Impact of grassland degradation on soil multifunctionality: Linking to protozoan network complexity and stability

Soil protozoa, as predators of microbial communities, profoundly influence multifunctionality of soils. Understanding the relationship between soil protozoa and soil multifunctionality (SMF) is crucial to unraveling the driving mechanisms of SMF. However, this relationship remains unclear, particularly in grassland ecosystems that are experiencing degradation. By employing 18S rRNA gene sequencing and network analysis, we examined the diversity, composition, and network patterns of the soil protozoan community along a well-characterized gradient of grassland degradation at four alpine sites, including two alpine meadows (Cuona and Jiuzhi) and two alpine steppes (Shuanghu and Gonghe) on the Qinghai-Tibetan Plateau. Our findings showed that grassland degradation decreased SMF for 1-2 times in all four sites but increased soil protozoan diversity (Shannon index) for 13.82-298.01 % in alpine steppes. Grassland degradation-induced changes in soil protozoan composition, particularly to the Intramacronucleata with a large body size, were consistently observed across all four sites. The enhancing network complexity (average degree), stability (robustness), and cooperative relationships (positive correlation) are the responses of protozoa to grassland degradation. Further analyses revealed that the increased network complexity and stability led to a decrease in SMF by affecting microbial biomass. Overall, protozoa increase their diversity and strengthen their cooperative relationships to resist grassland degradation, and emphasize the critical role of protozoan network complexity and stability in regulating SMF. Therefore, not only protozoan diversity and composition but also their interactions should be considered in evaluating SMF responses to grassland degradation, which has important implications for predicting changes in soil function under future scenarios of anthropogenic change.

De novo transcriptomic analysis of Doum Palm (Hyphaene compressa) revealed an insight into its potential drought tolerance

BACKGROUND

Doum palms (Hyphaene compressa) perform a crucial starring role in the lives of Kenya's arid and semi-arid people for empowerment and sustenance. Despite the crop's potential for economic gain, there is a lack of genetic resources and detailed information about its domestication at the molecular level. Given the doum palm's vast potential as a widely distributed plant in semi-arid and arid climates and a source of many applications, coupled with the current changing climate scenario, it is essential to understand the molecular processes that provide drought resistance to this plant.

RESULTS

Assembly of the first transcriptome of doum palms subjected to water stress generated about 39.97 Gb of RNA-Seq data. The assembled transcriptome revealed 193,167 unigenes with an average length of 1655 bp, with 128,708 (66.63%) successfully annotated in seven public databases. Unigenes exhibited significant differentially expressed genes (DEGs) in well-watered and stressed-treated plants, with 45071 and 42457 accounting for up-regulated and down-regulated DEGs, respectively. GO term, KEGG, and KOG analysis showed that DEGs were functionally enriched cellular processes, metabolic processes, cellular and catalytic activity, metabolism, genetic information processing, signal transduction mechanisms, and posttranslational modification pathways. Transcription factors (TF), such as the MYB, WRKY, NAC family, FAR1, B3, bHLH, and bZIP, were the prominent TF families identified as doum palm DEGs encoding drought stress tolerance.

CONCLUSIONS

This study provides a complete understanding of DEGs involved in drought stress at the transcriptome level in doum palms. This research is, therefore, the foundation for the characterization of potential genes, leading to a clear understanding of its drought stress responses and providing resources for improved genetic modification.

Global potential distribution of mangroves: Taking into account salt marsh interactions along latitudinal gradients

