From selection to complementarity: the shift along the abiotic stress gradient in a controlled biodiversity experiment

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.

Darwin's naturalization conundrum reconciled by changes of species interactions

Although phylogenetic distance between native and exotic species has a close link with their interactions, it is still unclear how environmental stresses and species interactions influence the relationship between phylogenetic distance and biological invasions. Here we assessed the effect of invader-native phylogenetic distance on the growth of the invader (Symphyotrichum subulatum) under three levels of drought (no, moderate, or intense drought). Under no drought, interspecific competition between close relatives was the dominant process and native communities more closely related to the invader showed higher resistance to invasion, supporting Darwin's naturalization hypothesis. In contrast, under intense drought, facilitation between close relatives by mutualism with arbuscular mycorrhizal fungi (AMF) became more important, and the invader became more successful in their more closely related native communities, supporting the preadaptation hypothesis. The colonization rate of AMF of the invader was higher in more closely related native communities regardless of the drought treatment, but it was only positively related to invader biomass under intense drought. Therefore, the shift of species interactions from competition to facilitation may be ascribed to the promotion of AMF to invasion occurring under intense drought, which leads to the effect of closely related natives on the invader shifting from negative to positive. Our results provide a new angle to resolve Darwin's naturalization conundrum from the change of species interactions along a stress gradient, and provide important clues for invasion management when species interactions change in response to global climatic change.

Genotypic Diversity Improves Photosynthetic Traits of Hydrocotyle vulgaris and Alters Soil Organic Matter and N2O Emissions of Wetland Microecosystems

In plant communities, genotypic diversity can impact the plant community structure and ecosystem functions, but related research has focused on native plants. Therefore, whether genotypic diversity affects the growth of invasive plants and then changes the wetland microecosystem remains unresolved. In this study, six different genotypes of Hydrocotyle vulgaris, a common invasive plant in China, were selected to construct populations with three different genotypic diversity levels (one, three, and six genotype combinations, respectively) to explore the effects of different genotypic diversity levels on the growth and physiological traits of H. vulgaris, and soil nutrients and greenhouse gas emissions of the wetland microecosystem under flooding conditions. We found that genotypic diversity improved the leaf area, root to shoot ratio and photosynthetic physiological traits of H. vulgaris, especially under flooding. Moreover, genotypic diversity increased soil organic matter (SOM) contents in the wetland microecosystem, while it reduced the cumulative nitrous oxide emissions under flooding conditions. Overall, genotype diversity improved photosynthetic traits of H. vulgaris, further increased SOM, and reduced the N2O emissions of the wetland microecosystem. The results of this study can provide a theoretical basis for exploring how genotypic diversity levels affect the invasiveness of invasive plants and ecosystems in wetland microecosystems.

Facilitation drives the positive effects of plant richness on trace metal removal in a biodiversity experiment

Background

Phytoextraction is an environmentally acceptable and inexpensive technique for mine tailing rehabilitation that uses metallophyte plants. These plants reduce the soil trace metal contents to environmentally acceptable levels by accumulating trace metals. Recently, whether more trace metals can be removed by species-rich communities of these plants received great attention, as species richness has been reported having positive effects on ecosystem functions. However, how the species richness affects trace metals removal of plant communities of mine tailing is rarely known.

Methodology/Principal Findings

We examined the effects of species richness on soil trace metal removal in both natural and experimental plant communities. The root lengths and stem heights of each plant species were measured in order to calculate the functional diversity indices. Our results showed that trace metal (Cu, Cd, Pb and Zn) concentrations in mine tailing soil declined as species richness increased in both the natural and experimental plant communities. Species richness, rather than functional diversity, positively affected the mineralomass of the experimental plant communities. The intensity of plant-plant facilitation increased with the species richness of experimental communities. Due to the incremental role of plant-plant facilitation, most of the species had higher biomasses, higher trace metal concentrations in their plant tissues and lower malondialdehyde concentrations in their leaves. Consequently, the positive effects of species richness on mineralomass were mostly attributable to facilitation among plants.

Conclusions/Significance

Our results provide clear evidence that, due to plant-plant facilitation, species richness positively affects the removal of trace metals from mine tailing soil through phytoextraction and provides further information on diversity conservation and environmental remediation in a mine tailing environment.

Functional traits composition predict macrophytes community productivity along a water depth gradient in a freshwater lake

Functional trait composition of plant communities has been proposed as a helpful key for understanding the mechanisms of biodiversity effects on ecosystem functioning. In this study, we applied a step-wise modeling procedure to test the relative effects of taxonomic diversity, functional identity, and functional diversity on macrophytes community productivity along water depth gradient. We sampled 42 plots and 1513 individual plants and measured 16 functional traits and abundance of 17 macrophyte species. Results showed that there was a significant decrease in taxonomic diversity, functional identity (i.e., stem dry mass content, leaf [C] and leaf [N]), and functional diversity (i.e., floating leaf, mean Julian flowering date and rooting depth) with increasing water depth. For the multiple-trait functional diversity (FD) indices, functional richness decreased, while functional divergence increased with water depth gradient. Macrophyte community productivity was strongly determined by functional trait composition within community, but not significantly affected by taxonomic diversity. Community-weighted means (CWM) showed a two times higher explanatory power relative to FD indices in determining variations in community productivity. For nine of sixteen traits, CWM and FD showed significant correlations with community productivity, although the strength and direction of those relations depended on selected trait. Furthermore, functional composition in a community affected productivity through either additive or opposite effects of CWM and FD, depending on the particular traits being considered. Our results suggested both mechanisms of mass ratio and niche complementarity can operate simultaneously on variations in community productivity, and considering both CWM and FD would lead to a more profound understanding of traits-productivity relationships.