Propagule pressure and an invasion syndrome determine invasion success in a plant community model

The success of species invasions depends on multiple factors, including propagule pressure, disturbance, productivity, and the traits of native and non‐native species. While the importance of many of these determinants has already been investigated in relative isolation, they are rarely studied in combination. Here, we address this shortcoming by exploring the effect of the above‐listed factors on the success of invasions using an individual‐based mechanistic model. This approach enables us to explicitly control environmental factors (temperature as surrogate for productivity, disturbance, and propagule pressure) as well as to monitor whole‐community trait distributions of environmental adaptation, mass, and dispersal abilities. We simulated introductions of plant individuals to an oceanic island to assess which factors and species traits contribute to invasion success. We found that the most influential factors were higher propagule pressure and a particular set of traits. This invasion trait syndrome was characterized by a relative similarity in functional traits of invasive to native species, while invasive species had on average higher environmental adaptation, higher body mass, and increased dispersal distances, that is, had greater competitive and dispersive abilities. Our results highlight the importance in management practice of reducing the import of alien species, especially those that display this trait syndrome and come from similar habitats as those being managed.     

Propagule pressure and native species richness effects drive invasibility in tropical dry forest seedling layers

Understanding the factors that encourage or inhibit plant invasions is vital to focusing limited invasive control efforts within areas where they are most practical and cost-effective. To extend the range of contexts in which invasibility is studied and aid the development of practical strategies to limit damaging plant invasions, we set out to test the relative importance of native species richness, native seedling density, and invasive propagule pressure, on the invasibility of artificial assemblages of naturally occurring tropical woody seedling communities. Our greenhouse mesocosms included a species pool of twelve trees and woody shrubs native to South Florida's tropical hardwood hammocks, and an increasingly prevalent noxious woody invader of this system, Ardisia elliptica. We found that invader propagule pressure was the single most important factor determining community invasibility. We also revealed a positive relationship between invasibility and native species richness in our polyculture mesocosms. Because A. elliptica biomass production significantly differed among different native monocultures and was not related to overyielding in native polycultures, we suggest that the effect of species richness on invasibility in this experiment was the result of sampling effects rather than a true effect of diversity.Three broad findings hold potential for application in preventing and controlling plant invasions, especially in the seedling layers of tropical dry forests: (1) effective invasive control efforts will likely benefit from measures to minimize propagule pressure; (2) managers might do well to prioritize invasive monitoring and removal efforts on the most diverse habitats within a management region; and (3) while more data are necessary to further understand our finding of a lack of association between productivity and invasibility, management regimes aimed at maximizing primary productivity might not increase invasibility, and in fact, strategies for controlling invasive plants via the management of ecosystem productivity may be ineffective.     

Fluctuating resource availability increases invasibility in microbial microcosms

The fluctuating resource hypothesis (FRH) proposes that fluctuations in resource supply can temporally reduce competitive pressure from resident species, thereby providing ephemeral opportunities for invading species. Although FRH has the potential to integrate many existing hypotheses regarding mechanisms of community invasibility, previous tests and evaluations of FRH were based on single trophic level, did not take the timing effect into account, and had difficulties in distinguishing the effects of resource pulses from other simultaneous processes. Here we test FRH in multi‐trophic aquatic microcosms by creating resource pulses, by controlling resource quantity, propagule supply and pulse recurrence frequency, and by manipulating the timing of pulses relative to the timing of the arrival of new species (i.e. invaders) to local communities. The novelty of our work lies in that we directly manipulate resource pulse timing relative to invader introduction events and thus demonstrate the importance of this timing effect for community invasibility. Our study supports FRH in general: invasion success was positively related to resource pulses, and invaders had strong performance in treatments receiving coincident pulses, although not all invaders gained more benefit when resources were supplied at large‐magnitude than supplied at continuous rates. Since many ecosystems worldwide are experiencing high rates of anthropogenic nutrient input and increasing rates of precipitation, these ecosystems are potentially more fragile and susceptible to invasion. More experiments across multiple ecosystem types are needed to help formulate a general theory of community invasibility.     

