Above‐ and Belowground Impacts of European Buckthorn (Rhamnus cathartica) on Four Native Forbs

Fast‐growing European buckthorn (Rhamnus cathartica) shrubs are aggressively invading woodlands in eastern and midwestern North America. Buckthorn casts dense shade, alters soil conditions, and may be allelopathic. We used greenhouse and field experiments to investigate above‐ and belowground effects of Rhamnus on four herbaceous species native to southern Wisconsin. In the greenhouse, we assessed how Rhamnus leaves and fruit affected seed germination and seedling growth relative to sugar maple (Acer saccharum) leaves. Fewer seeds of Eurybia macrophylla and Thalictrum dasycarpum germinated under Rhamnus leaves, and those that germinated were slowed. We planted seedlings of the four natives into four treatments at three sites in the field to assess how their survival, growth, and flowering respond to the separate and combined effects of light levels, buckthorn canopies, and buckthorn soils. Buckthorn consistently reduced native plant performance by diminishing survival, flowering, and growth in Thalictrum, survival and flowering in Eurybia, and flowering and growth in Symphyotrichum lateriflorum. Removing buckthorn canopies enhanced growth of these native species, but buckthorn soils separately inhibited growth at least as much (despite being more fertile). Buckthorn's impacts on native plants exceed effects attributable to light levels and soil fertility, suggesting allelopathy. Buckthorn reduced performance more in the uncommon species (Thalictrum and Eurybia) than the common species (Geranium maculatum and Symphyotrichum). As we do not yet know how long these inhibitory soil effects persist, we need additional research to learn how best to control buckthorn's impacts on native plant communities.     

Nutrient addition affects AM fungal performance and expression of plant/fungal feedback in three serpentine grasses

Plant/soil microbial community feedback can have important consequences for species composition of both the plant and soil microbial communities, however, changes in nutrient availability may alter plant reliance on mycorrhizal fungi. In this research, we tested whether plant/soil community feedback occurs and if increased soil fertility altered the plant/soil community interactions. In two greenhouse experiments we assessed plant and AM fungal performance in response to different soils (and their microbial communities), collected from under three co-occurring plants in serpentine grasslands, and nutrient treatments. The first experiment consisted of two plant species (Andropogon gerardii, Sorghastrum nutans), their soil communities, and three nutrient treatments (control, calcium, N-P-K), while the second experiment used three plant species (first two and Schizachyrium scoparium), their soil communities collected from a different site, and two nutrient treatments (control, N-P-K). Plant/soil community feedback was observed with two of the three species and was significantly affected by nutrient enrichment. Negative Sorghastrum/soil feedback was removed with the addition of N-P-K fertilizer at both sites. Andropogon/soil feedback varied between sites and nutrient treatments, while no differential Schizachyrium growth relative to soil community was observed. Addition of N-P-K fertilizer to the nutrient poor serpentine soils increased plant biomass production and affected plant/soil community interactions. Calcium addition did not affect plant biomass, but was associated with significant increases in fungal colonization regardless of plant species or soil community. Our results indicate that nutrient enrichment affected plant/soil community feedback, which has the potential to affect plant and soil community structure.     

Soil Microbial Community Associated with an Invasive Grass Differentially Impacts Native Plant Performance

This study is one of the first to show that invasive plant-induced changes in the soil microbial community can negatively impact native plant performance. This greenhouse experiment tested whether soil microbial communities specific to the rhizospheres of an invasive grass (Aegilops triuncialis) and two native plants (Lasthenia californica and Plantago erecta) affected invasive and/or native plant performance. Each of these species were grown in separate pots for 2 months to prime the soils with plant-specific rhizosphere microbial communities. Each plant species was then planted in native- and invasive-primed soil, and effects on plant performance were monitored. At 5 months, differences in microbial biomarker fatty acids between invaded and native soils mirrored previous differences found in field-collected soil. L. californica performance was significantly reduced when grown in invaded soil compared to native soil (flowering date was delayed, aboveground biomass decreased, specific root length increased, and root mass ratio increased). In contrast, P. erecta and A. triuncialis performance were unaffected when grown in invaded vs native soil. These results suggest that in some cases, invasion-induced changes in the soil microbial community may contribute to a positive feedback loop, leading to the increased dominance of invasive species in an ecosystem.     

