Do trade-offs govern plant species' responses to different global change treatments?

Plants are subject to trade-offs among growth strategies such that adaptations for optimal growth in one condition can preclude optimal growth in another. Thus, we predicted that a plant species that responds positively to one global change treatment would be less likely than average to respond positively to another treatment, particularly for pairs of treatments that favor distinct traits. We examined plant species' abundances in 39 global change experiments manipulating two or more of the following: CO2 , nitrogen, phosphorus, water, temperature, or disturbance. Overall, the directional response of a species to one treatment was 13% more likely than expected to oppose its response to a another single-factor treatment. This tendency was detectable across the global data set, but held little predictive power for individual treatment combinations or within individual experiments. Although trade-offs in the ability to respond to different global change treatments exert discernible global effects, other forces obscure their influence in local communities.

Dark septate endophyte improves salt tolerance of native and invasive lineages of Phragmites australis.

Fungal endophytes can improve plant tolerance to abiotic stress. However, the role of these plant-fungal interactions in invasive species ecology and their management implications remain unclear. This study characterized the fungal endophyte communities of native and invasive lineages of Phragmites australis and assessed the role of dark septate endophytes (DSE) in salt tolerance of this species. We used Illumina sequencing to characterize root fungal endophytes of contiguous stands of native and invasive P. australis along a salinity gradient. DSE colonization was assessed throughout the growing season in the field, and effects of fungal inoculation on salinity tolerance were investigated using laboratory and greenhouse studies. Native and invasive lineages had distinct fungal endophyte communities that shifted across the salinity gradient. DSE colonization was greater in the invasive lineage and increased with salinity. Laboratory studies showed that DSE inoculation increased P. australis seedling survival under salt stress; and a greenhouse assay revealed that the invasive lineage had higher aboveground biomass under mesohaline conditions when inoculated with a DSE. We observed that P. australis can establish mutualistic associations with DSE when subjected to salt stress. This type of plant-fungal association merits further investigation in integrated management strategies of invasive species and restoration of native Phragmites.

Sediment clays are trapping heavy metals in urban lakes: An indicator for severe industrial and agricultural influence on coastal wetlands at the Mediterranean coast of Egypt.

Coastal wetlands of the northern coast of Egypt have been impacted with higher loads of runoff, especially the large urbanized lakes of the Nile deltaic coast. Five urban lakes spanning the northern coast of Egypt (from east to west: Bardawil, Manzala, Burullus, Edku, and Mariut) were sampled for quantifying concentrations of heavy metals in their sediment and plant tissues. Sediment and plant tissues in lake Bardawil were the least contaminated, and the other lakes were moderately to highly polluted with Ni, Co, Cr, Pb, Zn, and Cu. Edku had the highest concentrations of Co, Cr, and Cu (19.83, 45.42 and 68.60 mg kg-1, respectively). The proportion of clay in sediment was significantly and positively correlated with Co and Ni in sediment (r = 0.7 and P ≤ 0.001), suggesting an important role of clay cation exchange capacity in the sorption of metals and removing them from the water column.

Wetland soil microplastics are negatively related to vegetation cover and stem density.

Microplastics are a complex group of ubiquitous environmental contaminants of emerging concern. These particles degrade slowly, release plasticizers, and can be transferred between trophic levels. In aquatic systems, they have been identified suspended in the water column, along shorelines, and within sediment. However, the abundance and distribution of microplastics in vegetated wetlands, which are transitional ecosystems between terrestrial and aquatic environments, are poorly understood. Here we describe the spatial distribution of soil microplastics in habitats of varying vegetation density in an urban tidal wetland. Samples were wet-sieved, organic matter was oxidized using hydrogen peroxide, and microplastics separated under a dissecting microscope, counted, and weighed. A fraction (n = 175) were analyzed via FTIR for validation. Positive microplastics identification was 81%-93%. Dominant polymers were polystyrene (29%) and polyethylene and synthetic rubber (both 8%). Average microplastic number to a 5-cm depth (23,200 ±â€¯2,500 m-2 or 1,270 ±â€¯150 kg-1) varied between habitat types, where mudflat, channel edge, and drift line habitats all had significantly more total microplastics than the interior of dense stands of vegetation, suggesting that emergent wetland plants are a highly effective filter of microplastics. Microfibers were about eight times as abundant as microfragments, and fibers and fragments differed in their distribution patterns, with microfibers most abundant in vegetation-free mudflats and microfragments in vegetated channel edges. Our results demonstrate that vegetated wetlands are important locations for microplastic accumulation and that wetland vegetation and hydrodynamics affect spatial distribution of microplastics between habitats.