Mangrove is one of the most productive and sensitive ecosystems in the world. Due to the complexity and specificity of mangrove habitat, the development of mangrove is regulated by several factors. Species distribution models (SDMs) are effective tools to identify the potential habitats for establishing and regenerating the ecosystem. Such models usually include exclusively environmental factors. Nevertheless, recent studies have challenged this notion and highlight the importance of including biotic interactions. Both factors are necessary for a mechanistic understanding of the mangrove distribution in order to promote the protection and restoration of mangroves. Thus, we present a novel approach of combining environmental factors and interactions with salt marsh for projecting mangrove distributions at the global level and within latitudinal zones. To test the salt marsh interaction, we fit the MaxEnt model with two predicting sets: (1) environments only and (2) environments + salt marsh interaction index (SII). We found that both sets of models had good predictive ability, although the SII improved model performance slightly. Potential distribution areas of mangrove decrease with latitudes, and are controlled by biotic and abiotic factors. Temperature, precipitation and wind speed are generally critical at both global scale and ecotones along latitudes. SII is important on global scale, with a contribution of 5.9%, ranking 6th, and is particularly critical in the 10-30°S and 20-30°N zone. Interactions with salt marsh, including facilitation and competition, are shown to affect the distribution of mangroves at the zone of coastal ecotone, especially in the latitudinal range from 10° - 30°. The contribution of SII to mangrove distribution increases with latitudes due to the difference in the adaptive capacity of salt marsh plants and mangroves to environments. Totally, this study identified and quantified the effects of salt marsh on mangrove distribution by establishing the SII. The results not only facilitate to establish a more accurate mangrove distribution map, but also improve the efficiency of mangrove restoration by considering the salt marsh interaction in the mangrove management projects. In addition, the method of incorporating biotic interaction into SDMs through establish the biotic interaction index has contributed to the development of SDMs.

Facilitation and competition deconstructed: a mechanistic modelling approach to the stress gradient hypothesis applied to drylands

Facilitative interactions among species are key in plant communities. While experimental tests support the Stress Gradient Hypothesis (SGH) as an association between facilitation and stress, whether the shape of net effects along stress gradients can be predicted is controversial, with no available mathematical modelling approaches. We proposed a novel test, using a modification of the R* model to study how negative and positive partial effects of plant interactions in drylands combine along two common stress gradients. We modelled different interactions: competition for water and light, amelioration of soil infiltration and/or grazing protection, obtaining that intensity and importance of facilitation did not generally increase along stress gradients, being dependent on the interaction type. While along the water stress gradient net interactions became more positive, reaching a maximum and then waning again, various outcomes were observed along the grazing gradient. Shape variety was mainly driven by the various shapes of the partial positive effects. Under resource stress, additive interaction effects can be expected, whereas when including grazing, the effects were non-additive. In the context of the SGH, deconstructing the effect of positive and negative interaction in a pairwise mechanistic models of drylands does not show a unique shape along stress gradients.

Natural and regenerated saltmarshes exhibit different bulk soil and aggregate-associated organic and inorganic carbon contents but similar total carbon contents

Saltmarshes are considered to be one of the planet's most efficient carbon sinks. The continued loss of saltmarshes and induced ecological consequences promoted their restoration worldwide. Previous efforts aimed to evaluate the success of restoration in terms of organic carbon accumulation, but inorganic carbon and carbon contents within soil aggregates, which are essential for making a comprehensive assessment of the carbon sink function, were rarely studied. To fill this gap, a range of metrics including bulk and aggregate-associated soil organic and inorganic carbon contents together with the soil's physical, chemical and microbiological parameters were measured to compare natural and a 15-year restoration effort in saltmarsh habitats within the Yellow River Delta region in eastern China. The results showed that regenerated saltmarsh exhibited significantly higher soil organic carbon (SOC) contents but significantly lower soil inorganic carbon contents, resulting in no notable change in total carbon contents between the regenerated and natural saltmarshes. SOC contents within the silt and clay fractions and their contribution to the bulk SOC contents were significantly lower in the regenerated saltmarsh than those in the natural ones (P < 0.05). In regenerated saltmarsh, significantly lower soil microbial biomass and distinct microbial community composition with reduced Gram-negative to Gram-positive bacteria ratios were observed compared to natural saltmarsh. These findings indicate the stability of SOC fraction and soil microbe-mediated carbon biogeochemical processes differed between naturally occurring and artificially regenerated saltmarshes. As interest in blue carbon programs gains global attention, further research on the generation and transformation processes of different carbon fractions during restoration are needed, which can be conducive to elucidating more details in coastal carbon cycling processes.

