Relative Interaction Index Research Articles

Phenotypic plasticity variations in Phragmites australis under different plant–plant interactions influenced by salinity

Abstract Coastal wetland ecosystems are increasingly threatened by escalating salinity levels, subjecting plants to salinity stress coupled with interactions in the community. Abiotic factors can disrupt the balance between competition and facilitation among plant species. Investigating the effects of different neighboring species and trait plasticity could extend the stress gradient hypothesis and enhance understanding of vegetation distribution and diversity in salt marshes. We conducted a greenhouse experiment and investigated the plastic response of wetland grass Phragmites australis to seven neighboring plants of three functional types (conspecifics, graminoids and forbs) under soil salinity (0 and 10 g/L). Plant height, base diameter, density, leaf thickness, specific leaf area and total and part biomasses were measured. Additionally, the relative interaction index (based on biomass) and the relative distance plasticity index (RDPI) were calculated. Salinity significantly reduced the biomass, height, density and diameter of P. australis. The functional types of neighboring plants also significantly affected these growth parameters. The influence of graminoids on P. australis was negative under 0 g/L, but this negative effect shifted to positive facilitation under 10 g/L. The facilitation effect of forbs was amplified under salinity, both supporting the stress gradient hypothesis. The growth traits of P. australis had a plastic response to salinity and competition, such as increasing belowground biomass to obtain more water and resources. The RDPI was higher under salt conditions than in competitive conditions. The plant–plant interaction response to stress varies with plant functional types and trait plasticity.

Rhizosphere microbial community changes due to weed-weed competition

Many species of weeds are present in agricultural areas, but weeds with greater competitive ability normally become dominant in the field. Rhizosphere soil microbiota can influence weed-weed interactions. However, the role of rhizosphere soil microorganisms in weed-weed interactions remains largely unexplored. In this study, we investigated the ecological relationships and microbial taxa present in the rhizosphere of weeds in monoculture and coexistence systems. The weed species Ageratum conyzoides, Ipomoea ramosissima, and Bidens pilosa were grown in monoculture and coexistence pairs under greenhouse conditions for 80 days. The ecological relationships between weeds were analyzed by calculating the Relative Interaction Index (RII) based on the total dry mass of the plants. The rhizosphere microbiome was analyzed after extracting the metagenomic DNA from rhizosphere microbial populations, followed by PCR amplification and sequencing of the 16S rDNA gene and ITS region, using the Illumina MiSeq platform. Competitive interactions were observed for all combinations of weed species. Ageratum conyzoides showed the greatest decrease in dry matter production due to competition. Weed-weed competition changed rhizosphere microbial community composition and bacterial diversity. The abundance of different bacterial genera in rhizosphere soil varied according to the treatments. When comparing the competition between A. conyzoides and B. pilosa and their respective monocultures, the abundances of Opitutus, Diplorickettisiales uncultured and Bdellovibrio increased in B. pilosa monoculture. When comparing the competition between A. conyzoides and I. ramosissima and their respective monocultures, the abundance of Pseudonocardia increased while the abundance of Fimbriiglobus decreased in A. conyzoides monoculture. Already the abundances of Actinospica, Chitinophaga, Gemmatirosa, 1921-2 and Hymenobacter decreased I. ramosissima monoculture. When comparing the competition between B. pilosa and I. ramosissima, and their respective monocultures, the abundances of Flexibacter and Candidatus Xiphinematobacter decreased I. ramosissima monoculture. The abundances of Clostridium and Rhodobacter increased in competition while the abundance of Pajaroellobacter increased in B. pilosa monoculture. The fungal genera that had their abundances significantly altered were Scytalidium, whose abundance decreased in competition of A. conyzoides and B. pilosa when compared to the respective monocultures, while the abundance of Robillarda increased in B. pilosa monoculture. Already the abundance of Myriodontium increased in B. pilosa monoculture when compared with I. ramosissima monoculture and competition between both plants. Weeds can recruit specific taxa depending on the competition situation in which they are involved. Such changes in the rhizobiome are possibly related to the lower competitive ability and dry matter production of A. conyzoides. B. pilosa have a greater capacity to determine the structure of the rhizobiome when in competition with A. conyzoides. I. ramosissima seem to be less affected by rhizobiome taxon-specific changes.