Effects of Detritus on the Mosquito Culex pipiens: Phragmites and Schedonorus (Festuca) Invasion Affect Population Performance.

Species interactions that influence the performance of the exotic mosquito Culex pipiens can have important effects on the transmission risk of West Nile virus (WNV). Invasive plants that alter the vegetation communities of ephemeral ground pools may facilitate or resist the spread of C. pipiens (L.) by altering allochthonous inputs of detritus in those pools. To test this hypothesis, we combined field surveys of roadside stormwater ditches with a laboratory microcosm experiment to examine relationships between C. pipiens performance and water quality in systems containing detritus from invasive Phragmites australis (Cav.) Trin. Ex Steud., introduced Schedonorusarundinaceus (Schreb.) Dumort., or native Juncus effusus L. or Typha latifolia L. In ditches, C. pipiens abundance was unrelated to detritus species but female C. pipiens were significantly larger from ditches with S. arundinaceus and smaller with J. effusus. Larger and smaller C. pipiens were also produced in microcosms provisioned with S. arundinaceus and J. effusus, respectively, yet the per capita rate of population of change did not vary. Larger females from habitats with S. arundinaceus were likely caused by faster decay rates of S. arundinaceus and resultant increases in microbial food, but lower survival as a result of fouling and higher tannin-lignin concentrations resulted in little changes to overall population performance. Larger female mosquitoes have been shown to have greater potential for transmitting arboviruses. Our findings suggest that changed community-level interactions from plant invasions in urban ephemeral ground pools can affect the fitness of C. pipiens and possibly increase WNV risk.

Global change effects on plant communities are magnified by time and the number of global change factors imposed.

Global change drivers (GCDs) are expected to alter community structure and consequently, the services that ecosystems provide. Yet, few experimental investigations have examined effects of GCDs on plant community structure across multiple ecosystem types, and those that do exist present conflicting patterns. In an unprecedented global synthesis of over 100 experiments that manipulated factors linked to GCDs, we show that herbaceous plant community responses depend on experimental manipulation length and number of factors manipulated. We found that plant communities are fairly resistant to experimentally manipulated GCDs in the short term (<10 y). In contrast, long-term (≥10 y) experiments show increasing community divergence of treatments from control conditions. Surprisingly, these community responses occurred with similar frequency across the GCD types manipulated in our database. However, community responses were more common when 3 or more GCDs were simultaneously manipulated, suggesting the emergence of additive or synergistic effects of multiple drivers, particularly over long time periods. In half of the cases, GCD manipulations caused a difference in community composition without a corresponding species richness difference, indicating that species reordering or replacement is an important mechanism of community responses to GCDs and should be given greater consideration when examining consequences of GCDs for the biodiversity-ecosystem function relationship. Human activities are currently driving unparalleled global changes worldwide. Our analyses provide the most comprehensive evidence to date that these human activities may have widespread impacts on plant community composition globally, which will increase in frequency over time and be greater in areas where communities face multiple GCDs simultaneously.

Urbanization Altered Bacterial and Archaeal Composition in Tidal Freshwater Wetlands Near Washington DC, USA, and Buenos Aires, Argentina.