Relative contribution of canopy and soil effects between plants with different metal tolerance along a metal pollution gradient

Multiple effects, operating either on the long-term (soil-engineering effects) or on the short-term during plant life (microclimate modification or resources pre-emption), can act simultaneously and determine the outcome of plant-plant interactions. These diverse effects have not been disentangled along a gradient of metal/metalloid pollution, although this is crucial for understanding the dominant species turnover along the gradient, and thus the driving processes of facilitation recurrently found in metalliferous ecosystems, which could help improving ecological restoration of these degraded ecosystems. Here, we experimentally assessed different short-term effects of two dominant forbs of highly polluted habitats (Hutchinsia alpina and Arenaria multicaulis, tolerant to metal stress) and two grasses of less polluted habitats (Agrostis capillaris and Festuca rubra, less tolerant to metal stress) on target plant species (the same as the dominant species mentioned above) transplanted along a large metal pollution gradient. Additionally, in highly polluted environments, we differentiated short- from long-term effects of the two metallicolous forbs, which had different abilities to concentrate metals in their leaves. In line with other studies along metal gradients, variation of short-term interactions appeared to follow the Stress Gradient Hypothesis for plants less adapted to metal pollution (p = 0.030), with positive interactions dominating in most severe areas. Regarding long-term effects, the species with highest leaf metal-accumulation showed no negative effect contrary to the Elemental allelopathy Hypothesis. Long-term effects of the species with lower leaf-metal accumulation could not be determined because of the occurrence of an unexpected difference in micro-habitat conditions (soil depth and humidity) for this species along the metal pollution gradient. Increasing short-term facilitation along metal pollution gradients, which confirmed previous studies, is promising for improving conditions and restoring the most polluted environments. However, long-term results stressed the difficulty to quantify these effects given that these areas are highly fragmented and heterogeneous.

It takes a village to grow a tree: Most tree species benefit from dissimilar neighbors

Scientific consensus is that diverse tree species positively impact forest productivity, especially when species are functionally dissimilar. Under the complementarity hypothesis, differences in species traits reduce competition among neighboring tree species. However, while this relationship has been extensively studied at the community level, there is a lack of understanding regarding how individuals of different species specifically respond to a functionally dissimilar neighborhood. In this study, we used permanent plots from Quebec, Canada, and 19 focal tree species to test whether: (1) tree growth response to neighborhood dissimilarity varies with their identity and competition intensity, and (2) focal tree species' traits explain their response to neighborhood dissimilarity. We demonstrate that: tree growth is primarily influenced by competition, species identity, and their interactions, but that dissimilarity, alone and in interaction with the main drivers of tree growth, explains an additional 1.8% of the variation in species growth. Within this context, (1) most species' respond positively to neighborhood dissimilarity, with magnitude being species and competition dependent, and (2) focal tree traits partly explain these dependencies, with shade-intolerant species benefiting most from dissimilar neighbors under high competition. Our study provides empirical support for the complementarity hypothesis, emphasizing the small but consistent positive effect of functional dissimilarity on tree growth in local neighborhoods. Our findings identify the species with the highest potential of benefiting from dissimilar neighbors but also demonstrate that the positive effect of neighborhood dissimilarity is not limited to a select few species with specific traits; rather, it is observed across a diverse range of species. The cumulative growth responses of individuals to functionally dissimilar neighbors may help explain the commonly observed higher productivity in more diverse communities.

Disentangling the roles of chance, abiotic factors and biotic interactions among epiphytic bryophyte communities in a tropical rainforest (Yunnan, China)

Epiphytes offer an appealing framework to disentangle the contributions of chance, biotic and abiotic drivers of species distributions. In the context of the stress-gradient theory, we test the hypotheses that (i) deterministic (i.e., non-random) factors play an increasing role in communities from young to old trees, (ii) negative biotic interactions increase on older trees and towards the tree base, and (iii) positive interactions show the reverse pattern. Bryophyte species distributions and abiotic conditions were recorded on a 1.1 ha tropical rainforest canopy crane site. We analysed co-occurrence patterns in a niche modelling framework to disentangle the roles of chance, abiotic factors and putative biotic interactions among species pairs. 76% of species pairs resulted from chance. Abiotic factors explained 78% of non-randomly associated species pairs, and co-occurrences prevailed over non-coincidences in the remaining species pairs. Positive and negative interactions mostly involved species pairs from the same versus different communities (mosses versus liverworts) and life forms, respectively. There was an increase in randomly associated pairs from large to small trees. No increase in negative interactions from young to old trees or from the canopy to the base was observed. Our results suggest that epiphytic bryophyte community composition is primarily driven by environmental filtering, whose importance increases with niche complexity and diversity. Biotic interactions play a secondary role, with a very marginal contribution of competitive exclusion. Biotic interactions vary among communities (mosses versus liverworts) and life forms, facilitation prevailing among species from the same community and life form, and competition among species from different communities and life forms.