Assessing the occurrence of soil improvement and its relationship to the dominant life form in the high mountains of Central Spain

Soil in mountainous regions is vital to the health and preservation of these unique and diverse ecosystems. In dry and semi-arid regions, vegetation patches play a crucial role in soil nutrient heterogeneity through continuous feedback with soils, acting as barriers to collect runoff water, sediments, and nutrients from bare soil regions. Soil amelioration, an enhancement of soil biogeochemical processes, results in the formation of “fertility islands,” the extent of which is contingent on the plant species in question, as well as nutrient dynamics and water availability. Here, we collected soil from across a two-peak altitudinal gradient in the Sierra de Guadarrama high-mountains, each peak featuring a different dominant vegetation type (herbs vs. cushion-like) to compare soil nutrients and properties between bare soil and vegetation-covered patches (microhabitats). Soil improvement was assessed in the microhabitats using the Relative Interaction Index (RII). Fertility islands were shown to be prevalent in high-mountain ecosystems, as soil quality and properties were higher beneath vegetation-covered regions than bare soils. There was a difference in RIIs between the transects, with greater soil improvement in the cushion-dominated transect than the herb-dominated one. Changes in nutrient levels were unrelated to patch successional stage, indicaing that plant generations may not shape the spatial variability of soil attributes. Instead, species variety or the presence of dominant clonal species increased soil nutrients and aggregate stability, highlighting the importance of root shape and high biomass in nutrient retention and soil reinforcement. Finally, our findings imply that the poor, shallow soils in the examined peaks, in comparison to other mountains, may account for the poor facilitative interactions. Competition for the scarce resources at these peaks may intensify as climate warms. Thus, while these plants may grow with minimum assistance under current climate circumstances, their associations may be especially vulnerable to climate change.

Does the fertile island effect in tamarisk nebkhas enhance in the northern Tarim Basin in the context of global warming?

AbstractWith the intensification of global warming, uncertainty regarding ecosystem evolution trends in drylands has increased. The objective of this article was to test whether the desertification process of tamarisk shrubland was intensified by comparing fertile island effects and soil ecological stoichiometry characteristics at different time periods. The tamarisk shrubland in the “shrubland” and “shrub duneland” stages in the northern Tarim Basin was selected, and soil samples were collected from the tamarisk nabkhas and from the interspaces between tamarisk nebkhas at the 0‐ to 30‐cm soil layers in 2009 and 2018. Soil carbon (C), nitrogen (N), and phosphorous (P) were analyzed, and the C/N, C/P, and N/P ratios were calculated. In comparison with 2009, fertile island effects were completely collapsed in the shrub duneland stage, with no obvious changes in the shrubland stage in 2018. The average soil C/N ratio decreased from a value greater than 25 in both stages. The average value of soil C/P ratio was below 200 in the shrubland and shrub duneland stages, with almost no marked change after nine years. The soil N/P ratio in our study significantly increased in 2018 than in 2009, but the soil N/P ratio was still much less than 11.9. Significantly positive correlations were observed between the relative interaction index (RII) of soil organic carbon (SOC) and soil total nitrogen (TN) and the soil C/P and N/P ratios in the tamarisk nebkhas. These results suggested that (1) fertile island effects were positive in relation to soil stoichiometry in tamarisk nebkhas in most cases; (2) the desertification process was accelerated in the shrub duneland stage, with no obvious progress during the shrubland stage; and (3) the loss of soil C from the collapsed fertile islands could therefore accelerate global warming.

Leaf carbon isotope tracks the facilitation pattern of legume shrubs shaped by water availability and species replacement along a large elevation gradient in Trans-Himalayas.

Understanding patterns and mechanisms of nurse plant facilitation is important to predict the resilience of arid/semi-arid ecosystems to climate change. We investigate whether water availability and nurse species turnover interact to shape the facilitation pattern of widespread legume shrubs along a large elevation gradient. We also investigate whether leaf δ13C of nurse plants can track the facilitation pattern. We measured the relative interaction index (RII) of the number of species within and outside the canopy of two widespread legume shrub species (Caragana gerardiana and Caragana versicolor) alternatively distributed along a large elevation gradient in the Trans-Himalayas. We also assessed the proportional increase of species richness (ISR) at the community level using the paired plot data. To determine site-specific water availability, we measured the leaf δ13C of nurse shrubs and calculated the Thornthwaite moisture index (MI) for each elevation site. Elevational variations in RII, ISR and δ13C were mainly explained by the MI when the effects of soil nitrogen and plant traits (leaf nitrogen and shrub size) were controlled. Variations in RII and ISR across the two nurse species were explained better by δ13C than by smoothly changing climatic factors along elevation. At the transition zone between the upper limit of C. gerardiana (4100 m) and the lower limit of C. versicolor (4200 m), RII and ISR were much higher in C. versicolor than in C. gerardiana under a similar MI. Such an abrupt increase in facilitation induced by nurse species replacement was well tracked by the variation of δ13C. Water availability and nurse species replacement are crucial to shaping facilitation patterns by legume shrubs along a large elevation gradient in dry mountainous regions, such as the Trans-Himalayas. Turnover in nurse species under global change might significantly alter the pattern of nurse plant facilitation associated with water availability, which can be well tracked by leaf δ13C.

