceous layer may not be needed in restoration programmes for this region. Moreover, herbs may benefit restoration of woody cover in mesic habitats.     

Ecotypic differentiation determines the outcome of positive interactions in a dryland annual plant species

Positive interactions among plants have rarely been investigated with respect to their evolutionary consequences and vice versa. The outcome of facilitative interactions depends on the competitive ability and stress tolerance of the species. We tested whether this also applies to populations of conspecifics that are locally adapted to different environments and thereby differ in these traits. We hypothesised that ecotypes from less stressful environments experience a greater effect of facilitation when grown in stressful environments compared to populations adapted to these conditions.Seeds of two ecotypes of the annual grass species, Brachypodium distachyon, were collected from Mediterranean and arid origins and transplanted at an arid environment within the species’ distribution range. To examine the effect of biotic interactions on these ecotypes, we transplanted the individuals with and without the presence of the shrub Gymnocarpos decander (underneath or away from the shrub), and with and without the presence of annual vegetation (removal experiment). We examined the effect of these interactions on the two B. distachyon ecotypes by comparison of emergence success, biomass, and survival to reproduction.The presence of shrubs had a positive effect on all three variables in both ecotypes. Facilitation by shrubs enabled individuals from Mediterranean origin to grow and reproduce in arid conditions. Unlike the locals, they failed to survive to reproduction away from the shrubs, because of the markedly shorter growing season in open areas. The annual vegetation did not affect emergence or survival to reproduction in either ecotype; however, the positive effect of shrubs on biomass was reduced in the presence annual vegetation in the Mediterranean ecotype.This demonstrates that ecotypes adapted to arid conditions respond differently to these biotic interactions compared to Mediterranean populations. We argue that facilitation may have important evolutionary consequences by enabling maladapted ecotypes to invade and colonize stressful habitats.     

Plant–plant interactions,environmental gradients and plant diversity: A global synthesis of community-level studies

Previous syntheses on the effects of environmental conditions on the outcome of plant–plant interactions summarize results from pairwise studies. However, the upscaling to the community-level of such studies is problematic because of the existence of multiple species assemblages and species-specific responses to both the environmental conditions and the presence of neighbors. We conducted the first global synthesis of community-level studies from harsh environments, which included data from 71 alpine and 137 dryland communities to: (i) test how important are facilitative interactions as a driver of community structure, (ii) evaluate whether we can predict the frequency of positive plant–plant interactions across differing environmental conditions and habitats, and (iii) assess whether thresholds in the response of plant–plant interactions to environmental gradients exists between “moderate” and “extreme” environments. We also used those community-level studies performed across gradients of at least three points to evaluate how the average environmental conditions, the length of the gradient studied, and the number of points sampled across such gradient affect the form and strength of the facilitation-environmental conditions relationship. Over 25% of the species present were more spatially associated to nurse plants than expected by chance in both alpine and dryland areas, illustrating the high importance of positive plant–plant interactions for the maintenance of plant diversity in these environments. Facilitative interactions were more frequent, and more related to environmental conditions, in alpine than in dryland areas, perhaps because drylands are generally characterized by a larger variety of environmental stress factors and plant functional traits. The frequency of facilitative interactions in alpine communities peaked at 1000 mm of annual rainfall, and globally decreased with elevation. The frequency of positive interactions in dryland communities decreased globally with water scarcity or temperature annual range. Positive facilitation-drought stress relationships are more likely in shorter regional gradients, but these relationships are obscured in regions with a greater species turnover or with complex environmental gradients. By showing the different climatic drivers and behaviors of plant–plant interactions in dryland and alpine areas, our results will improve predictions regarding the effect of facilitation on the assembly of plant communities and their response to changes in environmental conditions.     

Resource limitation,habitat segregation,and species interactions of british tree-hole mosquitoes in nature

Summary The insect fauna of water-filled tree holes in southern Britain consists primarily of the mosquitoes Aedes geniculatus, Anopheles plumbeus, Culex torrentium, and a benthic detritivorous fauna that includes primarily the scirtid beetle Prionocyphon serricornis and the chironomid midge Metriocnemus martinii. Culex torrentium has been documented only relatively recently in tree holes but all three species of mosquitoes partition the resource in space and time. When mosquito larvae were forced to coexist in natural tree holes at limiting densities and at higher than natural levels of interspecific encounter, there was no evidence that Aedes geniculatus or Anopheles plumbeus affected pupation success, pupal weight, or development time of the other or that either Aedes geniculatus or C. torrentium affected the survivorship, pupation success, pupal weight, and biomass yield of the other. When A. geniculatus at limiting densities were forced in natural tree holes to live without or to coexist with natural or twice natural densities of P. serricornis and M. martinii, the presence, absence, or superabundance of the benthic insects did not affect pupation success or pupal weight of A. geniculatus; development time of A. geniculatus was faster when a superabundance of the benthic fauna was present. Effects of the benthic fauna on A. geniculatus are slight and the only significant interaction is facilitative, not competitive. The pattern of habitat segregation among treehole mosquitoes in southern Britain is characteristic of their respective genera and we propose that this pattern is more likely (but not certain) to have arisen through a process of independent evolution than through competitively driven niche shifts among already coexisting species.     