Dose-Dependent and Species-Specific Effects of Wood Distillate Addition on the Germination Performance of Threatened Arable Plants

Wood distillate (WD) is a bio-based product applied to crop plants for its known action in terms of growth promotion and yield enhancement, but studies are lacking on its effects on the germination of arable plants. To test such effects, we applied WD at six different concentrations on the diaspores of three threatened arable plants: Bromus secalinus, Centaurea cyanus, and Legousia speculum-veneris. For all the studied species, the effect of WD was dose-dependent and species-specific. In B. secalinus, the germination percentage (GP) decreased at 0.125% WD but then remained stable at higher concentrations up to 1%. At 2% WD, almost no germination was observed. Mean germination time (MGT) was not influenced up to 1% WD but significantly increased at 2% WD. The germination rate index (GRI) and germination energy (GE) remained unaffected up to 1% WD but decreased at 2% WD. In C. cyanus, WD had no effects on GP and GE at any concentration. MGT showed no difference with the control up to 1% WD, but significantly increased at 2% WD. GRI increased only at low concentrations (0.125% and 0.25%). The germination performance of L. speculum-veneris was unaffected up to 0.25% WD for all the tested parameters. From 0.5% WD, a reduction in GP, GRI, and GE and an increase in MGT were observed. At 2% WD, germination was totally blocked. Our results suggest that using WD at low concentrations (<0.5%), those commonly used in arable crops, does not affect the germination of the three investigated plant species.

Invasive Spartina alterniflora habitat forms high energy fluxes but low food web stability compared to adjacent native vegetated habitats

Invasive Spartina spp. mostly colonizes a bare tidal flat and then establishes a new vegetated habitat, where it promotes the productivity of local ecosystems. However, it was unclear whether the invasive habitat could well exhibit ecosystem functioning, e.g. how its high productivity propagates throughout the food web and whether it thereby develops a high food web stability relative to native vegetated habitats. By developing quantitative food webs for a long-established invasive Spartina alterniflora habitat and adjacent native salt marsh (Suaeda salsa) and seagrass (Zostera japonica) habitats in China's Yellow River Delta, we investigated the distributions of energy fluxes, assessed the stability of food webs, and investigated the net trophic effects between trophic groups by combining all direct and indirect trophic interactions. Results showed that the total energy flux in the invasive S. alterniflora habitat was comparable to that in the Z. japonica habitat, whereas 4.5 times higher than that in the S. salsa habitat. While, the invasive habitat had the lowest trophic transfer efficiencies. Food web stability in the invasive habitat was about 3 and 40 times lower than that in the S. salsa and Z. japonica habitats, respectively. Additionally, there were strong net effects caused by intermediate invertebrate species in the invasive habitat rather than by fish species in both native habitats. This study revealed the contradiction between the promotion of energy fluxes and the decrease of food web stability resulting from the invasion of S. alterniflora, which provides new insights into the community-based management of plant invasions.

Phylogenetic and functional constraints of plant facilitation rewiring

Facilitative interactions bind community species in intricate ecological networks, preserving species that would otherwise be lost. The traditional understanding of ecological networks as static components of biological communities overlooks the fact that species interactions in a network can fluctuate. Analyzing the patterns that cause those shifts can reveal the principles that govern the identity of pairwise interactions and whether they are predictable based on the traits of the interacting species and the local environmental contexts in which they occur. Here we explore how abiotic stress and phylogenetic and functional affinities constrain those shifts. Specifically, we hypothesize that rewiring the facilitative interactions is more limited in stressful than in mild environments. We present evidence of a distinct pattern in the rewiring of facilitation-driven communities at different stress levels. In highly stressful environments with a firm reliance on facilitation, rewiring is limited to growing beneath nurse species with traits to overcome harsh stressful conditions. However, when environments are milder, rewiring is more flexible, although it is still constrained to nurses that are close relatives. Understanding the ability of species to rewire their interactions is crucial for predicting how communities may respond to the unprecedented rate of perturbations on Earth.