Association between bryophytes and seedlings in Nothofagussub‐Antarctic forests

AbstractBryophytes, which include mosses, liverworts and hornworts, indeed play a significant role in the natural dynamics of native forests. They contribute to fundamental ecological processes such as tree regeneration. In this context, the objective was to analyse the association of bryophytes with Nothofagus seedlings, as well as whether the substrates where bryophytes grew influenced bryophyte‐tree seedling association, in sub‐Antarctic forests at two contrasting landscapes (coastal and mountain) of Tierra del Fuego (Argentina), to better understand their impacts on the natural regeneration processes. We hypothesized that bryophytes act as a substrate for seed germination and initial regeneration growth in pure and mixed forests of N. pumilio and N. betuloides, resulting in widespread tree seedlings–bryophyte associations in all landscapes. In each forest and landscape location, 60 transects were established (10 m in length) to evaluate bryophyte cover (by point intercept method) and substrate type (bare soil, decaying wood and litter cover) where they were growing. An adaptation of the relative interaction index (RII) based on tree seedling cover associated with bryophyte species (%) and tree seedling cover in the absence of bryophytes (%) was calculated and analysed to assess competition or facilitation between bryophytes and tree seedlings. Nothofagus pumilio seedlings were less abundant in bryophytes compared to other substrates, suggesting an inhibitory effect on the germination and/or survival of N. pumilio. In contrast, the seedlings of N. betuloides, in both pure and mixed forests, exhibited higher abundance on bryophyte substrates, particularly in mosses at mountain landscape (RII = 0.83 ± 0.08 in litter and 0.62 ± 0.11 in decaying wood). These findings suggest that bryophytes play a facilitating role for N. betuloides seedlings during germination and initial phases of growth, mainly in the mountain. Therefore, their conservation promotes N. betuloides forest continuity, both in pure and mixed structures.

A novel capacity-based approach for measuring alternative interactions in multicriteria decision analysis

The issue of criteria interactions has been well addressed in multicriteria decision analysis research, whereas the interactive effects of alternatives on decision results are yet to be further investigated. To provide new insights into the issue of alternative interactions, we propose a novel approach for measuring alternative interactions by using the λ-capacity, Choquet integral, and criteria interactions. With the notion of alternative interactions being reflected and measured by criteria interactions, the approach first develops an absolute interaction index and a relative interaction index to objectively measure the degree of interactive effects between alternatives under different decision contexts. It then testifies the impact of criteria interactions on alternative interactions. To examine the effectiveness and applicability of the approach, an empirical study on city environmental performance evaluation is conducted. The empirical results show that the interaction between cities as alternatives provides valuable insights into city planning for developing effective city alliance forms.

Nurse shrubs influence plant biodiversity across environmental gradients in rocky outcrops and surrounding rangelands