The importance of facilitation in the zonation of shrubs along a coastal salinity gradient

Question: What are the interactive roles of abiotic stress and plant interactions in mediating the zonation of the shrub Tamarix chinensis along a salinity gradient? Location: Yellow River estuary (37°46′N, 119°09′E), northeast China. Methods: We surveyed the zonation of T. chinensis along a salinity gradient and quantified its salt tolerance using a pot experiment. In two field experiments, we transplanted T. chinensis seedlings into salt marsh, transitional zone and upland habitats, manipulated neighbours and quantified survivorship and biomass to examine neighbour effects. We also quantified vegetation effects on abiotic conditions in each zone. Results: Tamarix chinensis dominated the transitional zone, but was absent in upland and salt marsh habitats. In the pot experiment, T. chinensis grew well in freshwater treatments, but was inhibited by increasing salinity. Field experiments revealed that competition from neighbours limited T. chinensis growth in the uplands, while T. chinensis transplants were limited, with or without neighbours, in the salt marsh by high soil salinity. In the transitional zone, however, T. chinensis transplants performed better with than without neighbours. Vegetation removal significantly elevated soil salinity in the transitional zone, but not in other zones. Conclusions: Competition, facilitation and abiotic stress are all important in mediating the zonation of T. chinensis. Within its physiological stress tolerance range, or fundamental niche, it is limited by plant competition in low salinity habitats, and facilitated by neighbours in high salt stress habitats, but cannot survive in salt marshes having salinities above its salt stress tolerance limit. Our results have implications for understanding the relationships between facilitation and stress gradients.     

Dealing with scarce data to understand how environmental gradients and propagule pressure shape fine‐scale alien distribution patterns on coastal dunes

Questions: On sandy coastal habitats, factors related to substrate and to wind action vary along the sea–inland ecotone, forming a marked directional disturbance and stress gradient. Further, input of propagules of alien plant species associated to touristic exploitation and development is intense. This has contributed to establishment and spread of aliens in coastal systems. Records of alien species in databases of such heterogeneous landscapes remain scarce, posing a challenge for statistical modelling. We address this issue and attempt to shed light on the role of environmental stress/disturbance gradients and propagule pressure on invasibility of plant communities in these typical model systems. Location: Sandy coasts of Lazio (Central Italy). Methods: We proposed an innovative methodology to deal with low prevalence of alien occurrence in a data set and high cost of field‐based sampling by taking advantage, through predictive modelling, of the strong interrelation between vegetation and abiotic features in coastal dunes. We fitted generalized additive models to analyse (1) overall patterns of alien occurrence and spread and (2) specific patterns of the most common alien species recorded. Conclusion: Even in the presence of strong propagule pressure, variation in local abiotic conditions can explain differences in invasibility within a local environment, and intermediate levels of natural disturbance and stress offer the best conditions for spread of alien species. However, in our model system, propagule pressure is actually the main determinant of alien species occurrence and spread. We demonstrated that extending the information of environmental features measured in a subsample of vegetation plots through predictive modelling allows complex questions in invasion biology to be addressed without requiring disproportionate funding and sampling effort.     

Herbivory on invasive exotic plants and their non-invasive relatives

The Enemy Release Hypothesis links exotic plant success to escape from enemies such as herbivores and pathogens. Recent work has shown that exotic plants that more fully escape herbivores and pathogens are more likely to become highly invasive, compared to plants with higher enemy loads in their novel ranges. We predicted that highly invasive plants from the Asteraceae and the Brassicaceae would be less acceptable, in laboratory no-choice feeding trials, to the generalist herbivore the American grasshopper, Schistocerca americana. We also compared herbivory on invasive and non-invasive plants from the genus Centaurea in no-choice feeding trials using the red-legged grasshopper Melanoplus femurrubrum and in a common garden in the field. In accordance with our predictions, highly invasive plants were fed on less by grasshoppers in the laboratory. They also received less damage in the field, suggesting that they contain feeding deterrents that render them less acceptable to generalist herbivores than non-invasive plants.     

Competition and facilitation between the alien mussel Mytilus galloprovincialis and indigenous species: Moderation by wave action

As the alien species that most dominates space along the South African coast, the Mediterranean mussel Mytilus galloprovincialis has radically altered community composition on invaded shores. We experimentally assessed interspecific interactions between this invasive species and dominant indigenous species in conjunction with considering how wave action moderates such interactions. The density of both M. galloprovincialis and the limpet Scutellastra granularis increased with wave action. Conversely, the tube-building polycheate Gunnarea capensis was negatively affected by wave exposure, being most abundant on sheltered shores. The influence of wave action on the indigenous mussel Aulacomya ater, however, remains unclear. M. galloprovincialis outcompeted both G. capensis and A. ater at moderate to high exposure levels, whereas it had both positive and negative effects on S. granularis. It outcompeted adult limpets on primary rock space on semi-exposed and exposed shores, reducing densities of this portion of the population. However, recruitment of S. granularis was facilitated by M. galloprovincialis, as greater numbers recruited to the secondary substratum offered by mussel shells. Again this interaction intensified with wave action. Due to the extremely high density of recruits on secondary space, the net effect of M. galloprovincialis on S. granularis was positive. Thus, wave action not only influences the abundance of individual species, but also mediates both positive and negative interspecific interactions in rocky shore communities, including the impact of alien species such as M. galloprovincialis.