An evolutionary framework for understanding habitat partitioning in plants

Many plant species with overlapping geographic ranges segregate at smaller spatial scales. This spatial segregation-zonation when it follows an abiotic gradient and habitat partitioning when it does not-has been experimentally investigated for over a century often using distantly related taxa, such as different genera of algae or barnacles. In those foundational studies, trade-offs between stress tolerance and competitive ability were found to be the major driving factors of habitat partitioning for both animals and plants. Yet, the evolutionary relationships among segregating species are usually not taken into account. Since close relatives are hypothesized to compete more intensely and are more likely to interact during mating compared to distant relatives, the mechanisms underlying habitat partitioning may differ depending on the relatedness of the species in question. Here, I propose an integration of ecological and evolutionary factors contributing to habitat partitioning in plants, specifically how the relative contributions of factors predictably change with relatedness of taxa. Interspecific reproductive interactions in particular are understudied, yet important drivers of habitat partitioning. In spatially segregated species, interspecific mating can reduce the fitness of rare immigrants, preventing their establishment and maintaining patterns of spatial segregation. In this synthesis, I review the literature on mechanisms of habitat partitioning in plants within an evolutionary framework, identifying knowledge gaps and detailing future directions for this rapidly growing field of study.

Aquatic ecosystem responds differently to press and pulse nutrient disturbances as revealed by a microcosm experiment

Due to climate change and increasing anthropogenic activities, lakes are disturbed frequently, usually by press (e.g., diffused pollution, rising temperatures) or pulse (e.g., storms, rainfall, pollution events) disturbances. Both press and pulse disturbances can affect abiotic and biotic environments, changing the structure of ecosystems and affecting ecosystem services. To confront with the effects of climate change and increasing anthropogenic activities, understanding the different effects of press and pulse disturbances on lake ecosystems is essential. This study assessed the effect of press and pulse disturbances of phosphorus on a microcosmic aquatic ecosystem by measuring the total phosphorus (TP), algae density, and physiological indicators of submerged macrophytes. We found that the microcosmic aquatic ecosystem responded differently to press and pulse disturbances. Our results suggested that it had a lower resistance to pulse phosphorus disturbances than to press phosphorus disturbances. There were significantly higher nutrient concentrations and algal densities in the pulse treatment than in the press treatment. Positive feedback was found between the biomass of submerged macrophytes and the water quality. There was a higher submerged macrophytes biomass at low TP concentration and algal density. In the context of climate change, press and pulse disturbances could have severe impacts on lake ecosystems. Our findings will provide some insight for further research and lake management.

The salt secretion of leaves promotes the competitiveness of Reaumuria soongarica in a desert grassland

BACKGROUND

For better understanding the mechanism of Reaumuria soongarica community formation in a salt stressed grassland ecosystem, we designed a field experiment to test how leaves salt secretion changes the competitive relationship between species in this plant communities.

RESULTS

Among the three species (R. soongarica, Stipa glareosa and Allium polyrhizum) of the salt stressed grassland ecosystem, the conductivity of R. soongarica rhizosphere soil was the highest in five soil layers (0-55 cm depth). The high soil conductivity can increase the daily salt secretion rate of plant leaves of R. soongarica. In addition, we found the canopy size of R. soongarica was positively related to the distance from S. glareosa or A. polyrhizum. The salt-tolerance of R. soongarica was significantly higher than the other two herbs (S. glareosa and A. polyrhizum). Moreover, there was a threshold (600 µS/cm) for interspecific competition of plants mediated by soil conductivity. When the soil conductivity was lower than 600 µS/cm, the relative biomass of R. soongarica increased with the soil conductivity increase.

CONCLUSIONS

The efficient salt secretion ability of leaves increases soil conductivity under the canopy. This leads the formation of a "saline island" of R. soongarica. Meanwhile R. soongarica have stronger salt tolerance than S. glareosa and A. polyrhizum. These promote the competitiveness of R. soongarica and inhibit interspecies competition advantage of the other two herbs (S. glareosa and A. polyrhizum) in the plant community. It is beneficial for R. soongarica to establish dominant communities in saline regions of desert grassland.