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.     

Species specific plant-soil interactions influence plant distribution on serpentine soils

Where serpentine soils exist, variation in soil properties affects plant species distribution at both coarse and fine spatial scales. The New Idria (California, USA) serpentine mass has barren areas, supporting only sparse shrub and tree islands, adjacent to areas of densely-vegetated serpentine chaparral. To identify factors limiting growth on barren relative to vegetated serpentine soils, we analyzed soils from barren, shrub-island within barren, and vegetated areas and foliage from shrub-island and vegetated areas. We also grew Ceanothus cuneatus (native evergreen shrub), Achillea millefolium (native perennial forb), and Bromus madritensis ssp. rubens (invasive annual grass) in soils from barren and vegetated areas amended factorially with N, K, and Ca in a pot study. In well-watered pots, biomass was greater by 5-, 14-, and 33-fold for Ceanothus, Achillea, and Bromus, respectively, on vegetated-area-collected soils than on barren-collected soils, indicating a strong soil chemistry effect. Although field soil data suggested nutrient deficiency and not heavy metal toxicity, pot study plant data indicated otherwise for two of the three species. On barren-collected soils, only Ceanothus responded positively to added N and Ca and did not show greater foliar Mg or heavy metal (Fe, Ni, Cr, Co, Zn) concentrations than on vegetated-area-collected soils. Ceanothus maintained lower root Mg and heavy metal (Fe, Ni, Cr, Co) concentrations on barren soils and translocated less heavy metal (Fe, Ni, Cr, Co, Mn, Cu) from roots to foliage than Achillea and Bromus. Achillea and Bromus showed significant log-log biomass relationships with foliar Ca:Mg (+), Mg (-), and heavy metals (Fe, Ni, Cr, Co, Mn, Cu, Zn) (-), while Ceanothus showed relationships only with Ca:Mg (+) and Mg (-). The New Idria barren-vegetated pattern appears to be maintained by different factors for different species or functional types— low Ca:Mg ratios on barrens for all species tested, high heavy metal concentrations for Achillea and Bromus, and low macronutrient (N) concentrations for Ceanothus. Combined data from this and other studies suggest high heavy metal concentrations more strongly affect herbaceous than woody species, contributing to variation in species distribution on serpentine soils.     

An unfortunate alliance: Native shrubs increase the abundance,performance, and apparent impacts of Bromus tectorum across a regional aridity gradient

Interspecific facilitation contributes to the assembly of desert plant communities. However, we know little of how desert communities invaded by exotic species respond to facilitation along regional-scale aridity gradients. These measures are essential for predicting how desert plant communities might respond to concomitant plant invasion and environmental change. Here, we evaluated the potential for Bromus tectorum (a dominant invasive plant species) and the broader herbaceous plant community to form positive associations with native shrubs along a substantial aridity gradient across the Great Basin, Mojave, and San Joaquin Deserts in North America. Along this gradient, we sampled metrics of abundance and performance for B. tectorum, all native herbaceous species combined, all exotic herbaceous species combined, and the total herbaceous community using 180 pairs of shrub and open microsites. Across the gradient, B. tectorum formed strong positive associations with native shrubs, achieving 1.6–2.2 times greater abundance, biomass, and reproductive output under native shrubs than away from shrubs, regardless of relative aridity. In contrast, the broader herbaceous community was not positively associated with native shrubs. Interestingly, increasing B. tectorum abundance corresponded to decreasing native abundance, native species richness, exotic species richness, and total species richness under but not away from shrubs. Taken together, these findings suggest that native shrubs have considerable potential to directly (by increasing abundance and performance) and indirectly (by increasing competitive effects on neighbors) facilitate B. tectorum invasion across a large portion of the non-native range.     