AbstractQuestionRocky outcrops generally restrict recruitment and survival of plant species due to their environmental conditions including low soil moisture, nutrient shortages and microclimate harshness. Under such severe conditions, nurse plants may play critical roles in preserving plant biodiversity. Although the nurses' effects on plant biodiversity are fully understood in different major biomes, the relative effects of abiotic factors and nurse plants as important factors have rarely been studied in rocky outcrops, and compared to those in the surrounding rangelands. Therefore, we sought to assess the effects of two dominant shrub species (Artemisia aucheri and Prunus pseudoprostrata) on all biodiversity components including taxonomic, functional and phylogenetic diversity across some environmental gradients such as precipitation and soil gradients in rocky outcrops and their surrounding rangelands.LocationThe study was conducted on six sites located in rocky outcrops and their surrounding rangelands across a precipitation gradient (160–910 mm) and under different soil fertility levels in northern Iran.MethodsWe established 80 1‐m2 (1 m × 1 m) plots in each site with respect to the presence of nurse shrubs, and measured biodiversity components within the plots using Rao Q entropies (q0, q1 and q2) and mean pairwise distance (MPD) indices. The relative interaction index (RII) was also calculated to quantify the interaction type and intensity. Finally, variation partitioning analysis was performed to determine the relative importance of biotic interactions and abiotic factors such as precipitation, electrical conductivity (EC) and phosphorus on structuring biodiversity components.ResultsPlant–plant interactions and abiotic factors consistently influenced different components of biodiversity in rocky outcrops and their surrounding rangelands. However, nurse shrubs showed stronger effects on plant biodiversity than abiotic factors by providing suitable microenvironments to harbor less‐tolerant plant species under the severe conditions of outcrops. In addition, microhabitats beneath nurse shrubs significantly responded to soil EC and precipitation gradients, with a decrease in competition intensity under intermediate levels of the gradients. In contrast, nurse shrubs negatively affected different components of biodiversity, with a decrease in biodiversity under intermediate levels of environmental gradients in surrounding rangelands.ConclusionsOur findings show critical roles of nurse shrubs for maintaining plant biodiversity in rocky outcrops depending on their growth form and the biodiversity component considered. In this case, nurse shrubs could reduce double environmental filtering, consistent with establishing a functional paradox trade‐off to support a wide range of plant species with functionally divergent relatedness.

Phosphorus availability mediates plant–plant interaction and field productivity in maize-grass pea intercropping system: Field experiment and its global validation

CONTEXTPlant–plant competitive and facilitative interactions might strongly affect the biodiversity effect and field productivity of intercropping system under the condition of limited resources such as available phosphorus (P). To test this hypothesis, the maize–grass pea intercropping system with five P application gradients (from low- to high-P) was experimentally investigated and the global meta-analysis validation was simultaneously performed. OBJECTIVEThe main objectives of this study included as: 1) to examine the characteristics of plant–plant interaction variations along soil P addition gradients; 2) to identify the shift from interspecific complementarity to selection effect regarding the productivity and nutrient uptake and its driving mechanism; 3) to verify the universality of above phenomenon via global meta-analysis and explore the general pattern of P-dependent plant–plant interactions in the intercropping systems. RESULTS AND CONCLUSIONSRegardless of growing seasons, the total net effect of intercropping system was always >0, showing net facilitative effects of plant–plant interactions on field productivity and nutrient uptake efficiency, relative to monoculture systems. On average, the yield, biomass, N and P uptake were elevated by 3.0–13.7%, 3.2–12.8%, 7.5–20.1% and 5.6–18.9% in intercropping respectively, relative to those of monoculture. On the other hand, the contribution of complementarity effect to the total net effect was up to 78.7%, much more than that of selection effect (21.3%). The relative interaction index tended to decline remarkably with increasing soil P availability (from 0.097–0.126 to 0.027–0.029), suggesting that the facilitation (rather than the competition) dominated in intercropping under the low-P condition. Furthermore, the intercropped grass pea decreased the harvest index and allocated more biomass to vegetative growth as a result of the enhanced interspecific competition under the P-sufficient condition. Finally, the meta-analysis demonstrated that intercropping system had greater relative benefits regarding field productivity and N/P acquisition and utilization than monoculture system. In the meanwhile, the intensity of interspecific facilitative effect was enhanced under the P-deficient condition. SIGNIFICANCEField observations presented the evidences regarding the P-dependent plant-plant interactions and its effects on field productivity in the maize-grass pea intercropping system. The meta-analysis confirmed that high P application decreased the interspecific facilitative effects in terms of field productivity and nutrient utilization in the intercropping systems. But the low-P condition led to the opposite trend, i.e. facilitative effect dominated in the intercropping for greater productivity efficiency.

Disentangling the effects of biomass and productivity in plant competition.

