New occurrence of reed bed decline in southern Europe: do permanent flooding and chemical parameters play a role?

Vegetation dieback in the Mississippi River Delta triggered by acute drought and chronic relative sea-level rise

Vegetation dieback and recovery may be dependent on the interplay between infrequent acute disturbances and underlying chronic stresses. Coastal wetlands are vulnerable to the chronic stress of sea-level rise, which may affect their susceptibility to acute disturbance events. Here, we show that a large-scale vegetation dieback in the Mississippi River Delta was precipitated by salt-water incursion during an extreme drought in the summer of 2012 and was most severe in areas exposed to greater flooding. Using 16 years of data (2007-2022) from a coastwide network of monitoring stations, we show that the impacts of the dieback lasted five years and that recovery was only partial in areas exposed to greater inundation. Dieback marshes experienced an increase in percent time flooded from 43% in 2007 to 75% in 2022 and a decline in vegetation cover and species richness over the same period. Thus, while drought-induced high salinities and soil saturation triggered a significant dieback event, the chronic increase in inundation is causing a longer-term decline in cover, more widespread losses, and reduced capacity to recover from acute stressors. Overall, our findings point to the importance of mitigating the underlying stresses to foster resilience to both acute and persistent causes of vegetation loss.

Salt Water Exposure Exacerbates the Negative Response of Phragmites australis Haplotypes to Sea-Level Rise

The response of coastal wetlands to sea-level rise (SLR) largely depends on the tolerance of individual plant species to inundation stress and, in brackish and freshwater wetlands, exposure to higher salinities. Phragmites australis is a cosmopolitan wetland reed that grows in saline to freshwater marshes. P. australis has many genetically distinct haplotypes, some of which are invasive and the focus of considerable research and management. However, the relative response of P. australis haplotypes to SLR is not well known, despite the importance of predicting future distribution changes and understanding its role in marsh response and resilience to SLR. Here, we use a marsh organ experiment to test how factors associated with sea level rise-inundation and seawater exposure-affect the porewater chemistry and growth response of three P. australis haplotypes along the northern Gulf of Mexico coast. We planted three P. australis lineages (Delta, European, and Gulf) into marsh organs at five different elevations in channels at two locations, representing a low (Mississippi River Birdsfoot delta; 0-13 ppt) and high exposure to salinity (Mermentau basin; 6-18 ppt) for two growing seasons. Haplotypes responded differently to flooding and site conditions; the Delta haplotype was more resilient to high salinity, while the Gulf type was less susceptible to flood stress in the freshwater site. Survivorship across haplotypes after two growing seasons was 42% lower at the brackish site than at the freshwater site, associated with high salinity and sulfide concentrations. Flooding greater than 19% of the time led to lower survival across both sites linked to high concentrations of acetic acid in the porewater. Increased flood duration was negatively correlated with live aboveground biomass in the high-salinity site (χ2 = 10.37, p = 0.001), while no such relationship was detected in the low-salinity site, indicating that flood tolerance is greater under freshwater conditions. These results show that the vulnerability of all haplotypes of P. australis to rising sea levels depends on exposure to saline water and that a combination of flooding and salinity may help control invasive haplotypes.

Dieback and dredge soils of Phragmites australis in the Mississippi River Delta negatively impact plant biomass

Phragmites australis is exhibiting extensive dieback in the Lower Mississippi River Delta (MRD). We explored the potential for restoration of these marshes by (1) characterizing the chemical profiles of soils collected from healthy and dieback stands of P. australis and from sites recently created from dredge-disposal soils that were expected to be colonized by P. australis and (2) experimentally testing the effects of these soil types on the growth of three common P. australis lineages, Delta, Gulf and European. Soil chemical properties included Al, Ca, Cu, Fe, K, Mg, Mn, Na, P, S, Zn, % organic matter, % carbon, % nitrogen, and pH. Dieback soils were characterized by higher % organic matter, % carbon, % nitrogen, and higher S and Fe concentrations, whereas healthy soils had higher Cu, Al, P and Zn. In comparison, dredge sites were low in nutrients and organic matter compared to healthy soils. Rhizomes of each P. australis lineage were planted in each soil type in a common garden and greenhouse and allowed to grow for five months. Aboveground biomass was 16% lower in dieback and 44% lower in dredge soils than in healthy soils. However, we could detect no significant differences in response to soil types among lineages. Although dredge and dieback sites are not optimal for P. australis growth, plants can thrive on these soils, and we recommend restorative measures be initiated as soon as possible to minimize soil erosion.

Applying predictive models to decipher rhizobacterial modifications in common reed die-back affected populations

The microbiota inhabiting the soil, as well as the rhizosphere, represents a key determinant of several plant functions. Like for humans, dysbiosis of the plant-associated microbiota may be a co-causal agent in disease with still obscure eziology. In the last decades, the common reed Phragmites australis has been deeply studied for its disappearance from natural stands, but no clear causative agents have been identified and no laboratory models of such "reed die-back syndrome" (RDBS) have been developed. In this study, we try to shed light on the RDBS, by comparing the rhizosphere microbiota of five Italian P. australis populations with different degrees of decline. Results obtained showed a biogeographical meaningful pattern of rhizosphere microbiota, coupled with an impact of RDBS. Obtained data allowed to construct a two-steps predictive model which enabled the prediction of the plant health status from the microbiota taxonomic composition, independently from their geographic location. In conclusion, this study represents one of the first overviews that statistically links RDBS to alteration of rhizosphere microbiota and suggests a model for the analysis of plant-bacteria relationships in nature.