The physicochemical approaches of altering growth and biochemical properties of medicinal plants in saline soils

Medicinal plants are important sources of biochemical compounds affecting human health. However, because large areas of the world are subjected to different stresses including salinity, it is important to find methods, which may control the growth and biochemical properties of medicinal plants in such conditions. Another aspect of cropping medicinal plants in saline soils is the alteration of their biochemical properties by stress. Due to the significance of planting medicinal plants in saline soils, the objective of the present review article is to investigate and analyze the physicochemical approaches including soil leaching, organic fertilization, mineral nutrition, ozonated water, magnetism, superabsorbent polymers, and zeolite, which may control the effects of salinity stress on the growth and biochemical properties (production of secondary metabolites) of medicinal plants. In our just-published review article, we investigated the biological approaches, which may affect the growth and biochemical properties of medicinal properties in saline soils. Although salinity stress may induce the production of biochemical products in medicinal plants, the use of physicochemical approaches is also recommendable for the improved growth and biochemical properties of medicinal plants in saline soils. More has yet to be indicated on the use of the physicochemical approaches, which may affect the growth and biochemical properties of medicinal plants in salt stress conditions. KEY POINTS: • Growth and physiological alteration of medicinal plants in salt stress conditions. • The physicochemical approaches of such alteration have been reviewed. • More has yet to be indicated on the approaches, which may affect such properties.

Diversity and composition of macroinvertebrate communities in a rare inland salt marsh

Inland salt marshes are rare habitats in the Great Lakes region of North America, formed on salt deposits from the Silurian period. These patchy habitats are abiotically stressful for the freshwater invertebrates that live there, and provide an opportunity to study the relationship between stress and diversity. We used morphological and COI metabarcoding data to assess changes in diversity and composition across both space (a transect from the salt seep to an adjacent freshwater area) and time (three sampling seasons). Richness was significantly lower at the seep site with both datatypes, while metabarcoding data additionally showed reduced richness at the freshwater transect end, consistent with a pattern where intermediate levels of stress show higher diversity. We found complementary, rather than redundant, patterns of community composition using the two datatypes: not all taxa were equally sequenced with the metabarcoding protocol. We identified taxa that are abundant at the salt seep of the marsh, including biting midges (Culicoides) and ostracods (Heterocypris). We conclude that (as found in other studies) molecular and morphological work should be used in tandem to identify the biodiversity in this rare habitat. Additionally, salinity may be a driver of community membership in this system, though further ecological research is needed to rule out alternate hypotheses.

Niche separation and weak interactions in the high tidal zone of saltmarsh-mangrove mixing communities

Saltmarsh-mangrove ecotones occur at the boundary of the natural geographic distribution of mangroves and salt marshes. Climate warming and species invasion can also drive the formation of saltmarsh-mangrove mixing communities. How these coastal species live together in a "new" mixed community is important in predicting the dynamic of saltmarsh-mangrove ecosystems as affected by ongoing climate change or human activities. To date, the understanding of species interactions has been rare on adult species in these ecotones.Two typical coastal wetlands were selected as cases to understand how mangrove and saltmarsh species living together in the ecotones. The leaves of seven species were sampled from these coastal wetlands based on their distribution patterns (living alone or coexisting) in the high tidal zone, and seven commonly used functional traits of these species were analyzed.We found niche separation between saltmarsh and mangrove species, which is probably due to the different adaptive strategies they adopted to deal with intertidal environments.Weak interactions between coexisting species were dominated in the high tidal zone of the two saltmarsh-mangrove communities, which could be driven by both niche differentiation and neutral theory.Synthesis. Our field study implies a potential opportunity to establish a multispecies community in the high tidal zone of saltmarsh-mangrove ecotones, where the sediment was characterized by low salinity and high nitrogen.