The relationship between competition and productivity in plant communities is unclear, and this is likely to be due to (1) a confusion in the literature between productivity and biomass, (2) the lack of studies assessing variation in competition in all combinations of biomass and productivity. We assessed the outcome of plant-plant interactions by removing the neighbors around five focal species in 14 herbaceous communities with contrasting biomasses and productivities: meadows with high biomass and productivity, heathlands with high biomass and low productivity, understory communities of deciduous forests with low biomass and high productivity and calcareous grasslands with low biomass and low productivity. Competition intensity was quantified with the relative interaction index (RII) calculated for both survival and growth of the transplanted targets assessed with the increase in leaf number. To examine which traits better explain variation in competition and what drives variation in diversity, we also quantified litter decomposition rate, species composition and diversity and six morphological traits related to plant size and growth rate for eight dominant species of each community. Our main questions were: (1) Is competition mostly related to biomass or productivity? (2) Which traits of the community dominants better explain variation in competition? (3) Is variation in competition and related traits correlated with variation in diversity? Competition for survival significantly increased with increasing community biomass (but not productivity). In addition, competition for survival increased with the size traits and competitive effects of the dominant species of the communities, whereas diversity decreased. Competition for growth also increased with increasing productivity, but only for high-biomass communities. Additionally, the increase in competition for growth with increasing soil fertility, as measured with litter decomposition rate, was only due to an increase in target growth in plots without neighbors and was unrelated to community competitive effects and species diversity. The results of our study illustrate how the confusion between productivity and biomass could have contributed to the long-standing debate on variation in competition along productivity gradients and its consequence for diversity.

Temporal stability of productivity is associated with complementarity and competitive intensities in intercropping.

Year-to-year stability in crop production is a crucial aspect of feeding a growing global population. Evidence from natural ecosystems shows that increasing plant diversity generally increases the temporal stability of productivity; however, we have little knowledge of the mechanisms by which diversity affects stability. In fact, understanding the drivers of stability is a major knowledge gap in our understanding of biodiversity and ecosystem function in general. We varied resource inputs into crop monocultures and intercropping of maize/pea and maize/rapeseed for 3 years in field experiments to create a wide range of values for temporal stability, complementarity effects, selection effects, competition, and facilitation. We correlated whole-system temporal stability in productivity with these values and the stability of competitively subordinate species and competitively dominant species in the intercrops. We then used structural equation modeling (SEM), which combines complex path models with latent variables, to estimate how interspecific interactions for water, nitrogen, and phosphorus affected the relationships between stability and these values. Intercropping treatments did not increase stability, but the wide range of stability created by our experiments allowed us to explore the relationship of many factors with stability. Complementarity correlated positively with the temporal stability of grain yield and aboveground biomass, suggesting that either facilitative interactions or niche partitioning shifted over time in ways that promoted stability. Furthermore, the temporal stability of total productivity of intercropping relied most on the stability of more productive species. However, facilitation tested by relative interaction index independently did not correlate with stability, but the temporal stability of the whole system increased as the competitive effects of competitively dominant species (pea and rapeseed) on competitively subordinate species (maize) decreased and was highest when these competitive effects were virtually zero. SEM indicated that as competition for soil nitrogen from competitively dominant species on competitively subordinate species decreased, the overall temporal stability of whole-system aboveground biomass increased. This stability then led to greater stability in grain production. Our findings indicate that complex shifts in complementarity and competitive intensities are likely to be key mechanisms that maintain temporal stability in species-diverse agriculture and, potentially, in natural systems.

Higher facilitation for stress‐intolerant ecotypes along a metal pollution gradient are due to a decrease in performance in absence of neighbours

The study of variation in plant–plant interactions along metal‐pollution gradient is in its infancy, although this is worth to be assessed for both restoration and theoretical perspectives. Additionally, the mechanisms of facilitation at stake in these particular stressed conditions are poorly known. We aim at understanding the importance of species and ecotypes stress‐tolerance in competitive and facilitative responses to neighbours along metal‐pollution gradients. We addressed this goal in a field experiment conducted in a former mining area located in a subalpine grassland of the French pyrenees. Two ecotypes of Armeria muelleri (a highly tolerant species to metal stress) and Agrostis capillaris (a less stress‐tolerant species) were harvested at the highest and lowest levels of pollution within their distribution range and transplanted with and without neighbours along a strong metal‐pollution gradient. The relative interaction index (RII) was used to assess both the effect of neighbours at different stress levels and the effect of metallic stress with and without neighbours. With increasing pollution, plant–plant interactions shifted from negative to positive for Agrostis, but did not vary significantly for Armeria. At high pollution level, the unpolluted Agrostis ecotype was more facilitated than the polluted one, and in benign habitats the polluted Armeria ecotype was more sensitive to competition than the less polluted ecotype. Interestingly, the increase in facilitation with increasing metal stress for the stress‐intolerant Agrostis ecotype was primarily due to a decrease in performance without neighbours, whereas the increase in competition with decreasing metal stress for the stress‐tolerant Agrostis ecotype was primarily due to an increase in performance with neighbours. More generally, these results suggest that the high size‐plasticity of competitive (and stress‐intolerant) species or ecotypes may explain both their competitive effects in benign habitats through neighbour‐trait effects and facilitative responses in stressed habitats in the context of environmental‐severity effects.

