Relative Neighbor Effect Index Research Articles

Invasion of Trifolium repens L. aggravated by biodegradable plastics: adjustable strategy for foraging N and P

The invasion of alien plant and the pollution caused by soil microplastics have emerged as significant ecological threats. Recent studies have demonstrated aggravating effect of non-biodegradable microplastics on plant invasion. However, the impact of biodegradable microplastics (BMPs) on plant invasion remains unclear. Therefore, it is imperative to explore the impact of BMPs on plant invasion. In this study, a 30-day potting experiment with Trifolium repens L. (an invasive plant) and Oxalis corniculata L. (a native plant) was conducted to evaluate the influence of BMPs on T. repens's invasion. The findings revealed that BMPs results in a reduction in available N and P contents, thereby facilitating the colonization of arbuscular mycorrhizal fungi on T. repens 's roots. Consequently, T. repens adjusted its N and P foraging strategy by increasing P absorption ratio, and enhancing the accumulation of N and P in leaves. This ultimately led to the decrease of relative neighbor effect index of T. repens, indicating an aggravated invasion by T. repens. This study significantly enhances and expands the understanding of mechanisms by which microplastics aggravate plant invasion.

Effects of above- and below-ground herbivore interactions on interspecific relationship between the invasive plant Alternanthera philoxeroides and its native congener Alternanthera sessilis

Biological invasion is a major threat to global biodiversity. The relative interspecific competition abilities of invasive species compared to those native species determine their invasion success. In this study, we examined the effects of the specialist leaf beetle Agasicles hygrophila and the nematode Meloidogyne incognita on the growth and interspecific relationship between the invasive plant Alternanthera philoxeroides and its native congener A. sessilis. Compared without herbivory, nematode herbivory alone significantly reduced shoot height of A. sessilis by 28.1%, but conversely significantly increased the shoot height of A. philoxeroides by 52.8% and aboveground biomass of A. sessilis by 63.7%. Beetle herbivory alone significantly reduced shoot height of A. sessilis by 40.7%, but did not affect that of A. philoxeroides. The combination of beetle and nematode herbivory significantly reduced shoot height of A. sessilis by 35.3% as well as the belowground biomass of A. philoxeroides by 62.2%, but significantly increased the aboveground biomass of A. sessilis by 69.1%. Herbivore stress did not affect stem diameter, branch number, and root length of both species. The relative neighbor effect index (RNE) of the two species without herbivory were positive, and the RNE value of A. philoxeroides was 21.3% higher than that of A. sessilis. However, the RNE values of A. philoxeroides were negative under all above- and below-ground herbivory treatments. The RNE values of A. sessilis were positive under the beetle or the nematode herbivory alone and negative under the beetle + nematode herbivory combination. These results indicated that above- and below-ground herbivore interactions could change the interspecific relationship between the two species, and in turn might accelerate the invasion of A. philoxeroides.

Competition between two wetland macrophytes under different levels of sediment saturation

Plant-plant interactions have been widely studied under various environmental conditions. However, in wetland ecosystems how plant interactions change in response to variation in sediment saturation remains largely unclear, even though different levels of sediment saturation play important roles in determining plant growth performance in wetland ecosystems. To this end, a competition experiment with two typical wetland species, Carex brevicuspis (neighbor plant) and Polygonum hydropiper (target plant), was conducted in a target-neighbor design. Two water levels (0 cm and -40 cm water levels representing waterlogged and drained sediments, respectively) and three neighbor plant densities (0 plants m -2 , 400 plants m -2 , and 1600 plants m -2 ) were tested in a factorial design. Biomass accumulation of P. hydropiper decreased along with enhanced C. brevicuspis density in the waterlogged treatment. However, in the drained treatment, biomass accumulation did not change under two C. brevicuspis densities. Above-ground relative neighbor effect index (ARNE) and relative neighbor effect index (RNE) of C. brevicuspis on P. hydropiper increased along with enhanced C. brevicuspis density only under waterlogged conditions. The below-ground relative neighbor effect index (BRNE) was not affected at the different water level and density treatments. The below-ground mass fraction of P. hydropiper was much higher in the waterlogged treatment than it was in the drained one, especially with no C. brevicuspis treatment. However, the leaf mass fraction displayed the opposite pattern. The longest root length of P. hydropiper was much shorter under waterlogged treatment than under the drained treatment. These results suggest that the competition intensity of C. brevicuspis to P. hydropiper increased along with increasing C. brevicuspis density only under waterlogged conditions. Moreover, this study also confirms that P. hydropiper can acclimate to water stress mainly through modulating its root architecture, for example reducing the root system length or increasing the root mass fraction.

Does land use change affect the interactions between two dry grassland species?