Urban expansion causes coastal wetland loss, and environmental stressors associated with development can lead to wetland degradation and loss of ecosystem services. This study investigated the effect of urbanization on prokaryotic community composition in tidal freshwater wetlands. Sites in an urban, suburban, and rural setting were located near Buenos Aires, Argentina, and Washington D.C., USA. We sampled soil associated with two pairs of functionally similar plant species, and used Illumina sequencing of the 16S rRNA gene to examine changes in prokaryotic communities. Urban stressors included raw sewage inputs, nutrient pollution, and polycyclic aromatic hydrocarbons. Prokaryotic communities changed along the gradient (nested PerMANOVA, Buenos Aires: p = 0.005; Washington D.C.: p = 0.001), but did not differ between plant species within sites. Indicator taxa included Methanobacteria in rural sites, and nitrifying bacteria in urban sites, and we observed a decrease in methanogens and an increase in ammonia-oxidizers from rural to urban sites. Functional profiles in the Buenos Aires communities showed higher abundance of pathways related to nitrification and xenobiotic degradation in the urban site. These results suggest that changes in prokaryotic taxa across the gradient were due to surrounding stressors, and communities in urban and rural wetlands are likely carrying out different functions.

Asynchrony among local communities stabilises ecosystem function of metacommunities.

Temporal stability of ecosystem functioning increases the predictability and reliability of ecosystem services, and understanding the drivers of stability across spatial scales is important for land management and policy decisions. We used species-level abundance data from 62 plant communities across five continents to assess mechanisms of temporal stability across spatial scales. We assessed how asynchrony (i.e. different units responding dissimilarly through time) of species and local communities stabilised metacommunity ecosystem function. Asynchrony of species increased stability of local communities, and asynchrony among local communities enhanced metacommunity stability by a wide range of magnitudes (1-315%); this range was positively correlated with the size of the metacommunity. Additionally, asynchronous responses among local communities were linked with species' populations fluctuating asynchronously across space, perhaps stemming from physical and/or competitive differences among local communities. Accordingly, we suggest spatial heterogeneity should be a major focus for maintaining the stability of ecosystem services at larger spatial scales.

Tidal Marshes across a Chesapeake Bay Subestuary Are Not Keeping up with Sea-Level Rise.

Sea-level rise is a major factor in wetland loss worldwide, and in much of Chesapeake Bay (USA) the rate of sea-level rise is higher than the current global rate of 3.2 mm yr-1 due to regional subsidence. Marshes along estuarine salinity gradients differ in vegetation composition, productivity, decomposition pathways, and sediment dynamics, and may exhibit different responses to sea-level rise. Coastal marshes persist by building vertically at rates at or exceeding regional sea-level rise. In one of the first studies to examine elevation dynamics across an estuarine salinity gradient, we installed 15 surface elevation tables (SET) and accretion marker-horizon plots (MH) in tidal freshwater, oligohaline, and brackish marshes across a Chesapeake Bay subestuary. Over the course of four years, wetlands across the subestuary decreased 1.8 ± 2.7 mm yr-1 in elevation on average, at least 5 mm yr-1 below that needed to keep pace with global sea-level rise. Elevation change rates did not significantly differ among the marshes studied, and ranged from -9.8 ± 6.9 to 4.5 ± 4.3 mm yr-1. Surface accretion of deposited mineral and organic matter was uniformly high across the estuary (~9-15 mm yr-1), indicating that elevation loss was not due to lack of accretionary input. Position in the estuary and associated salinity regime were not related to elevation change or surface matter accretion. Previous studies have focused on surface elevation change in marshes of uniform salinity (e.g., salt marshes); however, our findings highlight the need for elevation studies in marshes of all salinity regimes and different geomorphic positions, and warn that brackish, oligohaline, and freshwater tidal wetlands may be at similarly high risk of submergence in some estuaries.

Site history and edaphic features override the influence of plant species on microbial communities in restored tidal freshwater wetlands.

