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Barrier islands are complex and dynamic systems that provide critical ecosystem services to coastal populations. Stability of these systems is threatened by rising sea level and the potential for coastal storms to increase in frequency and intensity. Recovery of dune-building grasses following storms is an important process that promotes topographic heterogeneity and long-term stability of barrier islands, yet factors that drive dune recovery are poorly understood. We examined vegetation recovery in overwash zones on two geomorphically distinct (undisturbed vs. frequently overwashed) barrier islands on the Virginia coast, USA. We hypothesized that vegetation recovery in overwash zones would be driven primarily by environmental characteristics, especially elevation and beach width. We sampled species composition and environmental characteristics along a continuum of disturbance from active overwash zones to relict overwash zones and in adjacent undisturbed environments. We compared species assemblages along the disturbance chronosequence and between islands and we analyzed species composition data and environmental measurements with Canonical Correspondence Analysis to link community composition with environmental characteristics. Recovering and geomorphically stable dunes were dominated by Ammophila breviligulata Fernaud (Poaceae) on both islands while active overwash zones were dominated by Spartina patens (Aiton) Muhl. (Poaceae) on the frequently disturbed island and bare sand on the less disturbed island. Species composition was associated with environmental characteristics only on the frequently disturbed island (p = 0.005) where A. breviligulata was associated with higher elevation and greater beach width. Spartina patens, the second most abundant species, was associated with larger sediment grain size and greater sediment size distribution. On the less frequently disturbed island, time since disturbance was the only factor that affected community composition. Thus, factors driving the abundance of dune-building grasses and subsequent recovery of dunes varied between the two geomorphically distinct islands.

Encroachment of woody plants into grasslands has occurred worldwide and includes coastal ecosystems. This conversion process is mediated by seed dispersal patterns, environmental filtering, and biotic interactions. As spatiotemporally heterogeneous, harsh environments, barrier islands present a unique set of challenges for dispersal and establishment. Environmental conditions act as filters on dispersed seeds, thereby influencing encroachment and distribution patterns. Seldom have patterns of propagule dispersal been considered in the context of woody encroachment. We quantified dispersal and post‐dispersal processes of an encroaching woody population of Morella cerifera relative to directional rate of encroachment and observed distribution patterns on an Atlantic coastal barrier island with strong environmental filtering. We analyzed historic foredune elevation as a proxy for reduced interior environmental stress. The dispersal kernel was leptokurtic, a common characteristic of expanding populations, but rate of encroachment has slowed since 2005. Expansion pattern was related to foredune elevation, which limits encroachment below a threshold elevation. This difference between dispersal kernel behavior and encroachment rate is due to limited availability of suitable habitat for Morella and temporal variability in chlorides during the time of germination. Our results demonstrate that processes mediating seeds and seedling success must be accounted for to better understand establishment patterns of encroaching woody plants.

Understanding the complex and unpredictable ways ecosystems are changing and predicting the state of ecosystems and the services they will provide in the future requires coordinated, long‐term research. This paper is a product of a U.S. National Science Foundation funded Long Term Ecological Research (LTER) network synthesis effort that addressed anticipated changes in future populations and communities. Each LTER site described what their site would look like in 50 or 100 yr based on long‐term patterns and responses to global change drivers in each ecosystem. Common themes emerged and predictions were grouped into state change, connectivity, resilience, time lags, and cascading effects. Here, we report on the “state change” theme, which includes examples from the Georgia Coastal (coastal marsh), Konza Prairie (mesic grassland), Luquillo (tropical forest), Sevilleta (arid grassland), and Virginia Coastal (coastal grassland) sites. Ecological thresholds (the point at which small changes in an environmental driver can produce an abrupt and persistent state change in an ecosystem quality, property, or phenomenon) were most commonly predicted. For example, in coastal ecosystems, sea‐level rise and climate change could convert salt marsh to mangroves and coastal barrier dunes to shrub thicket. Reduced fire frequency has converted grassland to shrubland in mesic prairie, whereas overgrazing combined with drought drive shrub encroachment in arid grasslands. Lastly, tropical cloud forests are susceptible to climate‐induced changes in cloud base altitude leading to shifts in species distributions. Overall, these examples reveal that state change is a likely outcome of global environmental change across a diverse range of ecosystems and highlight the need for long‐term studies to sort out the causes and consequences of state change. The diversity of sites within the LTER network facilitates the emergence of overarching concepts about state changes as an important driver of ecosystem structure, function, services, and futures.