Over the last 20 years, a change in traditional land use practices has taken place in central Germany. Formerly species-rich dry grassland communities have been converted into communities with greatly reduced diversity in many places. Whereas grass species have expanded, several forbs have declined in abundance. For the present study, plant–plant interactions were assessed between the expanding grass Festuca rupicola and the forb Dianthus carthusianorum – two typical, companion grassland species – to ascertain any associated effects of land use change. A competition experiment was set up with replacement design, in which pots were placed in the open field and monitored over two consecutive years. In order to assess the performance of both species in monoculture (D9, F9) and mixture (D3F6, D6F3) the parameters biomass, length of flowering stems, number of flowering stems and number of flowers per individual, or length of panicles, were analyzed. Positive and negative interactions were evaluated using the “Relative Neighbour Effect” index (RNE). To simulate land use by grazing or mowing, individuals in half of the pots were clipped in the first year after sowing. At this time, performance of D. carthusianorum – in terms of length of flowering stems, number of flowering stems and number of flowers – was significantly affected: whereas the first parameter increased, both of the other parameters decreased with increasing presence of the grass species. However, RNE indicated facilitation with a low number of F. rupicola individuals, and competition with a high number of F. rupicola individuals. In contrast, F. rupicola did not show any significant differences in performance with increasing presence of D. carthusianorum, and the RNE switched from neutral interaction to facilitation. In the second year after sowing, the performance of D. carthusianorum in the various treatments did not differ, except for the length of the flowering stems. The simulation of land use practices did not affect the performance of D. carthusianorum, but F. rupicola showed significant differences in biomass and in length of the panicles, as was the case also between monocultures and mixtures. The performance of the grass species was clearly improved in the unclipped treatments. The RNE reflected competition against D. carthusianorum which became reduced in unclipped treatments, while F. rupicola became facilitated. There were no interactions between the different treatments (monocultures and mixtures) and the simulated land use change (clipped and unclipped). Our data suggest that although traditional land use practices, i.e. grazing and mowing, do not affect the development of D. carthusianorum, the abandonment of these practices has improved F. rupicola performance, which has led to the increasingly invasive character of this grass species over the last 20 years.

Competitive Ability of Black Henbane (Hyoscyamus niger) When Grown with Three Native Grasses

AbstractBlack henbane is a poisonous, invasive plant in the family Solanaceae, and is typically associated with highly disturbed environments, such as pipelines, roadsides, and mammalian burrows. Often, such disturbances require reseeding for successful restoration; thus, the potential exists for competition between henbane and perennial grasses commonly used in restoration projects. These competitive interactions have not, to our knowledge, been evaluated. We conducted a greenhouse study to compare the response of henbane when grown alone and in combination with three common, cool season, perennial, northern mixed prairie grass species. We examined both seedling and mature grass response to the presence or absence of henbane and the response of henbane to the grasses. Using the relative neighbor-effect index, black henbane was found to be a very poor competitor with mature grasses and two out of three seedling grasses tested. All measures of henbane growth were significantly lower among plants grown with a mature grass pot companion. Total biomass of henbane was up to 99% lower when grown with mature grasses. Mature grasses were not negatively affected when grown in combination with henbane. Western wheatgrass (Pascopyrum smithii) was the only seedling grass that was competitive with henbane but was also the only seedling grass negatively affected by henbane in both biomass and tiller production. These experiments suggest that henbane is not well suited for invasion of mature grass stands but may negatively influence some perennial grass seedlings in restoration situations.

Is Campanula glomerata threatened by competition from expanding grasses? Results from a 5-year pot-experiment

The disappearance of low-intensity types of agricultural land use such as mowing and grazing is a global phenomenon which leads to changes in species composition and interactions in plant communities. In central Germany, formerly species-rich, semi-natural dry grasslands have been converted into species-poor communities dominated by Festuca rupicola or Poa angustifolia. As a consequence, several forbs have become endangered, and biodiversity has decreased. In a 5-year competition experiment (replacement design; pots were placed outdoors at Halle Botanical Garden), we evaluated the performance of plant–plant interactions between the rare forb Campanula glomerata, F. rupicola and P. angustifolia, respectively. The response of C. glomerata to the presence of the two grass species were measured by stem length, number of flowers and above-ground biomass annually harvested. We did not find significant differences in C. glomerata stem lengths between years and treatments, but flower production, which was highest in the first year, strongly decreased in the subsequent years. Moreover, flower production was lower in combination with P. angustifolia than with F. rupicola. Biomass production showed significant differences between years, but not between treatments. The positive and negative interactions between C. glomerata and the two grass species was estimated by calculating Relative Neighbour Effect Index (RNE). In combination with F. rupicola, interactions switched between facilitation and competition from year to year. In the presence of P. angustifolia, competitive effects on C. glomerata were highest in the second year, but declined from year to year. Curiously, there were no significant correlations between positive and negative interactions with climatic conditions (annual temperature, precipitation and relative air humidity). Our results showed that the response of the rare forb C. glomerata varies with neighbour species, and that fecundity is more prominently affected by competition than stem height or biomass. Our data indicate that biomass removal by traditional land use methods would facilitate the restoration of species-rich dry grassland communities.