Asymmetric responses of spatial variation of different communities to a salinity gradient in coastal wetlands

Various ecological communities are susceptible to the salinity gradients in coastal wetlands. Remane diagram has well described the macrozoobenthos diversity pattern along salinity gradients. Yet, further research is still needed, that is, the changes in diversity and biomass of other communities (e.g. plants, fish) along salinity gradients, and whether these changes are consistent or different among different communities. In this study, using China's Yellow River Delta wetland as a case study, we analyzed the variation of the community composition, species richness, and biomass of plant, macrozoobenthos, and fish communities along a salinity gradient from <0.5 to 30 ppt. We found that plant community composition exhibited more distinct variation along the salinity gradient than macrozoobenthos, with the least distinction for fish. Plant species richness decreased greatly along the gradient, whereas macrozoobenthos richness first decreased and then increased with increasing salinity, with the low richness occurring at a salinity of 0.9-12.3 ppt. Fish had the highest richness at a salinity of 14.8-16.0 ppt. The sum of plant, macrozoobenthos, and fish species and macrozoobenthos richness were both similar to the Remane diagram. Plants had higher biomass in low-salinity zones than in high-salinity zones, except for high biomass at a salinity of 14.8-16.0 ppt, whereas macrozoobenthos and fish showed the opposite trend. Principal-coordinate analysis showed an obvious dissimilarity map based on the composition, richness, and biomass of the plant, macrozoobenthos, and fish communities. Overall, the effects of salinity gradient differed among different communities. These findings demonstrate the asymmetric responses of different communities to salinity gradients, and have practical implications for maintaining a salinity gradient in coastal wetlands.

Facilitation of an invader by a native habitat-former increases along interacting gradients of environmental stress

Native habitat-forming species can facilitate invasion by reducing environmental stress or consumer pressure. However, the intensity of one stressor along a local gradient may differ when expanding the scale of observation to encompass major variations in background environmental conditions. In this study, we determined how facilitation of the invasive porcelain crab, Petrolisthes elongatus, by the native tube-forming serpulid, Galeolaria caespitosa, varied with environmental gradients at local (tidal height) and larger (wave exposure) spatial scales. G. caespitosa constructs a complex calcareous matrix on the underside of intertidal boulders and we predicted that its positive effects on P. elongatus density would increase in intensity with shore height and be stronger at wave-sheltered than wave-exposed locations. To test these predictions, we conducted two experiments. First, we determined the effects of serpulid presence (boulders with live or dead serpulid matrix vs. bare boulders) at six shore heights that covered the intertidal distribution of P. elongatus. Second, we determined the effects of serpulid presence (present vs. absent), shore height (high vs. low) and wave exposure (sheltered vs. exposed) on crabs across six locations within the invaded range in northern Tasmania, Australia. In Experiment 1, the presence of serpulids (either dead or alive) enhanced P. elongatus densities at all shore heights, with facilitation intensity (as determined by a relative interaction index; RII) tending to increase with shore height. In Experiment 2, serpulids facilitated P. elongatus across shore heights and wave exposures, although crab densities were lower at high shore levels of wave-sheltered locations. However, the intensity of crab facilitation by serpulids was greater on wave-sheltered than on wave-exposed shores, but only at the high shore level. This study demonstrates that local effects of native habitat-formers on invasive species are dependent on prevailing environmental conditions at larger spatial scales and that, under more stressful conditions, invaders become increasingly reliant on positive interactions with native habitat-formers. Increased strength of local-scale facilitation by native species, dampening broader scale variations in environmental stressors, could enhance the ability of invasive species to establish self-sustaining populations in the invaded range.

Theory predicts plants grow roots to compete with only their closest neighbours

The combination of individual-based selection with shared access to resources drives individuals to invest more than necessary in taking up their share of resources due to the threat of other individuals doing the same (competitive overinvestments). This evolutionary escalation of investment is common, from deer antlers and peacock feathers to tree height and plant roots. Because plant roots seem to be well intermingled belowground, the simplifying assumption that belowground resources are perfectly well mixed is often made in models-a condition that favours maximal fine-root overinvestments. Here, I develop simple models to investigate the role of space in determining the overlap among individuals belowground and resulting fine-root biomass. Without costs of growing roots through space, evolutionary optimization leads individuals to intermingle their fine roots perfectly and to invest just as much in these roots, whether there are two individuals competing or many. However, if there are any costs of sending roots through soil, investment in fine roots is constrained in amount and spatial extent. Dominant individuals are those that keep their roots in the soil closest to their own stem and the stems of their closest neighbours. These results highlight the importance of space in determining individual strategies as well as competitive networks.