Coastlines have traditionally been engineered to maintain structural stability and to protect property from storm-related damage, but their ability to endure will be challenged over the next century. The use of vegetation to reduce erosion on ocean-facing mainland and barrier island shorelines - including the sand dunes and beaches on these islands - could be part of a more flexible strategy. Although there is growing enthusiasm for using vegetation for this purpose, empirical data supporting this approach are lacking. Here, we identify the potential roles of vegetation in coastal protection, including the capture of sediment, ecological succession, and the building of islands, dunes, and beaches; the development of wave-resistant soils by increasing effective grain size and sedimentary cohesion; the ability of aboveground architecture to attenuate waves and impede through-flow; the capability of roots to bind sediments subjected to wave action; and the alteration of coastline resiliency by plant structures and genetic traits. We conclude that ecological and engineering practices must be combined in order to develop a sustainable, realistic, and integrated coastal protection strategy.

Woody encroachment into herbaceous ecosystems is emerging as an important ecological response to global change. A primary concern is alterations in C and N cycling and associated variations across a variety of ecosystems. We quantified seasonal variation in litterfall and litter N concentration in Morella cerifera shrub thickets to assess changes in litterfall and associated N input after shrub expansion on an Atlantic coast barrier island. We also used the natural abundance of ¹⁵N to estimate the proportion of litterfall N originating from symbiotic N fixation. Litterfall for shrub thickets ranged from 8,991 ± 247 to 3,810 ± 399 kg ha-¹ year-¹ and generally declined with increasing thicket age. Litterfall in three of the four thickets exceeded previous estimates of aboveground annual net primary production in adjacent grasslands by 300-400%. Leaf N concentration was also higher after shrub expansion and, coupled with low N resorption efficiency and high litterfall, resulted in a return of as much as 169 kg N ha-¹ year-¹ to the soil. We estimated that ~70% of N returned to the soil was from symbiotic N fixation resulting in an ecosystem input of between 37 and 118 kg ha-¹ year-¹ of atmospheric N depending on site. Considering the extensive cover of shrub thickets on Virginia barrier islands, N fixation by shrubs is likely the largest single source of N to the system. The shift from grassland to shrub thicket on barrier islands results in a substantial increase in litterfall and foliar N concentration that will likely have a major impact on the size and cycling of ecosystem C and N pools. Increasing C and N availability in these nutrient-poor soils is likely to permanently reduce cover of native grasses and alter community structure by favoring species with greater N requirements.

Drivers of shrub primary production and associated landscape impacts of encroachment are well known in drylands but have not been thoroughly studied in mesic and coastal habitats. The native, nitrogen-fixing shrub, Morella cerifera, has expanded into coastal grassland along the US Atlantic coast due to warming temperatures, but impacts on ecosystem function are not well known. Annual net primary production (ANPP) of Morella cerifera and key environmental drivers were measured long-term (1990 – 2007) across a chronosequence of shrub age on a mid-Atlantic barrier island. Soil and groundwater nutrients were compared with un-encroached grassland soil to evaluate impacts of vegetation on nutrient dynamics. Shrub ANPP declined with age at the same rate among all thickets, but there was variability from year to year. When climate variables were included in models, shrub age, precipitation, and freshwater table depth were consistent predictors of ANPP. Water table depth decreased over time, reducing ANPP. This may be due to rising sea-level, as well as to feedbacks with shrub age and evapotranspiration. Soil N and C increased with shrub age and were higher than adjacent grassland sites; however, there was a significant loss of N and C to groundwater. Our results demonstrate that drivers influencing the encroachment of shrubs in this coastal system (i.e., warming temperature) are not as important in predicting shrub primary production. Rather, interactions between shrub age and hydrological properties impact ANPP, contributing to coastal carbon storage.

