Distinguishing the Effects of Stress Intensity and Stress Duration in Plant Responses to Salinity.

Species-specific variation in response to stress is a key driver of ecological patterns. As climate change alters stress regimes, coastal plants are experiencing intensifying salinity stress due to sea-level rise and more intense storms. This study investigates the variation in species' responses to presses and pulses of salinity stress in five glycophytic and five halophytic species to determine whether salinity intensity, duration, or their interaction best explain patterns of survival and performance. In salinity stress exposure experiments, we manipulated the intensity and duration of salinity exposure to challenge species' expected salinity tolerances. Salinity intensity best explained patterns of survival in glycophytic species, while the interaction between intensity and duration was a better predictor of survival in halophytic species. The interaction between intensity and duration also best explained biomass and chlorophyll production for all tested species. There was interspecific variability in the magnitude of the interactive effect of salinity intensity and duration, with some glycophytic species (Persicaria maculosa, Sorghum bicolor, and Glycine max) having a more pronounced, negative biomass response. For the majority of species, prolonged stress duration exacerbated the negative effect of salinity intensity on biomass. We also observed an unexpected, compensatory response in chlorophyll production in two species, Phragmites australis and Kosteletzkya virginica, for which the effect of salinity intensity on chlorophyll became more positive with increasing duration. We found the regression coefficient of salinity intensity versus biomass at the highest stress duration, i.e., as a press stressor, to be a useful indicator of salinity tolerance, for which species' salinity-tolerance levels matched those in the literature. In conclusion, by measuring species-specific responses to stress exposure, we were able to visualize the independent and interactive effects of two components of a salinity stress regime, intensity, and duration, to reveal how species' responses vary in magnitude and by tolerance class.

Sexual reproduction is light-limited as marsh grasses colonize maritime forest.

Climate change is driving abiotic shifts that can threaten the conservation of foundation species and the habitats they support. Directional range shifting is one mechanism of escape, but requires the successful colonization of habitats where interspecific interactions may differ from those to which a species has adapted. For plants with multiple reproductive strategies, these range-edge interactions may alter the investment or allocation toward a given reproductive strategy. In this study, we quantified sexual reproduction of the clonal marsh grass Spartina patens across an inland colonization front into maritime forest being driven by sea-level rise. We find that flowering is variable across S. patens meadows, but consistently reduced in low light conditions like those of the forest understory. Observational surveys of S. patens flowering at four sites in the Delmarva Peninsula agreed with the results of two experimental manipulations of light availability (shading experiment in S. patens-dominated marsh and a forest dieback manipulation). These three approaches pinpointed light limitation as a principal control on S. patens flowering capacity, suggesting that light competition with taller upland species can suppress S. patens flowering along its upland migration front. Consequently, all propagation in shaded conditions must occur clonally or via seeds from the marsh, a reproductive restriction that could limit the potential for local adaptation and reduce genetic diversity. Future research is needed to determine whether the lack of flowering is the result of a trade-off between sexual and clonal reproduction or results from insufficient photosynthetic products needed to achieve either reproductive method.

Species-specific responses of a marsh-forest ecotone plant community responding to climate change.

Ecotones are responsive to environmental change and pave a path for succession as they move across the landscape. We investigated the biotic and abiotic filters to species establishment on opposite ends of a tidal marsh-forest ecotone that is moving inland in response to sea level rise. We transplanted four plant species common to the ecotone to the leading or trailing edge of the migrating ecotone, with and without caging to protect them from ungulate herbivores. We found that species exhibited an individualistic response to abiotic and biotic pressures in this ecotone; three species performed better at the leading edge of the ecotone in the coastal forest, whereas one performed better at the trailing edge in the marsh. Specifically, grass species Phragmites australis and Panicum virgatum grew more in the low light and low salinity conditions of the leading edge of the ecotone (forest), whereas the shrub Iva frutescens grew better in the high light, high salinity conditions of the trailing edge of the ecotone (marsh). Furthermore, of the four species, only P. australis was affected by the biotic pressure of herbivory by an introduced ungulate, Cervus nippon, which greatly reduced its biomass and survival at the leading edge (forest). P. australis is an aggressive invasive species and has been observed to dominate in the wake of migrating marsh-forest ecotones. Our findings detail the role of lower salinity stress to promote and herbivory pressure to inhibit the establishment of P. australis during shifts of this ecotone, and also highlight an interaction between two nonnative species, P. australis and C. nippon. Understanding migration of the marsh-forest ecotone and the factors controlling P. australis establishment are critical for marsh conservation in the face of sea level rise. More generally, our findings support the conclusion that the abiotic and biotic filters of a migrating ecotone shape the resulting community.

