Drivers of dune formation control ecosystem function and response to disturbance in a barrier island system.

Barrier islands are landscape features that protect coastlines by reducing wave energy and erosion. Quantifying vegetation-topographic interactions between adjacent habitats are essential for predicting long-term island response and resilience to sea-level rise and disturbance. To understand the effects of dune dynamics on adjacent interior island ecosystem processes, we quantified how sediment availability and previous disturbance regime interact with vegetation to influence dune building and ease of seawater and sediment movement into the island interior on two US mid-Atlantic coast barrier islands. We conducted field surveys of sediment accretion, vegetative cover, and soil characteristics in dune and swale habitats. Digital elevation models provided assessment of water flow resistance from the mean high water mark into the island interior. We found that geographic location impacted sediment accretion rates and Panicum amarum (a species increasing in abundance over time in the Virginia barrier islands) accreted sediment at a significantly lower rate compared to other dune grasses. Dune elevation impacted the ease of seawater flow into the island interior, altering soil chlorides, annual net primary productivity, and soil carbon and nitrogen. Our work demonstrates the importance of incorporating biological processes and cross-island connectivity into future scenario modeling and predictions of rising sea-levels and increased disturbance.

Plant community re-organization and increased productivity due to multi-year nutrient enrichment of a coastal grassland.

Nutrient enrichment alters plant community structure and function at a global scale. Coastal plant systems are expected to experience increased rates of nitrogen and phosphorus deposition by 2100, caused mostly by anthropogenic activity. Despite high density of studies investigating connections between plant community structure and ecosystem function in response to nutrient addition, inconsistencies in system response based on the ecosystem in question calls for more detailed analyses of nutrient impacts on community organization and resulting productivity response. Here, we focus on nutrient addition impacts on community structure and organization as well as productivity of different lifeforms in a coastal grassland. We established long-term nutrient enrichment plots in 2015 consisting of control (C), nitrogen (N), phosphorus (P), and nitrogen + phosphorus (NP) treatments. In 2017 we collected graminoid and forb productivity, root productivity, and community composition for each plot. We found no N x P interaction, but N enrichment was a significant main effect on productivity, highlighting N limitation in coastal systems. Importantly, nutrient enrichment treatments did not alter root productivity. However, all treatments caused significant differences in community composition. Using rank abundance curves, we determined that community composition differences were driven by increased dominance of nitrophilous graminoids, re-organization of subordinate species, and species absences in N and NP plots. Results of this study highlight how coastal grassland communities are impacted by nutrient enrichment. We show that community re-organization, increased dominance, and absence of critical species are all important mechanisms that reflect community-level impacts of nutrient enrichment in our coastal grassland site.

Microclimate feedbacks sustain power law clustering of encroaching coastal woody vegetation.

The spatial pattern of vegetation patchiness may follow universal characteristic rules when the system is close to critical transitions between alternative states, which improves the anticipation of ecosystem-level state changes which are currently difficult to detect in real systems. However, the spatial patterning of vegetation patches in temperature-driven ecosystems have not been investigated yet. Here, using high-resolution imagery from 1972 to 2013 and a stochastic cellular automata model, we show that in a North American coastal ecosystem where woody plant encroachment has been happening, the size distribution of woody patches follows a power law when the system approaches a critical transition, which is sustained by the local positive feedbacks between vegetation and the surrounding microclimate. Therefore, the observed power law distribution of woody vegetation patchiness may be suggestive of critical transitions associated with temperature-driven woody plant encroachment in coastal and potentially other ecosystems.

Decreased temperature variance associated with biotic composition enhances coastal shrub encroachment.

Regime shift from grasslands to shrub-dominated landscapes occur worldwide driven by altered land-use and climate change, affecting landscape function, biodiversity, and productivity. Warming winter temperatures are a main driver of expansion of the native, evergreen shrub, Morella cerifera, in coastal landscapes. Shrub establishment in these habitats alters microclimate, but little is known about seasonal differences and microclimate variance. We assessed influence of shrubs on microclimate variance, community composition, and community physiological functioning across three vegetation zones: grass, transitional, and shrub in a coastal grassland. Using a novel application of a time-series analysis, we interpret microclimatic variance modification and elucidate mechanisms of shrub encroachment at the Virginia Coast Reserve, Long-Term Ecological Research site. As shrub thickets form, diversity is reduced with little grass/forb cover, while transpiration and annual productivity increase. Shrub thickets significantly reduced temperature variance with a positive influence of one day on the next in maximum air, minimum air, and maximum ground temperature. We also show that microclimatic temperature moderation reduces summer extreme temperatures in transition areas, even before coalescence into full thickets. Encroachment of Morella cerifera on the Virginia barrier islands is driven by reduced local exposure to cold temperatures and enhanced by abiotic microclimatic modification and biotic physiological functioning. This shift in plant community composition from grassland to shrub thicket alters the role of barrier islands in productivity and can have impacts on the natural resilience of the islands.