Transition in plant–plant facilitation in response to soil water and phosphorus availability in a legume-cereal intercropping system

BackgroundThe tradeoff between negative and positive interactions of facilitated species and facilitators may depend on the degree of resource availability in agroecosystems. However, the rhizospheric mechanisms driving trade-offs that occur along phosphorus (P) and water availability gradients have not yet been systematically clarified. We established three types of root isolation conditions (no barrier, nylon barrier and solid barrier) at different P and water addition levels to address the above issue in a maize-grass pea intercropping system.ResultsThe total yield and biomass net effect (NE) and the relative interaction index (RII) were significantly higher than 0 under all environmental conditions, demonstrating that plant-plant interactions generated positive effects in the intercropping system. The maize yield and biomass RII were 0.029–0.095 and 0.018–0.066, respectively, which indicated that maize growth was constantly facilitated. However, the RII for grass pea yield and biomass exhibited a different trend in comparison with maize. It was higher than 0 (as the facilitated species) under low soil P and moisture conditions and transitioned to values lower than 0 (facilitator species) under high P and moisture conditions, which showed that the type and intensity of plant-plant interactions steadily shifted with the applied stressors. Direct interactions decreased the maize rhizospheric soil pH by 1.5% and 1.9% under Low-P conditions. Notably, the rhizospheric soil acid and alkaline phosphatase secretions of maize and grass pea increased by 17.4–27.4% and 15.3–27.7%, respectively, in P-deficient soils. These results show that plant-plant interactions can effectively relieve P stress by mineralizing organophosphorus in P-deficient soils. Furthermore, the above tendency became more pronounced under drought-stressed conditions. The nylon barrier partially restricted the exchange and utilization of available nutrients and decreased the total yield and biomass by 1.8–7.8% and 1.1–7.8%, respectively. The presence of a solid barrier completely restricted interspecific rhizospheric interactions and decreased the total yield and biomass by 2.1–13.8% and 1.6–15.7%, respectively. Phytate and KH2PO4 addition intensified asymmetric interspecific competition, and grass pea was consistently subjected to competitive pressures.ConclusionBriefly, the tradeoff between facilitation and competition was driven by rhizospheric interactions, and the transition in the intensity and type of interaction was highly dependent on resource availability in a biologically diverse system.

Density effect of Polygonum ferrugineum Wedd. (Polygonaceae) and nutrients concentration of sediment on establishment of Alternanthera philoxeroides (Mart.) Griseb. (Amaranthaceae)

Abstract Aim The effects of neighbor’s species density and nutrient availability in the sediment are essential to understand the structuring rules of emergent macrophyte communities. The objective of this paper was to investigate how the density of Polygonum ferrugineum and the availability of nutrients in the sediment influence the establishment of Alternanthera philoxeroides. Methods After collection, we sectioned the stems of each species so that each propagule obtained was composed of two nodes. These propagules were planted in trays with moist sediment for rooting and development of new leaves for 24 days, and only then were transferred to the experimental mesocosms. Our experimental design had an A. philoxeroides propagule submitted to the following treatments: I – control (planted alone); II – associated with three P. ferrugineum propagules; III – associated with five P. ferrugineum propagules. There were 36 mesocosms arranged inside the greenhouse, with half of them representing conditions of low nutrient availability and half representing conditions of high nutrient availability (with 12.5 g of NPK in the sediment). The experiment lasted 60 days, and the following response variables were measured: aerial length, root length, aerial, root and total dry biomass. The response variables related to biomass were obtained after the plants were dried in an oven at 60°C. We also calculated the relative interaction index (RII) for each treatment, in order to analyze the interactions between the species. Each response variable was analyzed using a two-way ANOVA. Results Among the main results obtained, we can highlight the lower accumulation of biomass in A. philoxeroides under conditions of low nutrient availability and high density of P. ferrugineum. Conclusions These results indicate the negative effects of P. ferrugineum density on the establishment of A. philoxeroides, contributing to the understanding of the dynamics and structuring rules of the emergent macrophyte community.