Cultivation legacies alter soil nutrients and differentially affect plant species performance nearly a century after abandonment

Cultivation legacies affect native vegetation in old fields of the Great Basin, USA for nearly a century after these fields are abandoned. We hypothesized that cultivation lowered soil nutrients and that this legacy would differentially impact plant performance of four representative Great Basin species. To test these hypotheses, we compared soil nutrients (C, N, P, K, Mg and Ca) between two formerly cultivated and adjacent noncultivated sites in two soil series. We then compared the plant growth and foliar nutrient content of an exotic grass (Bromus tectorum L.), two native grasses (Elymus elymoides [Raf.] Swezey and Achnatherum hymenoides [Roem. and Schult.] Barkworth), and a native forb (Sphaeralcea grossulariifolia [Hook. and Arn.] Rydb) grown in these soils in the greenhouse and in the field. Only one sampling site had reduced soil nutrients associated with cultivation legacies, where most of the negative effects on plant performance were found. E. elymoides appeared to be less affected by cultivation legacies than did A. hymenoides, which had a reduced survivorship and 20 % less above-ground biomass in cultivated soils. No species, including B. tectorum, were favored by cultivation. Our findings suggest that cultivation legacies can affect plant performance of different species in different ways and that altered soil nutrients may interact with other abiotic and biotic cultivation legacies in complex ways.     

The response of Mediterranean herbaceous community to soil disturbance by native wild boars

Native wild boar (Sus scrofa lybicus) populations in Israel increased in the last decades, and their rooting activity presents an apparent severe disturbance that may affect herbaceous plant communities. We studied the extent of wild boar rooting and its effect on herbaceous vegetation in an east Mediterranean garrigue. We compared the effects of rooting soil disturbance (RSD) between summer, winter, and RSD-free control plots on herbaceous plants. RSD affected four percent of the area covered by herbaceous vegetation in winter and spring, and only 2 % in summer. Summer RSD increased herbaceous species diversity in the following spring by decreasing the abundance of the dominant species, but it did not affect total herbaceous biomass. Winter RSD lowered herbaceous biomass but did not affect herbaceous plant abundance or diversity. We conclude that Mediterranean herbaceous plant community appears to be resilient to RSD, causing only minor changes in species composition and small decrease of their biomass. The contrast between the relatively large extent of wild boars RSD, and its limited impact on the herbaceous plant community, can be attributed to the long mutual evolutionary history of wild boars and these vegetation communities.     

Selection on soil microbiomes reveals reproducible impacts on plant function

Soil microorganisms found in the root zone impact plant growth and development, but the potential to harness these benefits is hampered by the sheer abundance and diversity of the players influencing desirable plant traits. Here, we report a high level of reproducibility of soil microbiomes in altering plant flowering time and soil functions when partnered within and between plant hosts. We used a multi-generation experimental system using Arabidopsis thaliana Col to select for soil microbiomes inducing earlier or later flowering times of their hosts. We then inoculated the selected microbiomes from the tenth generation of plantings into the soils of three additional A. thaliana genotypes (Ler, Be, RLD) and a related crucifer (Brassica rapa). With the exception of Ler, all other plant hosts showed a shift in flowering time corresponding with the inoculation of early- or late-flowering microbiomes. Analysis of the soil microbial community using 16 S rRNA gene sequencing showed distinct microbiota profiles assembling by flowering time treatment. Plant hosts grown with the late-flowering-associated microbiomes showed consequent increases in inflorescence biomass for three A. thaliana genotypes and an increase in total biomass for B. rapa. The increase in biomass was correlated with two- to five-fold enhancement of microbial extracellular enzyme activities associated with nitrogen mineralization in soils. The reproducibility of the flowering phenotype across plant hosts suggests that microbiomes can be selected to modify plant traits and coordinate changes in soil resource pools.     