Restored wetland soils differ significantly in physical and chemical properties from their natural counterparts even when plant community compositions are similar, but effects of restoration on microbial community composition and function are not well understood. Here, we investigate plant-microbe relationships in restored and natural tidal freshwater wetlands from two subestuaries of the Chesapeake Bay. Soil samples were collected from the root zone of Typha latifolia, Phragmites australis, Peltandra virginica, and Lythrum salicaria. Soil microbial composition was assessed using 454 pyrosequencing, and genes representing bacteria, archaea, denitrification, methanogenesis, and methane oxidation were quantified. Our analysis revealed variation in some functional gene copy numbers between plant species within sites, but intersite comparisons did not reveal consistent plant-microbe trends. We observed more microbial variations between plant species in natural wetlands, where plants have been established for a long period of time. In the largest natural wetland site, sequences putatively matching methanogens accounted for ∼17% of all sequences, and the same wetland had the highest numbers of genes coding for methane coenzyme A reductase (mcrA). Sequences putatively matching aerobic methanotrophic bacteria and anaerobic methane-oxidizing archaea (ANME) were detected in all sites, suggesting that both aerobic and anaerobic methane oxidation are possible in these systems. Our data suggest that site history and edaphic features override the influence of plant species on microbial communities in restored wetlands.

Livestock as a potential biological control agent for an invasive wetland plant.

Invasive species threaten biodiversity and incur costs exceeding billions of US$. Eradication efforts, however, are nearly always unsuccessful. Throughout much of North America, land managers have used expensive, and ultimately ineffective, techniques to combat invasive Phragmites australis in marshes. Here, we reveal that Phragmites may potentially be controlled by employing an affordable measure from its native European range: livestock grazing. Experimental field tests demonstrate that rotational goat grazing (where goats have no choice but to graze Phragmites) can reduce Phragmites cover from 100 to 20% and that cows and horses also readily consume this plant. These results, combined with the fact that Europeans have suppressed Phragmites through seasonal livestock grazing for 6,000 years, suggest Phragmites management can shift to include more economical and effective top-down control strategies. More generally, these findings support an emerging paradigm shift in conservation from high-cost eradication to economically sustainable control of dominant invasive species.

Warming increases plant biomass and reduces diversity across continents, latitudes, and species migration scenarios in experimental wetland communities.

Atmospheric warming may influence plant productivity and diversity and induce poleward migration of species, altering communities across latitudes. Complicating the picture is that communities from different continents deviate in evolutionary histories, which may modify responses to warming and migration. We used experimental wetland plant communities grown from seed banks as model systems to determine whether effects of warming on biomass production and species richness are consistent across continents, latitudes, and migration scenarios. We collected soil samples from each of three tidal freshwater marshes in estuaries at three latitudes (north, middle, south) on the Atlantic coasts of Europe and North America. In one experiment, we exposed soil seed bank communities from each latitude and continent to ambient and elevated (+2.8 °C) temperatures in the greenhouse. In a second experiment, soil samples were mixed either within each estuary (limited migration) or among estuaries from different latitudes in each continent (complete migration). Seed bank communities of these migration scenarios were also exposed to ambient and elevated temperatures and contrasted with a no-migration treatment. In the first experiment, warming overall increased biomass (+16%) and decreased species richness (-14%) across latitudes in Europe and North America. Species richness and evenness of south-latitude communities were less affected by warming than those of middle and north latitudes. In the second experiment, warming also stimulated biomass and lowered species richness. In addition, complete migration led to increased species richness (+60% in North America, + 100% in Europe), but this higher diversity did not translate into increased biomass. Species responded idiosyncratically to warming, but Lythrum salicaria and Bidens sp. increased significantly in response to warming in both continents. These results reveal for the first time consistent impacts of warming on biomass and species richness for temperate wetland plant communities across continents, latitudes, and migration scenarios.

A novel dual-compartment, continuous-flow wetland microcosm to assess cis-dichloroethene removal from the rhizosphere.