Connectivity in coastal systems: Barrier island vegetation influences upland migration in a changing climate.

Due to their position at the land-sea interface, barrier islands are vulnerable to both oceanic and atmospheric climate change-related drivers. In response to relative sea-level rise, barrier islands tend to migrate landward via overwash processes which deposit sediment onto the backbarrier marsh, thus maintaining elevation above sea level. In this paper, we assess the importance of interior upland vegetation and sediment transport (from upland to marsh) on the movement of the marsh-upland boundary in a transgressive barrier system along the mid-Atlantic Coast. We hypothesize that recent woody expansion is altering the rate of marsh to upland conversion. Using Landsat imagery over a 32 year time period (1984-2016), we quantify transitions between land cover (bare, grassland, woody vegetation, and marsh) and the marsh-upland boundary. We find that the Virginia Barrier Islands have both gains and losses in backbarrier marsh and upland, with 19% net loss from the system during the timeframe of the study and increased variance in marsh to upland conversion. This is consistent with recent work indicating a shift toward increasing rates of landward barrier island migration. Despite a net loss of upland area, macroclimatic winter warming resulted in 41% increase in woody vegetation in protected, low-elevation areas, introducing new ecological scenarios that increase resistance to sediment movement from upland to marsh. Our analysis demonstrates how the interplay between elevation and interior island vegetative cover influences landward migration of the boundary between upland and marsh (a previously underappreciated indicator that an island is migrating), and thus, the importance of including ecological processes in the island interior into coastal modeling of barrier island migration and sediment movement across the barrier landscape.

Literature-based latitudinal distribution and possible range shifts of two US east coast dune grass species (Uniola paniculata and Ammophila breviligulata).

Previous work on the US Atlantic coast has generally shown that coastal foredunes are dominated by two dune grass species, Ammophila breviligulata (American beachgrass) and Uniola paniculata (sea oats). From Virginia northward, A. breviligulata dominates, while U. paniculata is the dominant grass south of Virginia. Previous work suggests that these grasses influence the shape of coastal foredunes in species-specific ways, and that they respond differently to environmental stressors; thus, it is important to know which species dominates a given dune system. The range boundaries of these two species remains unclear given the lack of comprehensive surveys. In an attempt to determine these boundaries, we conducted a literature survey of 98 studies that either stated the range limits and/or included field-based studies/observations of the two grass species. We then produced an interactive map that summarizes the locations of the surveyed papers and books. The literature review suggests that the current southern range limit for A. breviligulata is Cape Fear, NC, and the northern range limit for U. paniculata is Assateague Island, on the Maryland and Virginia border. Our data suggest a northward expansion of U. paniculata, possibly associated with warming trends observed near the northern range limit in Painter, VA. In contrast, the data regarding a range shift for A. breviligulata remain inconclusive. We also compare our literature-based map with geolocated records from the Global Biodiversity Information Facility and iNaturalist research grade crowd-sourced observations. We intend for our literature-based map to aid coastal researchers who are interested in the dynamics of these two species and the potential for their ranges to shift as a result of climate change.

Non-linear shift from grassland to shrubland in temperate barrier islands.

Woody plant encroachment into grasslands is a major land cover change taking place in many regions of the world, including arctic, alpine and desert ecosystems. This change in plant dominance is also affecting coastal ecosystems, including barrier islands, which are known for being vulnerable to the effects of climate change. In the last century, the woody plant species Morella cerifera L. (Myricaceae), has encroached into grass covered swales in many of the barrier islands of Virginia along the Atlantic seaboard. The abrupt shift to shrub cover in these islands could result from positive feedbacks with the physical environment, though the underlying mechanisms remain poorly understood. We use a combination of experimental and modeling approaches to investigate the role of climate warming and the ability of M. cerifera to mitigate its microclimate thereby leading to the emergence of alternative stable states in barrier island vegetation. Nighttime air temperatures were significantly higher in myrtle shrublands than grasslands, particularly in the winter season. The difference in the mean of the 5% and 10% lowest minimum temperatures between shrubland and grassland calculated from two independent datasets ranged from 1.3 to 2.4°C. The model results clearly show that a small increase in near-surface temperature can induce a non-linear shift in ecosystem state from a stable state with no shrubs to an alternative stable state dominated by M. cerifera. This modeling framework improves our understanding and prediction of barrier island vegetation stability and resilience under climate change, and highlights the existence of important nonlinearities and hystereses that limit the reversibility of this ongoing shift in vegetation dominance.

Multiple metrics quantify and differentiate responses of vegetation to composition B.