Interaction analysis of the influence of different factors and benzene exposure on the white blood cell count of workers

Objective: To explore the main influencing factors of leucopenia in benzene workers. Methods: The data of 514 benzene hazardous enterprises and 97 non benzene hazardous enterprises in 16 municipal districts of Tianjin from January to December 2019 were collected. A total of 16113 benzene exposed workers and 5817 non benzene exposed workers were included in the study. The occupational health examination data of workers during on-the-job were analyzed, and they were divided into white blood cell count reduction group and white blood cell count normal group. Additive interaction was used to evaluate the effects of workers' general situation, occupational history, enterprise information and benzene exposure on leukocyte count reduction. Results: The age and length of service of workers in the white blood cell count reduction group were higher than those in the white blood cell count normal group (Z=-3.411, -4.539, P<0.01) . The detection rate of white blood cell count reduction in women was higher than that in men (χ(2)=211.631, P<0.01) . The detection rate of white blood cell count reduction in workers in enterprises located in urban areas, classified as manufacturing and small and micro enterprises was higher than that in workers in suburbs, non manufacturing and large and medium-sized enterprises (χ(2)=8.039, 4.232, 37.478, P<0.05) . The additive interaction analysis of the influencing factors of white blood cell count reduction showed that there was additive interaction between gender and benzene exposure, the relative excess risk ratio (RERI) , attribution ratio (AP) and interaction index (S) were 3.789 (95%CI: 2.280-5.299) , 0.571 (95%CI: 0.414-0.726) and 3.045 (95%CI: 1.699-5.459) , respectively. Conclusion: The management of benzene operation in the production environment of small and micro manufacturing enterprises in Tianjin should be strengthened, as well as the personal protective articles and occupational health protection measures for female operators, so as to protect the health of operators and prevent the occurrence of occupational diseases.

Phylogenetic distance and resource availability mediate direction and strength of plant interactions in a competition experiment.

Phylogenetic ecology uses evolutionary history to improve understanding of plant interactions. Phylogenetic distance can mediate plant interactions such as competition (e.g., via limiting similarity) and facilitation (e.g., via niche complementarity), influencing community assembly patterns. Previous research has found evidence both for and against a relationship between phylogenetic distance and the strength of plant interactions, and has found that other factors, such as trait differences, may be more influential. In addition to phylogenetic distance and species' traits, environmental conditions can also influence competition, with facilitative interactions-particularly among distantly related species-potentially becoming more pronounced under stressful, resource-limited conditions. We tested the prediction that greater phylogenetic distance is associated with decreased competition in a greenhouse experiment using plant species of the North American tallgrass prairie. We calculated the Relative Interaction Index for 81 species pairs using plant height, leaf length, and biomass as indicators of performance. We found that phylogenetic distance alone did not significantly affect competition. However, the interaction between phylogenetic distance and stressful conditions (sandier soils with low nutrient availability and water retention vs. resource-rich potting soil) altered plant traits and competition. Under stressful conditions, more distantly related species competed more strongly, leading to smaller plants. Conversely, under benign conditions more distantly related species pairs competed less and were larger. These results were contrary to our expectations that distant relatives would compete less under stressful conditions. Our experiment provides evidence that, while relatedness alone may not drive competition, phylogenetic distance can nonetheless be influential through interactions with environmental conditions.

Positive intercropping effects on biomass production are species-specific and involve rhizosphere enzyme activities: Evidence from a field study

Less attention has been given to soil enzymes that contribute to beneficial rhizosphere interactions in intercropping systems. Therefore, we performed a field experiment by growing faba bean, lupine, and maize in mono and mixed cultures in a moderately fertile soil. We measured shoot biomass and the kinetic parameters (maximal velocity (Vmax) and Michaelis-constant (Km)) of three key enzymes in the rhizosphere: Leucine-aminopeptidase (LAP), β-1,4-N-acetylglucosaminidase (NAG), and phosphomonoesterase (PHO). Faba bean benefitted in mixed cultures by greater shoot biomass production with both maize and lupine compared to its expected biomass in monoculture. Next, LAP and NAG kinetic parameters were less responsive to mono and mixed cultures across the crop species. In contrast, both the Vmax and Km values of PHO increased in the faba bean rhizosphere when grown in mixed cultures with maize and lupine. A positive relative interaction index for shoot P and N uptake for faba bean showed its net facilitative interactions in the mixed cultures. Overall, these results suggest that over-productivity in intercropping is crop-specific and the positive intercropping effects could be modulated by P availability. We argue that the enzyme activities involved in nutrient cycling should be incorporated in further research.