Seasonality and resource availability control bacterial and archaeal communities in soils of a temperate beech forest

It was hypothesized that seasonality and resource availability altered through tree girdling were major determinants of the phylogenetic composition of the archaeal and bacterial community in a temperate beech forest soil. During a 2-year field experiment, involving girdling of beech trees to intercept the transfer of easily available carbon (C) from the canopy to roots, members of the dominant phylogenetic microbial phyla residing in top soils under girdled versus untreated control trees were monitored at bimonthly intervals through 16S rRNA gene-based terminal restriction fragment length polymorphism profiling and quantitative PCR analysis. Effects on nitrifying and denitrifying groups were assessed by measuring the abundances of nirS and nosZ genes as well as bacterial and archaeal amoA genes. Seasonal dynamics displayed by key phylogenetic and nitrogen (N) cycling functional groups were found to be tightly coupled with seasonal alterations in labile C and N pools as well as with variation in soil temperature and soil moisture. In particular, archaea and acidobacteria were highly responsive to soil nutritional and soil climatic changes associated with seasonality, indicating their high metabolic versatility and capability to adapt to environmental changes. For these phyla, significant interrelations with soil chemical and microbial process data were found suggesting their potential, but poorly described contribution to nitrification or denitrification in temperate forest soils. In conclusion, our extensive approach allowed us to get novel insights into effects of seasonality and resource availability on the microbial community, in particular on hitherto poorly studied bacterial phyla and functional groups.     

The functional role of temperate forest understorey vegetation in a changing world

Temperate forests cover 16% of the global forest area. Within these forests, the understorey is an important biodiversity reservoir that can influence ecosystem processes and functions in multiple ways. However, we still lack a thorough understanding of the relative importance of the understorey for temperate forest functioning. As a result, understoreys are often ignored during assessments of forest functioning and changes thereof under global change. We here compiled studies that quantify the relative importance of the understorey for temperate forest functioning, focussing on litter production, nutrient cycling, evapotranspiration, tree regeneration, pollination and pathogen dynamics. We describe the mechanisms driving understorey functioning and develop a conceptual framework synthesizing possible effects of multiple global change drivers on understorey‐mediated forest ecosystem functioning. Our review illustrates that the understorey's contribution to temperate forest functioning is significant but varies depending on the ecosystem function and the environmental context, and more importantly, the characteristics of the overstorey. To predict changes in understorey functioning and its relative importance for temperate forest functioning under global change, we argue that a simultaneous investigation of both overstorey and understorey functional responses to global change will be crucial. Our review shows that such studies are still very scarce, only available for a limited set of ecosystem functions and limited to quantification, providing little data to forecast functional responses to global change.     

Ballooning propensity of canopy and understorey spiders in a mature temperate hardwood forest

1. Spiders frequently disperse and colonise habitats through ballooning, a passive aerial dispersal process. Ballooning is pre‐eminent in open habitat spider communities and its propensity can be modulated by habitat conditions and availability, and by life‐history traits such as body size, degree of specialisation, and feeding behaviour. 2. Using spiders from the canopy and understorey of a north‐temperate hardwood forest as a model system, our main objectives were to detect if foliage spiders of a mature forest disperse through ballooning, and identify life‐history traits that influence ballooning propensity. 3. Our results demonstrate that foliage spiders living in the canopy and understorey of a mature forest do balloon, and in some cases have very high ballooning propensities similarly to open field spiders. Species level models showed that small body size had a strong positive effect on ballooning for juveniles of species with large‐bodied adults, while individuals of small‐bodied species initiated ballooning regardless of size, habitat or development stage. A generalised linear mixed model indicated that small size web‐building spiders from the Retro Tibial Apophysis (RTA) and Orbicularia clades had the highest propensity for foliage spiders of this north‐temperate hardwood forest. 4. In conclusion, we provide the first demonstration that forest spiders can have high ballooning propensities and that ballooning propensity is negatively affected by body size and positively affected by the prominent use of silk to catch prey. However, spiders originating from the canopy and understorey of a north‐temperate hardwood forest did not differ in their ballooning propensities.     

Exotic vascular plant invasiveness and forest invasibility in urban boreal forest types

The riverine forests of the northern city of Edmonton, Alberta, Canada display strong resilience to disturbance and are similar in species composition to southern boreal mixedwood forest types. This study addressed questions such as, how easily do exotic species become established in urban boreal forests (species invasiveness) and do urban boreal forest structural characteristics such as, native species richness, abundance, and vertical vegetation layers, confer resistance to exotic species establishment and spread (community invasibility)? Eighty-four forest stands were sampled and species composition and mean percent cover analyzed using ordination methods. Results showed that exotic tree/shrub types were of the most concern for invasion to urban boreal forests and that exotic species type, native habitat and propagule supply may be good indicators of invasive potential. Native forest structure appeared to confer a level of resistance to exotic species and medium to high disturbance intensity was associated with exotic species growth and spread without a corresponding loss in native species richness. Results provided large-scale evidence that diverse communities are less vulnerable to exotic species invasion, and that intermediate disturbance intensity supports species coexistence. From a management perspective, the retention of native species and native forest structure in urban forests is favored to minimize the impact of exotic species introductions, protect natural succession patterns, and minimize the spread of exotic species.     