Size, activity and catabolic diversity of the soil microbial biomass in a wetland complex invaded by reed canary grass

Reed canary grass (Phalaris arundinacea, L.) invasion of wetlands is an ecological issue that has received attention, but its impact on soil microbial diversity is not well documented. The present study assessed the size (substrate-induced respiration), catabolic diversity (CLPP, community level physiological profiles) and composition (selective inhibition) of the soil microbial community in invaded (>95% P. arundinacea cover) and in non-invaded areas of a wetland occupied by native species grown either as a mixed assemblage (22 species) or as quasi-monotypic stands of Scirpus cyperinus (74% cover). The study also tested the hypothesis that decomposition of lignin- and phenolics-rich plant tissues would be fastest in soils exhibiting high catabolic diversity. Results showed that soil respiration, microbial biomass and diversity were significantly higher (P?<?0.03; 1.5 to 3 fold) in P. arundinacea-invaded soils than in soils supporting native plant species. Fungal to bacterial ratios were also higher in invaded (0.6) than in non-invaded (0.4) plots. Further, canonical discriminant analysis of CLPP data showed distinct communities of soil decomposers associated with each plant community. However, these differences in microbial attributes had no effect on decomposition of plant biomass which was primarily controlled by its chemical composition. While P. arundinacea invasion has substantially reduced plant diversity, this study found no parallel decline in the size and diversity of the soil microbial community in the invaded areas.     

Plant-induced changes in soil nutrient dynamics by native and invasive grass species

Alteration of soil nutrient dynamics has recently garnered more attention as both a cause and an effect of plant invasion. This project examines how nutrient dynamics are affected by native (Elymus elymoides, Pseudoroegneria spicata, and Vulpia microstachys) and invasive (Aegilops triuncialis, Agropyron cristatum, Bromus tectorum, and Taeniatherum caput-medusae) grass species. This research questions whether natives and invasives differ in their effects on nutrient dynamics. A greenhouse study was conducted using two field-collected soils. Effects on nutrient dynamics were compared using an integrated index that evaluates the total nutrients in soil and in plant tissue compared to an unplanted control. With this index, we evaluated whether soil nutrients increased or decreased as a result of plant growth, controlling for plant uptake. We found no consistent support for our hypothesis that invasive grass species as a group influence nutrient dynamics differently than native grass species as a group. Our results indicate species-specific effects on nutrient dynamics. Alteration of nutrient dynamics is not a trait shared by all of the invasive grass species in our study. However, alteration of nutrient dynamics may be a mechanism by which some individual species increase their invasive potential.     

The impact of two non-native plant species on native flora performance: potential implications for habitat restoration

Both Impatiens glandulifera and Fallopia japonica are highly invasive plant species that have detrimental impacts on native biodiversity in areas where they invade and form dense monocultures. Both species are weakly dependent on arbuscular mycorrhizal fungi (AMF) for their growth and, therefore, under monotypic stands, the AMF network can become depauperate. We evaluated the impact of I. glandulifera and F. japonica on the performance (expressed as shoot biomass) of three UK native species (Plantago lanceolata, Lotus corniculatus and Trifolium pratense) grown in soil collected from under stands of both invasive plants and compared to plants grown in soil from under stands of the corresponding native vegetation. All native species had a higher percentage colonisation of AMF when grown in uninvaded soil compared to the corresponding invaded soil. P. lanceolata and L. corniculatus had a higher biomass when grown in uninvaded soil compared to corresponding invaded soil indicating an indirect impact from the non-native species. However, for T. pratense there was no difference in biomass between soil types related to I. glandulifera, suggesting that the species is more reliant on rhizobial bacteria. We conclude that simply managing invasive populations of non-native species that are weakly, or non-dependent, on AMF is inadequate for habitat restoration as native plant colonisation and establishment may be hindered by the depleted levels of AMF in the soil below invaded monocultures. We suggest that the reintroduction of native plants to promote AMF proliferation should be incorporated into future management plans for habitats degraded by non-native plant species.     