The anaerobic biodegradation of tetrachloroethene commonly results in the accumulation of chlorinated intermediates such as cis-1,2-dichloroethene (cDCE). Frequently, groundwater contaminated with chlorinated ethenes discharges to natural wetlands. The goal of this study was to quantitatively evaluate the effects of wetland plants and microorganisms on the fate of cDCE in the wetland rhizosphere. To accomplish this goal, a novel dual-compartment wetland microcosm was designed. A Phragmites australis individual was maintained in the microcosm, which was operated with continuous flows of air and mineral medium through the foliar and rhizosphere compartments, respectively, to incorporate mass transfer/transport processes that are important in natural wetlands and allow steady-state assessment of changes in dissolved O2 and cDCE or [1,2-(14)C]cDCE levels. Substantial amounts of [14C]cDCE were phytovolatilized through a healthy P. australis individual to the foliar chamber. Rhizodegradation by native microorganisms associated with P. australis roots also converted substantial amounts of [14C]cDCE to 14C-labeled CO2 and non-volatile compounds, presumably through cometabolic reactions that could be enhanced by the release of O2 and exudates by P. australis. These results suggest that, in some cases, the intrinsic capacity of native wetland plants and microorganisms to remove cDCE from the rhizosphere may be substantial.

Variation in wetland seed banks across a tidal freshwater landscape.

Almost nothing is known about landscape-level variation in seed bank composition across complexes of hydrologically connected wetlands. We examined species composition of seed and spore banks in three habitats of tidal freshwater wetlands (marshes, swamp hollows, and swamp hummocks) along 48 km of tributaries throughout the tidal freshwater portions of the Nanticoke River watershed (Maryland and Delaware). Taxa and seedling density decreased with increasing distance upstream in the swamp hollows and hummocks, but increased or remained constant proceeding upstream in the marshes. Species rarefaction curves indicated equal taxa richness (28) between marshes and swamp hummocks at 175 individuals, with lower richness in swamp hollows (19). However, communities in swamp hollows were patchier and had an estimated total taxa richness of 52, similar to the marshes (50) and higher than swamp hummocks (41). Coefficients of variation for seedling emergence densities (136-180%) were greater than those of published seed bank studies conducted at smaller spatial scales in tidal freshwater marshes (36-117%). Our literature searches suggest that ours is the first study to document significant spatial trends in seed bank diversity and density across a wetland landscape.

Response of two oligohaline marsh communities to lethal and nonlethal disturbance.

Severity is recognized as an important attribute of disturbance in many plant communities. However, the effects of disturbances of different severity on patterns of regeneration in oligohaline marsh vegetation have not been experimentally examined. In these communities, a critical difference in the effects of disturbance severity may be whether the vegetation dies as a result of the disturbance or is merely damaged and hence capable of resprouting. We described the regeneration of vegetation in two Louisiana marsh community types, one dominated by Sagittaria lancifolia L. and the other by Spartina patens (Ait.) Muhl., following three levels of disturbance: no disturbance, a nonlethal disturbance, and a lethal disturbance. In the nonlethal disturbance, aboveground vegetation was clipped to simulate common disturbances such as fire and herbivory that remove aboveground vegetation but leave rhizomes intact. In the lethal disturbance vegetation was killed using herbicide to simulate disturbances causing plant mortality such as wrack deposition, sedimentation, scouring, and flooding following fire or herbivory. Regeneration was assessed over a 2-year period by measuring plant species richness, relative abundance, relative dominance, cover, and final biomass. To elucidate mechanisms for observed responses of vegetation, the species composition of the seed bank, light penetration, water level, salinity, and soil redox potential were evaluated. Despite differences in the structure of undisturbed vegetation in the two community types, they exhibited the same overall pattern of regeneration. Following nonlethal disturbance, the dominant species resprouted and quickly reestablished the structure of the vegetation. In contrast, recolonization following lethal disturbance occurred primarily via seedling recruitment, which resulted in marked shifts in community structure that persisted throughout the study. While the two communities responded similarly overall to disturbance, the response of individual species was not uniform; abundance, dominance, biomass, or cover increased for some species but decreased for others in response to disturbance. Seed bank species occurred in the vegetation following lethal disturbance in the Spartina community and in both disturbed and undisturbed plots in the Sagittaria community, indicating that the seed bank is a source of propagules for regeneration and maintenance of oligohaline marshes. Of the environmental variables measured, light level was most closely related to the effect of disturbance severity on community structure. Our results suggest that lethal and nonlethal disturbances have differential effects on regeneration of vegetation that can create pattern in oligohaline marshes communities.