Facilitation in an unproductive boreal forest understorey community

Questions: We tested the hypothesis that if competition had a significant influence in structuring this boreal plant community, removal of neighbours, addition of fertilizer and addition of water would all benefit the transplanted seedlings. Alternatively, if facilitation had a greater influence, then removal of neighbours would be detrimental to the transplants but fertilization and watering would still be beneficial. Location: Understorey of the boreal forest in southwestern Yukon Territory, Canada (138°22′W; 61°02′N). Methods: Ten of the most common species were transplanted as seedlings into transects from which all neighbours had been removed, and also into transects with intact vegetation. We used a factorial design with two levels of watering and two levels of fertilization; this allowed us to test effects at both species and community level. Results: The summed survival and total biomass of all transplants was significantly higher in the presence of neighbours than without neighbours, indicating a facilitative effect of neighbouring plants, but there were significant increases in only six of the ten species. The combined survival and biomass of all species increased with watering, survival decreased and biomass increased with fertilization, but only two species had significant responses to fertilization: Anenome parviflora decreased and Mertensia paniculata increased in biomass. Watering increased the biomass of Achillea millefolium, Festuca altaica and Solidago multiradiata; there were also some interaction effects. Conclusions: (1) The presence of neighbours was generally facilitative. (2) Fertilization had negligible effects, and watering had minor beneficial effects. (3) This study demonstrates the importance of facilitation in structuring this boreal understorey community.     

Recruitment and growth dynamics of a temperate forest understorey species following wildfire in southeast Australia

In southeast Australia, fire regimes are changing. Conserving species into the future under these changing fire regimes will require understanding their recruitment and growth dynamics following historical fires. Where monitoring is absent, dendroecology provides a tool for reconstructing and quantifying these dynamics. The use of dendroecology in southeast Australia has been limited due to presumptions that many of the species do not produce annual growth rings. In this study, we determined the dendroecological potential of a fire-sensitive understorey tree species, Pomaderris aspera, as a case study species, to explore the potential to use of understorey species to provide insights into past fire history. We used growth patterns of this species to understand the role of resource limitation on growth and senescence. We found that P. aspera had distinct growth rings and high within tree correlation when cross-dated.Recruitment events of this species aligned with three known fire events. We found that the impacts of historical fire on tree canopy cover could be estimated. P. aspera grew rapidly post fire and then become suppressed within 9–15 years. Cycles of growth release and suppression were found with increasing incidence of suppression occurring over time. Increased suppression and reduced growth rates aligned with patterns of recorded senescence over time in the understories of a maturing.Eucalyptus canopy. Our results highlight the potential to use dendroecology to confirm past fire extents and amount of canopy disturbances and the impacts that these events have on the recruitment and growth dynamics of understorey species in southeast Australia.     

Functional and phylogenetic diversity determine woody productivity in a temperate forest

Understanding the relationships between biodiversity and ecosystem productivity has become a central issue in ecology and conservation biology studies, particularly when these relationships are connected with global climate change and species extinction. However, which facets of biodiversity (i.e. taxonomic, functional, and phylogenetic diversity) account most for variations in productivity are still not understood very well. This is especially true with regard to temperate forest ecosystems. In this study, we used a dataset from a stem‐mapped permanent forest plot in northeastern China exploring the relationships between biodiversity and productivity at different spatial scales (20 × 20 m; 40 × 40 m; and 60 × 60 m). The influence of specific environmental conditions (topographic conditions) and stand maturity (expressed by initial stand volume and biomass) were taken into account using the multivariate approach known as structural equation models. The variable “Biodiversity” includes taxonomic (Shannon), functional (FDis), and phylogenetic diversity (PD). Biodiversity–productivity relationships varied with the spatial scales. At the scale of 20 × 20 m, PD and FDis significantly affected forest biomass productivity, while Shannon had only indirect effects. At the 40 × 40 m and 60 × 60 m scales, biodiversity and productivity were weakly correlated. The initial stand volume and biomass were the most important drivers of forest productivity. The local environmental conditions significantly influenced the stand volume, biomass, biodiversity, and productivity. The results highlight the scale dependency of the relationships between forest biodiversity and productivity. The positive role of biodiversity in facilitating forest productivity was confirmed at the smaller scales. Our findings emphasize the fundamental role of environmental conditions in determining forest ecosystem performances. The results of this study provide a better understanding of the underlying ecological processes that influence specific forest biodiversity and productivity relationships.     