Intraspecific and interspecific pair-wise seedling competition between exotic annual grasses and native perennials: plant–soil relationships

Few studies have examined plant–soil relationships in competitive arenas between exotic and native plants in the western United States. A pair-wise competitive design was used to evaluate plant–soil relationships between seedlings of the exotic annual grasses Bromus tectorum and Taeniatherium caput-medusae and the native perennial grasses Elymus elymoides and Pseudoroegneria spicata. Two soils were tested: an arid soil (argid) occupied by E. elymoides and presently invaded by B. tectorum and a high elevation, high organic matter, soil (aquept) where none of the tested species would typically occur. Plant growth proceeded for 85 days at which time above-ground biomass and tissue nutrient concentrations were quantified. Soil also was collected from the rooting zone beneath each species and analyzed for various nutrient pools. The exotic species had significantly greater above-ground biomass than the natives and grew far better in the aquept soil than the argid soil. Growth of B. tectorum, and to some degree, T. caput-medusae was suppressed in intraspecific competition and enhanced, especially in the aquept soil, when competing with the natives. Although not significant, biomass of natives strongly trended downward when competing with the exotic grasses. Overall, concentrations of tissue nutrients were minimally affected by competition, but natives tended to be more negatively affected by competition with exotics. Except for phosphorus (P), all species had significantly greater nutrient concentrations when growing in the aquept soil compared to the argid soil. In both soils, exotics had significant greater tissue concentrations of manganese (Mn), magnesium (Mg), and iron (Fe), while natives had significantly greater nitrogen (N). Species affects on soil nutrient pools occurred mostly in the aquept soil with exotic species significantly decreasing pools of available N, potentially available N, and soil-solution pools of calcium (Ca²⁺), potassium (K⁺), and magnesium (Mg²⁺) relative to natives. Overall, the data suggest that, in the seedling state, B. tectorum is a superior competitor. Moreover, when the natives compete intra- or interspecifically, particularly in the aquept soil, availability of N and other nutrients in their rooting zone is consistently greater than when they compete interspecifically with the exotic grasses. These data suggest the exotics are able to co-opt nutrients in the rooting zone of the natives and perhaps gain a competitive advantage.     

Exogenous application of plant defense hormones alters the effects of live soils on plant performance

The overall effect of a live soil inoculum collected from nature on plant biomass is often negative. One hypothesis to explain this phenomenon is that the overall net pathogenic effect of soil microbial communities reduces plant performance. Induced plant defenses triggered by the application of the plant hormones jasmonic acid (JA) and salicylic acid (SA) may help to mitigate this pathogenic effect of live soil. However, little is known about how such hormonal application to the plant affects the soil and how this, in turn, impacts plant growth. We grew four plant species in sterilized and inoculated live soil and exposed their leaves to two hormonal treatments (JA and SA). Two species (Jacobaea vulgaris and Cirsium vulgare) were negatively affected by soil inoculation. In these two species foliar application of SA increased biomass in live soil but not in sterilized soil. Two other species (Trifolium repens and Daucus carota) were not affected by soil inoculum and for these two species foliar application of SA reduced plant biomass in both the sterilized and live soil. Application of JA reduced plant biomass in both soils for all species. We subsequently carried out a multiple generation experiment for one of the plant species, J. vulgaris. In each generation, the live soil was a mixture of 10% soil from the previous generation and 90% sterilized soil and the same hormonal treatments were applied. The negative effects of live soil on plant biomass were similar in all four generations, and this negative effect was mitigated by the application of SA. Our research suggests that the application of SA can mitigate the negative effects of live soil on plant growth. Although the inoculum of soil containing a natural live soil microbial community had a strong negative effect on the growth of J. vulgaris, we found no evidence for an increase or decrease in negative plant-soil feedback in either the control or the SA treated plants. Also plant performance did not decrease consistently with succeeding generations.     