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.

Coastal ecosystems in general, and barrier islands in particular, offer unique opportunities to observe ecological patterns and processes in spatially and temporally compressed settings. Harsh abiotic conditions and frequent disturbances limit the number of resident species, and highly dynamic geomorphology may result in a space‐for‐time substitution as young shoreward communities give way to more developed later‐successional interior communities. We investigated relationships between lianas and woody plants at barrier island sites in Virginia (Hog Island) and North Carolina (Duck FRF ), to evaluate whether liana proliferation accelerates or delays succession from the dominant woody community, Morella shrub thicket, to the historic climax community, maritime forest. Using aerial imagery, Lidar data, community surveys, and edaphic sampling, we found a correlative relationship between abiotic environmental variables and woody community structure, and also between woody community distribution and liana success. Environmental variables demonstrated little predictive ability for liana distribution, but there was evidence of an association between lianas and the prominent woody species at Hog Island, though patchiness of the community and abiotic homogeneity prevented these associations from establishing at Duck FRF . We suggest that trends of woody plant expansion, compounded with current and predicted global change effects on growth and success of lianas, may contribute to a delay or prevention of further successional development at these and other temperate coastal sites.

Barrier islands provide the first line of defense against storms for millions of people living in coastal areas. Upland vegetation (that is, grassland, shrubland, and maritime forest) has received little attention, even though this land surface is most strongly affected by development pressures. We use remotesensing analysis to assess state change on seven undeveloped Virginia barrier islands over 27 years (1984–2011) that are free from direct human influence. Our analysis highlights the spatial–temporally dynamic nature of barrier island upland land area and vegetation, with rapidly changing ecosystem states. Over the time period, upland vegetation was dramatically reduced by 29% whereas woody vegetation cover increased 40% across all islands. Although conversions between sand, grassland, and woody vegetation were variable within each island, three major patterns of vegetative land-cover change were apparent: overall loss of vegetative cover, frequent transitions between grass and woody cover, and gain in woody cover. These patterns are valuable for understanding natural evolution of barrier islands in response to sea-level rise. Evaluation of temporal dynamics in barrier upland is needed to characterize underlying processes including island resilience or chronic stress, and is a prerequisite to sustainable coastal management- and resilience-based planning, especially when implementing ecosystem-based solutions.

Barrier islands are at the forefront of storms and sea-level rise. High disturbance regimes and sediment mobility make these systems sensitive and dynamic. Island foredunes are protective structures against storm-induced overwash that are integrally tied to dune grasses via biogeomorphic feedbacks. Shifts in dune grass dominance could influence dune morphology and susceptibility to overwash, altering island stability. In a glasshouse study, two dune grasses, Ammophila breviligulata and Uniola paniculata, were planted together and subjected to a 20 cm burial to quantify morphological and physiological responses. Burial had positive effects on both plants as indicated by increased electron transport rate and total biomass. Ammophila breviligulata performance declined when planted with U. paniculata. Uniola paniculata was not affected when planted with A. breviligulata but did have higher water use efficiency and nitrogen use efficiency. Planted in mixture, differential reallocation of biomass occurred between species potentially altering resource acquisition further. As U. paniculata migrates into A. breviligulata dominated habitat and A. breviligulata performance diminishes, biotic interactions between these and other species may affect dune formation and community structure. Our study emphasizes the importance of studying biotic interactions alongside naturally occurring abiotic drivers.