Fertilization,soil and plant community characteristics determine soil microbial activity in managed temperate grasslands

Background and aims

Contaminated soils can impede germination and growth of selected plant species, restricting effective phytoremediation strategies. The purpose of the present study was to enhance the germination and growth of saltgrass [Distichlis spicata (L.) Greene] by evaluating the efficacy of certain seed pretreatments and soil amendments.

Methods

Ten seed pretreatment methods, two amendments, three soil depths and five saline levels were tested under greenhouse conditions.

Results

Saltgrass germination and growth were negatively correlated with increasing salinity levels when NaCl > 85.6 mM. Among ten seed pretreatments (stratification + Proxy 24 h, hot water + Proxy 24 h, stratification, hot water + Proxy 48 h, Proxy 48 h, Proxy 24 h, hot water, scarification, gibberellins, and KMnO₄), the two best methods were stratification + Proxy 24 h and hot water + Proxy 24 h for enhancing saltgrass germination, with the latter pretreatment being especially useful because of its shorter preparation time and high germination rates. Proxy is a commercial ethephon product. Potting soil (5.0 cm depth) was found to be the best amendment for saltgrass germination and growth in hydrocarbon-contaminated soils.

Conclusion

We conclude that direct seeding of saline soils contaminated with petroleum hydrocarbons is a feasible phytoremediation strategy provided that appropriate seed pretreatments and amendments are utilized.

     

Effects of light and soil water availability on leaf photosynthesis and growth of Arisaema heterophyllum,a riparian forest understorey plant

The effects of soil-water availability on leaf light acclimation and whole-plant carbon gain were examined in Arisaema heterophyllum Blume, a riparian deciduous forest understorey plant. Photosynthesis, above-ground morphology and ramet biomass accumulation (relative growth rate: RGR of a corm for a full leaf life-span) were measured on plants raised under three light treatments combined with two soil water conditions. The two higher light treatments during growth (high: max. 550 μmol photons m^–2 s^–1; medium: 150 μmol photons m^–2 s^–1) resulted in a twofold increase in RGRs, 30% higher photosynthetic capacities and 20% less photosynthetic low-light use efficiency than those under a low light condition (50 μmol photons m^–2 s^–1). Leaf area was the smallest and leaf mass area ratio was the largest under the high light treatment. Water stress decreased both photosynthetic rate and leaf area and, hence, RGR in all the light regimes. However, water stress did not alter the general patterns of physiological and morphological responses to different light regimes. We estimated that higher photosynthetic low-light use efficiency and larger leaf area in the low light leaf would lead to a threefold carbon gain as compared with the high light leaf under simulated low light conditions. Both experimental and simulation results suggest that the physiological and morphological acclimations tend to be beneficial to carbon gain when light availability is low, whereas they favor increased water use efficiency when light availability is sufficiently high. Electronic Publication     

Interactive effects of fertilization and disturbance on community structure and resource availability in an old-field plant community

Summary The interactive effects of fertilization and disturbance on plant community structure and resource availability were studied by supplying four levels of nitrogen and applying four intensities of tilling to a 30 year old field in a factorial design for 2 year. Live above-ground biomass, root biomass, and litter generally increased with nitrogen supply and decreased with disturbance. Species composition varied significantly, with annuals increasing with both nitrogen and disturbance, but with perennials unaffected by nitrogen and decreased by disturbance. Species diversity decreased with disturbance, but decreased with nitrogen only in undisturbed vegetation. Root: shoot ratios decreased with added nitrogen, leaf allocation decreased with disturbance, and flowering allocation increased. Surprisingly, stem allocation was unaffected by disturbance. This result reflected a shift from vertical stems to horizontal stems as disturbance increased. Resource measurements suggested that the vegetation responded to interactions between the treatments as well as to direct treatment effects. Variation in light penetration was reduced by fertilization in undisturbed vegetation but not in tilled plots; variability was not directly affected by disturbance. The availability of nitrogen, the limiting soil nutrient, increased with fertilization but was not significantly affected by disturbance. In contrast, the ratio of ammonium to nitrate was significantly reduced by disturbance but unaffected by supply rates, suggesting that nitrogen may have had different effects under different disturbance regimes, even though its total availability was constant. While many community responses to fertilization and disturbance conformed to those reported earlier, resource and allocation measurements indicated that their interactions are not always predictable from their separate effects.     