Distribution and decline of endangered herbaceous heathland species in relation to the chemical composition of the soil

High atmospheric deposition of ammonium affects the physical and chemical status of the soil, increasing nitrogen availability, soil acidity and the mobilization of toxic metal ions. To investigate whether and how the decline of several herbaceous plant species in Dutch heathlands is associated with these processes, the chemical composition of the soil on which these species grow has been compared with the soil on which heathland species such asCalluna vulgaris (L.) Hull,Erica tetralix L. andMolinea caerulea (L.) Moench dominate. The discrimination between both soil types was primarily based on differences in pH (H₂O), pH (NaCl) and the aluminium/calcium ratio in the waterextracts. Within the group of endangered herbaceous heathland species these soil parameters also varied. This led to a division into 4 groups of species:

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Dominating species growing on acid soils

Herbaceous species growing together with dominating species on acid soils

Herbaceous species growing together with dominating species on moderately acid soils

Herbaceous species growing together with dominating species on weakly acid soils.

This study indicated that, unlike the decline of heather species, the decline of herbaceous species is not likely to be due to increased competition from grass species as a result of eutrophication. Soil acidification and the changed mineral balance in the soil are most likely to be responsible for the decline of all three groups of herbaceous plant species. ei]R F Huettl     

Flowering in grassland predicted by CO2 and resource effects on species aboveground biomass

Continuing enrichment of atmospheric CO₂ may change plant community composition, in part by altering the availability of other limiting resources including soil water, nutrients, or light. The combined effects of CO₂ enrichment and altered resource availability on species flowering remain poorly understood. We quantified flowering culm and ramet production and biomass allocation to flowering culms/ramets for 10 years in C₄‐dominated grassland communities on contrasting soils along a CO₂ concentration gradient spanning pre‐industrial to expected mid‐21st century levels (250–500 μl/L). CO₂ enrichment explained up to 77% of the variation in flowering culm count across soils for three of the five species, and was correlated with flowering culm count on at least one soil for four of five species. In contrast, allocation to flowering culms was only weakly correlated with CO₂ enrichment for two species. Flowering culm counts were strongly correlated with species aboveground biomass (AGB; R² = .34–.74), a measure of species abundance. CO₂ enrichment also increased soil moisture and decreased light levels within the canopy but did not affect soil inorganic nitrogen availability. Structural equation models fit across the soils suggested species‐specific controls on flowering in two general forms: (1) CO₂ effects on flowering culm count mediated by canopy light level and relative species AGB (species AGB/total AGB) or by soil moisture effects on flowering culm count; (2) effects of canopy light level or soil inorganic nitrogen on flowering and/or relative species AGB, but with no significant CO₂ effect. Understanding the heterogeneity in species responses to CO₂ enrichment in plant communities across soils in edaphically variable landscapes is critical to predict CO₂ effects on flowering and other plant fitness components, and species potential to adapt to future environmental changes.     

Earthworm effects on plant growth do not necessarily decrease with soil fertility

Earthworms are known to generally increase plant growth. However, because plant-earthworm interactions are potentially mediated by soil characteristics the response of plants to earthworms should depend on the soil type. In a greenhouse microcosm experiment, the responsiveness of plants (Veronica persica, Trifolium dubium and Poa annua) to two earthworm species (in combination or not) belonging to different functional groups (Aporrectodea. caliginosa an endogeic species, Lumbricus terrestris an anecic species) was measured in term of biomass accumulation. This responsiveness was compared in two soils (nutrient rich and nutrient poor) and two mineral fertilization treatments (with and without). The main significant effects on plant growth were due to the anecic earthworm species. L. terrestris increased the shoot biomass and the total biomass of T. dubium only in the rich soil. It increased also the total biomass of P. annua without mineral fertilization but had the opposite effect with fertilization. Mineral fertilization, in the presence of L. terrestris, also reduced the total biomass of V. persica. L. terrestris did not only affect plant growth. In P. annua and V. persica A. caliginosa and L. terrestris also affected the shoot/root ratio and this effect depended on soil type. Finally, few significant interactions were found between the anecic and the endogeic earthworms and these interactions did not depend on the soil type. A general idea would be that earthworms mostly increase plant growth through the enhancement of mineralization and that earthworm effects should decrease in nutrient-rich soils or with mineral fertilization. However, our results show that this view does not hold and that other mechanisms are influential.     