Artemisia sieberishrubs have contrasting specific effects on understory species in Iranian steppes

AbstractQuestions(a) Are there contrasting short‐ and long‐term effects of shrubs on understorey species that may balance at the community level? (b) Are these effects due to the responses of different understorey species groups? (c) Are these effects contributing to community composition?LocationAnArtemisiasteppe community in Golestan National Park (North‐East Iran).MethodsWe used both the removal and observational methods to separate short‐ from long‐term effects ofArtemisia sieberiBesser on the cover of 18 subordinate species and the relative interaction index (RII) to calculate species’ responses. We conducted a principal components analysis (PCA) on species RII and grouped species with a cluster analysis depending on their responses to the shrubs. Another PCA on understorey composition plots was conducted to assess the consequences of shrub effects for community composition.ResultsArtemisiahad negative short‐term and positive long‐term community‐level effects that balanced each other to produce neutral net effects. Short‐ and long‐term effects were due to the responses of different species groups, dominantly annuals and perennial grasses, respectively. Although balancing at the community level, the different shrub effects significantly contributed to explain community composition.ConclusionsOur study highlights that a dominant foundation shrub may have contrasting short‐ and long‐term effects on different groups of understorey species that contribute to explain community composition, although balancing at the community level.

Nutrient‐use strategy and not competition determines native and invasive species response to changes in soil nutrient availability

Non‐native invasive plants often outcompete native species under high resource availability. Restoration techniques that lower resources may, therefore, create favorable conditions for resource conservative native species over resource exploitative invasive species. Research on this topic has focused on temperate grass and forb‐dominated ecosystems and has rarely been tested for woody species or tropical vegetation. We evaluated growth, resource‐use efficiency (RUE), ecophysiology, and competition (i.e. a relative interaction index based on biomass) of four woody native and two dominant invasive species from Hawaiian wet and dry tropical ecosystems in a greenhouse experiment. Density of plants was constant and species were grown with either a conspecific or the invasive species from that ecosystem type across a gradient of soil nutrient availability. Instantaneous photosynthetic rates varied minimally across nutrient availability. However, both invasive and one native species increased leaf area as nutrients increased, providing more photosynthetic area and increasing total biomass. Nitrogen RUE decreased with increasing nutrient availability for all but one native species, while phosphorus RUE remained constant for all but one native species. Competitive interactions were weak, variable, and not significantly impacted by soil nutrients. Overall, plants categorized as invasive or resource exploitative had larger changes in response variables with increasing soil nutrients compared to those categorized as native or resource conservative. These results suggest that manipulating soil nutrient availability is a potentially viable restoration tool for at least some woody species in tropical ecosystems. However, predicting restoration success requires understanding species‐specific ecophysiological traits determining response to altered environmental conditions.

Fire slightly decreases the competitive effects of a thorny cushion shrub in a semi‐arid mountain steppe in the short term

AbstractQuestionsStudies in alpine environments have highlighted dominant facilitative effects of cushion species for diversity. Many fewer studies have assessed the effects of cushion shrubs in drier and less cold mountain habitats and how these effects vary within the shrub canopy and with increasing drought stress with exposure and disturbance by fire.LocationA mountain steppe of the Golestan National Park in the semi‐arid climate of northeast Iran.MethodsWe quantified the cover of understorey species at the center and edge of the canopy of the thorny cushion shrub Onobrychis cornuta and in paired open areas, in north and south exposures and unburned and burned plots, two years after a fire. Soil chemical variables and moisture were measured in treatments. We quantified cushions’ center and edge effects on understorey species cover with the Relative Interaction Index (RII). Treatment effects on RII cover were analyzed with generalized linear mixed‐effects models (GLMM) and species composition with Correspondence Analysis.ResultsIn unburned plots, competition for cover was very high at cushion centers and strongly decreased at cushion edges. Exposure stress did not affect competition in unburned plots. Fire slightly decreased competition but only at center cushion shrub position and at south exposure. Species composition was mostly affected by exposure and fire treatments, although there was subtle variation in species composition among patches due to fire. Cushion position, fire and exposure treatments affected soil moisture and chemical variables, but soil changes did not explain cushion effects on understorey cover and their variations along treatments.ConclusionsThe effects of the cushion shrub were highly negative and in particular at cushion center, likely due to interference and competition for light in a dry and not too cold environment. Fire only slightly decreased competition in the short term, likely due to the delayed response of understorey species to disturbance induced by fire.