Resilience of Tropical Ecosystems to Ocean Deoxygenation.

The impacts of ocean deoxygenation on biodiversity and ecosystem function are well established in temperate regions, and here we illustrate how the study of hypoxia in tropical ecosystems can offer insights of general importance. We first describe how mechanisms of resilience have developed in response to naturally occurring hypoxia across three tropical ecosystems: coral reefs, seagrass beds, and mangrove forests. We then suggest that the vulnerability of these systems to deoxygenation lies in interactions with other stressors that are increasing rapidly in the Anthropocene. Finally, we advocate for the adoption of a broader community- and ecosystem-level perspective that incorporates mutualisms, feedbacks, and mechanisms of self-rescue and recovery to develop a better predictive understanding of the effects of deoxygenation in coastal ecosystems.

Seeds of change: characterizing the soil seed bank of a migrating salt marsh.

BACKGROUND AND AIMS: The capacity for dispersal to promote or hinder species' responses to global change remains a major question in ecology. One ecosystem experiencing rapid change is the tidal marsh, which is migrating inland in response to accelerated sea level rise. Few studies to date have investigated the ecological dynamics that impact this large-scale migration. Seed dispersal and persistence in the soil seed bank is a component that can be strongly indicative of community trajectories. With this in mind, the aim of our study was to characterize the germinable seed bank across a marsh-forest ecotone in the Chesapeake Bay. METHODS: Soil samples were collected across transects that ran from the high marsh to the coastal loblolly pine forest in a brackish marsh in Blackwater National Wildlife Refuge, MD, USA. Samples were grown in a greenhouse and watered with either freshwater or 3 ppt seawater solution. We compared community composition across transects and between salinity treatments. Additionally, we compared the seed bank with standing vegetation and used seed trait data from the TRY Database to investigate changes in functional traits along this ecotone. KEY RESULTS: We found halophytic species dispersing up to 15 m into the forest and a general lack of obligate upland species, including near absence of Pinus taeda, the dominant species in the forest canopy. A majority of species detected in the seed bank were wetland species of various types, with species with wide salinity tolerance arising most frequently. Salinity addition had a significant negative influence on seed bank diversity. CONCLUSION: Overall, our seed bank results suggest that dispersal and germination under the conditions of saltwater intrusion will limit forest regeneration and favour marsh plant dispersal. This indicates that the ecological processes that determine the soil seed bank community will support continued migration of marsh species into uplands.

Climate change and dead zones.

Estuaries and coastal seas provide valuable ecosystem services but are particularly vulnerable to the co-occurring threats of climate change and oxygen-depleted dead zones. We analyzed the severity of climate change predicted for existing dead zones, and found that 94% of dead zones are in regions that will experience at least a 2 °C temperature increase by the end of the century. We then reviewed how climate change will exacerbate hypoxic conditions through oceanographic, ecological, and physiological processes. We found evidence that suggests numerous climate variables including temperature, ocean acidification, sea-level rise, precipitation, wind, and storm patterns will affect dead zones, and that each of those factors has the potential to act through multiple pathways on both oxygen availability and ecological responses to hypoxia. Given the variety and strength of the mechanisms by which climate change exacerbates hypoxia, and the rates at which climate is changing, we posit that climate change variables are contributing to the dead zone epidemic by acting synergistically with one another and with recognized anthropogenic triggers of hypoxia including eutrophication. This suggests that a multidisciplinary, integrated approach that considers the full range of climate variables is needed to track and potentially reverse the spread of dead zones.