Positive responses of coastal dune plants to soil conditioning by the invasive Lupinus nootkatensis

Invasive nitrogen-fixing plants drive vegetation dynamics and may cause irreversible changes in nutrient-limited ecosystems through increased soil resources. We studied how soil conditioning by the invasive alien Lupinus nootkatensis affected the seedling growth of co-occurring native plant species in coastal dunes, and whether responses to lupin-conditioned soil could be explained by fertilisation effects interacting with specific ecological strategies of the native dune species. Seedling performance of dune species was compared in a greenhouse experiment using field-collected soil from within or outside coastal lupin stands. In associated experiments, we quantified the response to nutrient supply of each species and tested how addition of specific nutrients affected growth of the native grass Festuca arundinacea in control and lupin-conditioned soil. We found that lupin-conditioned soil increased seedling biomass in 30 out of 32 native species; the conditioned soil also had a positive effect on seedling biomass of the invasive lupin itself. Increased phosphorus mobilisation by lupins was the major factor driving these positive seedling responses, based both on growth responses to addition of specific elements and analyses of plant available soil nutrients. There were large differences in growth responses to lupin-conditioned soil among species, but they were unrelated to selected autecological indicators or plant strategies. We conclude that Lupinus nootkatensis removes the phosphorus limitation for growth of native plants in coastal dunes, and that it increases cycling of other nutrients, promoting the growth of its own seedlings and a wide range of dune species. Finally, our study indicates that there are no negative soil legacies that prevent re-establishment of native plant species after removal of lupins.     

Competitive dominance mediates the effects of topography on plant richness in a mountain grassland

Small-scale landforms influence plant species richness, but their mechanisms and effects in semi-natural dry grasslands have been poorly investigated. In this study we compared vascular plant richness, species composition, plant traits, soil properties and biomass nutrient content of convex (hillocks) and concave (hollows) karst landforms in a mountain pasture of the Central Apennines (Italy), at a small spatial scale (1 m² plots). We found hillocks had significantly higher species richness than hollows. On hillocks, smaller Specific Leaf Area and Lateral Width, together with greater allocation of resources to Below-Ground Organs, indicated lower water availability, whereas hollows had deeper (thus moister), more acidic and more fertile soils, with aboveground plant biomass displaying higher nutrient levels. Partial correlation and regression tree models suggested that fine-scale richness patterns were not directly determined by abiotic properties, but were rather the result of competition levels associated with the cover of Agrostis capillaris (=A. tenuis) – a calcifuge and drought-sensitive grass able to achieve dominance only in hollows. The higher functional convergence exhibited by hollows suggests that A. capillaris is a strong competitor both above- and below-ground, mediating the effects of topography by imposing a biotic filter. On hillocks, competition is released by lower levels of available soil water in summer and higher soil pH, resulting in higher species richness and a more functionally divergent assemblage.     

Spatial variation of plant communities and shoot Cu concentrations of plant species at a timber treatment site

Plant species, spatial variability in plant diversity and vegetation cover were recorded at a French timber treatment site with Cu-contaminated soils (65–2600 mg/kg). Shoot biomass, shoot Cu concentration and accumulation were determined for each plant species found on 168 quadrats with increasing total Cu in soil and soil solution. A total of 91 species occurred on the site including four considered as invasive (Cyperus eragrostis, Phytolacca americana, Senecio inaequidens, and Sporobolus indicus). Species richness, Shannon index, vegetation cover and plant biomass decreased as soil Cu increased. At low soil Cu, members of the Poaceae were most frequent followed by Fabaceae, Rosaceae, and Asteraceae. At high soil Cu, Poaceae were again most frequent. Species known to form Cu-tolerant populations, i.e. Agrostis capillaris, A. stolonifera and Rumex acetosella were present. Shoot Cu concentration and accumulation were higher in plants growing in the most contaminated soils. At 2142 mg Cu/kg soil, shoot Cu accumulation peaked at 6 mg Cu/m² in A. capillaris, and its shoot Cu concentration (364 mg Cu/kg dry weight) exceeded the fodder Cu threshold for domestic livestock. In less Cu-contaminated soils some candidates were identified for sustainable phytoremediation with a potential financial return.