A bounded null model explains juvenile tree community structure along light availability gradients in a temperate rain forest

Ecologists have proposed that tree species may coexist by specialising on light environments associated with gaps of different sizes. Remarkably few community‐level studies, however, have actually examined juvenile tree distributions along light availability gradients. Here we describe distributions of juvenile trees in relation to canopy openness in a temperate rainforest, and test the hypothesis that competitive sorting causes coexisting species to overlap less in light environment occupancy than would be expected by chance. Average overlap of species’ interquartile ranges on the canopy openness gradient was tested against a bounded domain null model of community structure which used range‐size criteria to constrain random placement of species optima. Microsite availability was strongly skewed towards low light, with 43% of microsites occurring at <5% canopy openness. We therefore transformed canopy openness values to ranks, so that equal intervals on the transformed gradient represented equal areas of microsite availability. We then calculated the interquartile range (25–75%) of sample ranks occupied by juveniles of each species. About half the assemblage was non‐randomly distributed in relation to canopy openness, providing evidence of niche expression. Average overlap of species’ interquartile ranges did not depart significantly from that predicted by the bounded null model, indicating that community structure in relation to canopy openness was mainly explained by a mid‐domain effect. As predicted by the null model, species’ interquartile range mid‐points were concentrated in the centre of the rank‐transformed gradient, and species richness (overlap of interquartile ranges) peaked close to the median light environment. Most species therefore had intermediate light requirements. The apparent lack of constraints on pairwise overlap suggest that differences in light use are not a prerequisite for tree species coexistence. As far as we are aware, this is the first study to identify a mid‐domain effect on a resource availability axis.     

Light and warming drive forest understorey community development in different environments

Plant community composition and functional traits respond to chronic drivers such as climate change and nitrogen (N) deposition. In contrast, pulse disturbances from ecosystem management can additionally change resources and conditions. Community responses to combined environmental changes may further depend on land‐use legacies. Disentangling the relative importance of these global change drivers is necessary to improve predictions of future plant communities. We performed a multifactor global change experiment to disentangle drivers of herbaceous plant community trajectories in a temperate deciduous forest. Communities of five species, assembled from a pool of 15 forest herb species with varying ecological strategies, were grown in 384 mesocosms on soils from ancient forest (forested at least since 1850) and postagricultural forest (forested since 1950) collected across Europe. Mesocosms were exposed to two‐level full‐factorial treatments of warming, light addition (representing changing forest management) and N enrichment. We measured plant height, specific leaf area (SLA) and species cover over the course of three growing seasons. Increasing light availability followed by warming reordered the species towards a taller herb community, with limited effects of N enrichment or the forest land‐use history. Two‐way interactions between treatments and incorporating intraspecific trait variation (ITV) did not yield additional inference on community height change. Contrastingly, community SLA differed when considering ITV along with species reordering, which highlights ITV’s importance for understanding leaf morphology responses to nutrient enrichment in dark conditions. Contrary to our expectations, we found limited evidence of land‐use legacies affecting community responses to environmental changes, perhaps because dispersal limitation was removed in the experimental design. These findings can improve predictions of community functional trait responses to global changes by acknowledging ITV, and subtle changes in light availability. Adaptive forest management to impending global change could benefit the restoration and conservation of understorey plant communities by reducing the light availability.     

Effects of resource availability on plant recruitment at the community level in a Mediterranean mountain ecosystem

Coexisting plant species usually differ in resource requirements, which may also vary within species at successive demographic stages. Such differences become extremely important during the early life stages, since these are the most critical phases in woody-species recruitment, they depend heavily on resources, and they may determine future community composition. Under a global-change scenario, where climatic conditions, nutrient availability, and habitat characteristics are expected to be altered, it is difficult to predict the way in which plant recruitment will be affected. To understand the impact of different global-change drivers on community recruitment, we sowed a set of species representative of the different successional groups of a complete Mediterranean woody community under field conditions, and studied their emergence, growth, and survival along the main resource gradients of light, water, and nutrients. The light and nutrient gradients followed the natural range of conditions in the study area, but water availability was manipulated to simulate three contrasting climatic scenarios: wetter, drier, and current conditions. Structural equation modelling was used to provide a comprehensive analysis of the factors and relations governing plant recruitment. Overall, seedling emergence was determined directly by light; growth was determined by light and summer soil moisture; and survival was determined by summer soil moisture. Light was the main factor indirectly affecting the demographic stages of all species. However, the magnitude of the direct and indirect relationships varied among species. Particularly, species differed in their response to the expected drier climatic conditions, some (e.g. Pinus sylvestris, Acer opalus) being much more vulnerable than others (e.g. Cytisus scoparius, Salvia lavandulifolia). These differential responses could translate as major shifts in the structure of the overall plant community. Our results support the idea that the analysis of complex relations among essential resources is critical for accurate forecasts of the impact of climate change on community dynamics.