Quantifying vegetation response to explosive compounds has focused predominantly on morphological impacts and uptake efficiency. A more comprehensive understanding of the total impacts of explosives on vegetation can be gained using a multivariate approach. We hypothesized that multiple variables representing morphological and physiological responses will more clearly differentiate species and treatments than any single variable. Individuals of three plant species were placed in soils contaminated with Composition B, which comprises 60% hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 40% 2,4,6-trinitrotoluene (TNT), and grown for 2 months. Response metrics used included photosynthetic operation, water relations, growth characteristics, as well as nitrogen and carbon concentrations and isotopic compositions. Individual metrics showed high variability in response across the three species tested. Water relations and nitrogen isotopic composition exhibited the most consistent response across species. By comparing multiple variables simultaneously, better separation of both species and exposure was observed. The inclusion of novel metrics can reinforce previously established concepts and provide a new perspective. Additionally, the inclusion of various other metrics can greatly increase the ability to identify and differentiate particular groups. By using multivariate analyses and standard vegetation metrics, new aspects of the vegetation response to explosive compounds can be identified.

Impacts of explosive compounds on vegetation: A need for community scale investigations.

Explosive compounds are distributed heterogeneously across the globe as a result of over a century of human industrial and military activity. RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and TNT (2-methyl-1,3,5-trinitrobenzene) are the most common and most abundant explosives in the environment. Vegetation exhibits numerous physiological and morphological stress responses in the presence of RDX and TNT. Varied stress responses act as physiological filters that facilitate the proliferation of tolerant species and the extirpation of intolerant species. Contaminants alter community composition as they differentially impact plants at each life stage (i.e. germination, juvenile, adult), subsequently modifying larger scale ecosystem processes. This review summarizes the current explosives-vegetation literature, focusing on RDX and TNT as these are well documented in the literature, linking our current understanding to ecological theory. A conceptual framework is provided that will aid future efforts in predicting plant community response to residual explosive compounds.

Differential effects of two explosive compounds on seed germination and seedling morphology of a woody shrub, Morella cerifera.

Soils contaminated with explosive compounds occur on a global scale. Research demolition explosive (RDX) (hexahydro-1,3,5-trinitro-1,3,5-triazine) and trinitrotoluene (TNT) (2-methyl-1,3,5-trinitrobenzene) are the most common explosive compounds in the environment. These compounds, by variably impacting plant health, can affect species establishment in contaminated areas. Our objective was to quantify comparative effects of RDX and TNT on a woody shrub, Morella cerifera, commonly found on bombing ranges along the Atlantic Coast of the United States. Two life stages of M. cerifera, Seeds and juvenile plants, were exposed to soil amended with concentrations of RDX and TNT representative of field levels; RDX up to 1,500 ppm and TNT up to 900 ppm. Percent germination was recorded for 3 weeks; morphological metrics of necrotic, reduced, and curled leaves, in addition to shoot length and number measured at the end of the experiment (8 weeks) for juvenile plants. All concentrations of RDX inhibited seed germination while TNT did not have an effect at any concentration. As contaminant concentration increased, significant increases in seedling morphological damage occurred in the presence of RDX, whereas TNT did not affect seedling morphology at any concentration. Overall the plants were more sensitive to the presence of RDX. Species specific responses to explosive compounds in the soil have the potential to act as a physiological filter, altering plant recruitment and establishment. This filtering of species may have a number of large scale impacts including: altering species composition and ecological succession.

Physiological and transcriptional responses of Baccharis halimifolia to the explosive "composition B" (RDX/TNT) in amended soil.

Unexploded explosives that include royal demolition explosive (RDX) and trinitrotoluene (TNT) cause environmental concerns for surrounding ecosystems. Baccharis halimifolia is a plant species in the sunflower family that grows naturally near munitions sites on contaminated soils, indicating that it might have tolerance to explosives. B. halimifolia plants were grown on 100, 300, and 750 mg kg(-1) of soil amended with composition B (Comp B) explosive, a mixture of royal demolition explosive and trinitrotoluene. These concentrations are environmentally relevant to such munitions sites. The purpose of the experiment was to mimic contaminated sites to assess the plant's physiological response and uptake of explosives and to identify upregulated genes in response to explosives in order to better understand how this species copes with explosives. Stomatal conductance was not significantly reduced in any treatments. However, net photosynthesis, absorbed photons, and chlorophyll were significantly reduced in all treatments relative to the control plants. The dark-adapted parameter of photosynthesis was reduced only in the 750 mg kg(-1) Comp B treatment. Thus, we observed partial physiological tolerance to Comp B in B. halimifolia plants. We identified and cloned 11 B. halimifolia gene candidates that were orthologous to explosive-responsive genes previously identified in Arabidopsis and poplar. Nine of those genes showed more than 90% similarity to Conyza canadensis (horseweed), which is the closest relative with significant available genomics resources. The expression patterns of these genes were studied using quantitative real-time PCR. Three genes were transcriptionally upregulated in Comp B treatments, and the Cytb6f gene was found to be highly active in all the tested concentrations of Comp B. These three newly identified candidate genes of this explosives-tolerant plant species can be potentially exploited for uses in phytoremediation by overexpressing these genes in transgenic plants and, similarly, by using promoters or variants of promoters from these genes fused to reporter genes in transgenic plants for making phytosensors to report the localized presence of explosives in contaminated soils.