Interactions of Acacia raddiana with herbaceous vegetation change with intensity of abiotic stress

Theoretical models predict that the relative importance of facilitation and competition may vary inversely across gradients of abiotic stress. However, these predictions have not been thoroughly tested in the field, especially in arid and semi-arid environments. In this study, we evaluated how the net effect of Acacia tortilis subsp. raddiana (syn. A. raddiana) trees on the herbaceous species varies across a gradient of water stress in arid Tunisian ecosystems. Our results show that the influence of trees on the herbaceous stratum is beneficial. An Acacia tree improves the richness of herbaceous species around it by two to three times. The positive effect of Acacia raddiana on species composition is characterised in particular by a better development of some species of high pastoral value such as Cenchrus ciliaris, Cynodon dactylon, Eragrostis papposa, Sisymbrium irio and Chenopodium album. We fitted the relationship between seasonal rainfall and the relative neighbour effect index, which varied across this gradient, to a linear model. Our results show that herbaceous plant cover, plant biomass, plant density and diversity are higher under a tree canopy and this positive interaction still increase with higher abiotic stress conditions.

Competition and Facilitation in Three Marsh Plants in Response to a Water-Level Gradient

Biomass accumulation and neighbor effects (measured with a relative neighbor effect index, NEI) were investigated in three marsh plant species (Carex lasiocarpa, Glyceria spiculosa, and Deyeuxia angustifolia) grown at three water levels (0, 20, and 40 cm relative to the soil surface). The three species occur naturally along a water-level gradient in the Sanjiang Plain, the largest freshwater marsh in China. Both intraspecific and interspecific competition were measured using a target-neighbor design, which subjected the three species to different intensities of inter- and intra-specific competition. Biomass accumulation and NEI differences among water levels and competition treatments suggested that intra- and inter-specific competition was strongest at 0 cm inundation. In contrast, neighboring plants stimulated the growth of C. lasiocarpa and D. angustifolia at the 40 cm water level. These results indicate that the strength of intra- and inter-specific competition decreases and the strength of facilitation increases with increasing water levels. Our results support the prediction of the stress-gradient hypothesis (SGH) that plant-plant interactions should switch from competition to facilitation along an increasing water-level stress gradient.

The ecological value of Eryngium horridum in maintaining biodiversity in subtropical grasslands

AbstractThe role of facilitation in the structuring of plant communities has been often demonstrated in environments under high abiotic stress, especially in semi‐arid and arid ecosystems and high elevations. Few studies, however, analysed facilitation in systems that are highly productive and rich in species, which are thought to be theoretically unlikely to demonstrate strong effects of facilitation. Here, we investigate the importance of Eryngium horridum, a rosette species, on the maintenance of plant diversity in subtropical grasslands in southern Brazil. We evaluated facilitation in areas under two different types of management: abandonment and grazing. Plots were established in areas with and without individuals of E. horridum and all species were identified and had their cover estimated. The Relative Neighbour Effect index was calculated in order to verify the presence of competition or facilitation. Our results indicated facilitation in both abandoned and in grazed grasslands, but apparently through different mechanisms. In the first case, the plant's architecture opens the canopy and allows more light to reach small forbs in the grass matrix. In the second case, E. horridum appears to protect more palatable species from herbivores. Otherwise considered an obnoxious species, E. horridum plays an important ecological role in subtropical grasslands in southern Brazil by facilitating other species and consequently, increasing local richness. Areas with this rosette species are important sources of diaspores, which are able to colonize new open sites and thus, maintain biodiversity.

Contrasting community responses to fertilization and the role of the competitive ability of dominant species

AbstractQuestion: Do contrasting biotic contexts in nutrient‐poor grasslands affect the predictability of invasion by exploitative species following fertilization?Location: French Alps.Methods: We examined community responses after 2 years of nutrient addition for two nutrient‐poor European calcareous grasslands, a mesoxeric community dominated by the short bunchgrass Bromus erectus and a mesic community dominated by the tall rhizomatous grass Brachypodium rupestre. We also performed reciprocal transplantations of these two dominant slow‐growing species and Arrhenatherum elatius, a tall fast‐growing grass that dominates nutrient‐rich communities and is likely to invade nutrient‐poor communities after fertilization. Transplants were grown with or without neighbors, in order to measure their individual responses (without neighbors) and competition intensity (by comparing performances with and without neighbors using the Relative Neighbor Effect index – RNE) during one growing season in all three communities.Results: In the Bromus community, fertilization induced a strong increase in fast‐growing grasses (including A. elatius). Competition intensity was low for the three transplanted grasses, but strongly increased with resource addition, to reach values observed in the Arrhenatherum community. In the Brachypodium community, no change in competition intensity with fertilization was detected, because of the high mortality of the two “non‐resident” species, irrespective of the presence of neighbors.Conclusions: Community responses to nutrient improvement are context‐dependent and vary as a function of the biotic environment. Soil processes are proposed as the main drivers of community resistance to the invasion of fast‐growing species in the mesic, nutrient‐poor grassland dominated by the large conservative competitor B. rupestre.