AFLP Approach Reveals Variability in Phragmites australis: Implications for Its Die-Back and Evidence for Genotoxic Effects

Phragmites australis is a subcosmopolitan species typical of wetlands being studied in Europe for its disappearance from natural stands, a phenomenon called reed die-back syndrome (RDBS). Although it is conjectured that low genetic variability contributes to RDBS, this aspect remains neglected to this day. Using a molecular fingerprinting approach and a sequence analysis of the trnT-trnL/rbcL-psaI regions of cpDNA, this study aimed to compare the genetic structure of stable vs. RDBS-affected P. australis stands from five wetlands of central Italy. Beforehand, in order to characterize the health condition of reed populations, the occurrence of the main macromorphological descriptors for RDBS was considered on 40 reed stands. Soil samples were also collected to examine the total content of heavy metals. The current study analyzed cpDNA in 19 samples and AFLP profiles in 381 samples to investigate the genetic structure of Phragmites populations. Based on the multinomial-Dirichlet model, an analysis of candidate loci under selective pressure was also performed. The relationships among AFLP data, RDBS descriptors and chemicals were evaluated with the use of Linear Mixed Models. The analysis of the cpDNA shows the occurrence of the haplotypes M (the most widespread), and K here recorded for the first time in Italy. Three new haplotypes were also described. The DNA fingerprinting analysis has produced a total of 322 loci (98% polymorphic) and shows the medium-to-high amount of genetic diversity. The significant genetic differentiation among wetlands (Fst = 0.337) suggests either low gene flow or small effective population size. Moreover, the low amount of outlier loci (only 5; l.5% of the total), seems to indicate the scarce occurrence of selective pressure upon the reed's genome. Genetic diversity increased in relationship to the decrease in diameter and of flowering buds of the reed, two of the trends associated with the die-back. The current study rejects the hypothesis that genetic diversity massively contributed to RDBS. Moreover, significant relationships between genetic diversity and the total concentration of some heavy metals (Cr, Cu, and Zn) were highlighted, indicating possible genotoxic effects on P. australis. The current study represents a fact-finding background useful for the conservation of common reed.

Cosmopolitan Species As Models for Ecophysiological Responses to Global Change: The Common Reed Phragmites australis

Phragmites australis is a cosmopolitan grass and often the dominant species in the ecosystems it inhabits. Due to high intraspecific diversity and phenotypic plasticity, P. australis has an extensive ecological amplitude and a great capacity to acclimate to adverse environmental conditions; it can therefore offer valuable insights into plant responses to global change. Here we review the ecology and ecophysiology of prominent P. australis lineages and their responses to multiple forms of global change. Key findings of our review are that: (1) P. australis lineages are well-adapted to regions of their phylogeographic origin and therefore respond differently to changes in climatic conditions such as temperature or atmospheric CO2; (2) each lineage consists of populations that may occur in geographically different habitats and contain multiple genotypes; (3) the phenotypic plasticity of functional and fitness-related traits of a genotype determine the responses to global change factors; (4) genotypes with high plasticity to environmental drivers may acclimate or even vastly expand their ranges, genotypes of medium plasticity must acclimate or experience range-shifts, and those with low plasticity may face local extinction; (5) responses to ancillary types of global change, like shifting levels of soil salinity, flooding, and drought, are not consistent within lineages and depend on adaptation of individual genotypes. These patterns suggest that the diverse lineages of P. australis will undergo intense selective pressure in the face of global change such that the distributions and interactions of co-occurring lineages, as well as those of genotypes within-lineages, are very likely to be altered. We propose that the strong latitudinal clines within and between P. australis lineages can be a useful tool for predicting plant responses to climate change in general and present a conceptual framework for using P. australis lineages to predict plant responses to global change and its consequences.

Oomycete Communities Associated with Reed Die-Back Syndrome

Phragmites australis (Cav.) Trin. ex Steud. die-back is a widely-studied phenomenon that was first discovered in northern Europe and that, until recently, was almost unknown in the Mediterranean basin. It has been described as a complex syndrome affecting reed populations leading to their retreat and decline, with significant impacts on valuable ecosystem services. Among the factors that cause the decline, soil-living microorganisms can be crucial. The aims of this study were to analyze the diversity of oomycetes communities associated with reed stands, and to understand whether they could play a key role in the decline. Variations in the structure of oomycetes communities were studied by metabarcoding of the internal transcribed spacer (ITS) 1 region of ribosomal DNA, from the sediments of five Italian freshwater ecosystems. They were chosen to cover a large variability in terms of surface area, water depth, microclimate, and presence of documented reed retreat. From 96 samples collected from reed roots, rhizosphere, and bulk soil, we assembled 207661 ITS1 reads into 523 OTUs. We demonstrated that oomycete communities were structured by several factors, among which the most important was die-back occurrence. Our study also indicates that Pythiogeton spp. could be potentially involved in the development of die-back. The role of heavy metals in the soil was also explored, and cadmium concentration was shown to affect oomycetes distribution. This study represents a significant step forward for the characterization of microbial communities associated with reed die-back syndrome and helps to gain knowledge of the complexity of these important wet ecosystems.