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.

Substrate size mediates thermal stress in the rocky intertidal.

Variation in physical factors, such as slope, orientation, and wind exposure, shapes thermal conditions. Variation in substrate size is common in many habitats, but its thermal consequences for organisms are not well characterized. Larger substrates should remain more thermally stable and act as thermal refuges for associated organisms during short, thermally stressful periods such as midday temperature peaks or tidal exposure. In observations and a transplant and thermal integration experiment, we found that larger rock substrates stayed cooler and facilitated greater survival of the barnacle Semibalanus balanoides in the high intertidal relative to small substrates during the hot summer months in southern New England, USA. However, in thermally benign northern New England, rock substrate size had no effect on barnacle distributions, indicating that the thermal effects of substrate size are mediated by regional climate.

How will warming affect the salt marsh foundation species Spartina patens and its ecological role?

Foundation species structure environments and create refuge from environmental stress. In New England high salt marsh, the grass Spartina patens is a foundation species that reduces salinity, anoxia, desiccation, and thermal stresses through canopy shading and root proliferation. In a factorial S. patens-removal and warming field experiment, foundation species removal strongly impacted every aspect of the community, reiterating the important role of the foundation species S. patens in the high marsh. Given this central role, we hypothesized that facilitation by the foundation species would be even more important under warmer conditions by ameliorating more severe thermal stress. However, the ecological role of S. patens was unaffected by experimental warming, and, independent of the presence of the foundation species, warming had only weak effects on the salt marsh ecological community. Only the foundation species itself responded strongly to warming, by significantly increasing aboveground production in warmed plots. Apparently, amelioration of thermal stress is not as important for salt marsh ecosystem function as S. patens' moderation of salinity and desiccation stresses. From these experimental results, we anticipate that climate change-associated thermal stress will not greatly affect S. patens-dominated high marsh communities. In contrast, foundation species loss, an emergent conservation issue in Atlantic salt marshes, represents a critical threat to salt marsh ecosystem function.

Experimental warming causes rapid loss of plant diversity in New England salt marshes.

Anthropogenic climate change is predicted to cause widespread biodiversity loss due to shifts in species' distributions, but these predictions rarely incorporate ecological associations such as zonation. Here, we predict the decline of a diverse assemblage of mid-latitude salt marsh plants, based on an ecosystem warming experiment. In New England salt marshes, a guild of halophytic forbs occupies stressful, waterlogged pannes. At three sites, experimental warming of < 4 degrees C led to diversity declines in pannes and rapid takeover by a competitive dominant, Spartina patens. In Rhode Island, near their southern range limit, pannes were more sensitive to warming than farther north, and panne area also declined in control plots over the three-season experiment. These results suggest that warming will rapidly reduce plant diversity in New England salt marshes by eliminating a high diversity zone. Biodiversity in zoned ecosystems may be more affected by climate-driven shifts in zonation than by individual species' distribution shifts.

Small-mammal herbivore control of secondary succession in New England tidal marshes.

Secondary succession is impacted by both biotic and abiotic forces, but their relative importance varies due to environmental drivers. Across estuarine salinity gradients, physical stress increases with salinity, and biotic stresses are greater at lower salinities. In southern New England tidal marshes spanning a landscape-scale salinity gradient, we experimentally examined the effects of physical stress and consumer pressure by mammalian herbivores on secondary succession in artificially created bare patches. Recovery was slower in marshes exposed to full-strength seawater, where physical stress is high. Compared to full-strength salt marshes, recovery in low-salinity marshes was much faster and was influenced by small-mammal consumers. At lower salinities, small mammals selectively ate and prevented the establishment of several native and two invasive, nuisance species (Typha angustifolia and Phragmites australis) but were unable to control the expansion of established P. australis stands. By controlling the establishment of competitively dominant species and the trajectory of secondary succession in low-salinity marshes, small mammals may play a cryptic keystone role in estuarine plant communities and are a critical, overlooked consideration in the conservation and management of estuarine marshes.