Density-Dependent Plant-Plant Interactions Triggered by Grazing

Plant species performance in rangelands highly depends on the effect of grazing and also on the occurrence of unpalatable benefactor species that can act as biotic refuges protecting neighboring plants from herbivores. The balance between facilitation and competition may changes with the benefactor density. Despite the high number of studies on the role of biotic refuges, the density dependent effects of unpalatable herbaceous plants on the performance of other species, and on the habitat heterogeneity of rangelands are still unclear. Therefore, we performed a study to test the following hypotheses: (i) Performances of understory species follow a humped-back relationship along the density gradient of the unpalatable benefactor species. (ii) Small-scale heterogeneity of the vegetation decreases with increasing benefactor density. We studied meadow steppes with medium intensity cattle grazing in Hungary. We surveyed understory species' performance (number of flowering shoots and cover scores) along the density gradient of a common, native unpalatable species (Althaea officinalis). Our findings supported both hypotheses. We found unimodal relationship between the benefactor cover and both the flowering success and richness of understory species. Moreover, small-scale heterogeneity declined with increasing benefactor cover. In this study we detected a humped-back pattern of facilitation along the density gradient of an herbaceous benefactor in pastures. Indeed, this pattern was predictable based on such conceptual models like "consumer pressure-abiotic stress model," "humped-back model," "intermediate disturbance hypothesis," and "disturbance heterogeneity model"; but until now the validity of these relationships has not been demonstrated for herbaceous species. By the demonstration of this effect between herbaceous species we can better forecast the responses of grasslands to changes in management.

Facilitation from an intraspecific perspective - stress tolerance determines facilitative effect and response in plants

Plant-plant interactions are reciprocal and include effects on and response to neighbours. Distinct traits confer competitive effect and response ability, but how specific traits determine effect and response in facilitative interactions has not been studied experimentally. We utilized the model species Arabidopsis thaliana to test for trait dependence of facilitative interactions. Salt-sensitive (sos) mutants or salt-tolerant wild-types were exposed to an experimental salinity gradient with and without intraspecific neighbours and the intensity of plant-plant interactions was measured for three performance variables. We tested whether salt tolerance can predict facilitative effect and response and whether a tradeoff exists between competitive ability and tolerance to stress. Interactions shifted very clearly from negative to positive with increasing stress. Salt-sensitive genotypes were less negatively affected by competition but more dependent on facilitation than were wild-types, indicating a tradeoff between competitive ability and stress tolerance. Surprisingly, sensitive genotypes imposed stronger facilitative effects, despite being much smaller under stress, probably because they retrieved more salt from the soil. Stress tolerance defined facilitative effect and response via distinct mechanisms. We advocate more controlled experiments with model species to advance our understanding of the trait dependence of biotic interactions and their consequences for community organization.

Participatory coastal management through elicitation of ecosystem service preferences and modelling driven by "coastal squeeze"

The Baixo Vouga Lagunar (BVL) is part of Ria de Aveiro coastal lagoon in Portugal, which is classified as a Special Protection Area under the European Habitats and Birds Directives. This part of the system, corresponding to the confluence of the Vouga River with the lagoon, is very important culturally and socioeconomically for the local communities, taking place several human activities, especially agriculture. To prevent salt water intrusion from the Ria de Aveiro into agriculture fields, a floodbank was initiated in the 90's. In frame of ongoing changes in Ria de Aveiro hydrodynamics, the existing floodbank will be now extended, introducing further changes in the ecological dynamics of the BVL and its adjacent area. As a consequence, the water level in the floodbank downstream side is expected to rise, increasing the submersion period in tidal wetlands, and leading to coastal squeeze. The aim of this study is to apply an ecosystem based-management approach to mitigate the impacts on biodiversity resulting from the management plan. To do so, we have modelled the implications of the changes in several hydrological and environmental variables on four saltmarsh species and habitats distribution, as well as on their associated ecosystem services, both upstream and downstream of the floodbank. The ecosystem services of interest were prioritized by stakeholders' elicitation, which were then used as an input to a spatial multi-criteria analysis aimed to find the best management actions to compensate for the unintended loss of biodiversity and ecosystem services in the BVL. According to our results, the main areas to be preserved in the BVL were the traditional agricultural mosaic fields; the freshwater courses and the subtidal estuarine channels. By combining ecology with the analysis of social preferences, this study shows how co-developed solutions can support adaptive management and the conservation of coastal ecosystems.