Katharina A. M. Engelhardt

The role of landscape connectivity in assembling exotic plant communities: a network analysis

Landscape fragmentation and exotic species invasions are two modern-day forces that have strong and largely irreversible effects on native diversity worldwide. The spatial arrangement of habitat fragments is critical in affecting movement of individuals through a landscape, but little is known about how invasive species respond to landscape configuration relative to native species. This information is crucial for managing the global threat of invasive species spread. Using network analysis and partial Mantel tests to control for covarying environmental conditions, we show that forest plant communities in a fragmented landscape have spatial structure that is best captured by a network representation of landscape connectivity. This spatial structure is less pronounced in invasive species and exotic species dispersed by animals. Our research suggests that invasive species can spread more easily in fragmented landscapes than native species, which may make communities more homogeneous over time.

Annual species abundance in a tidal freshwater marsh: Germination and survival across an elevational gradient

Annual species contribute significantly to the standing biomass of tidal freshwater marshes, but they tend to be distributed unevenly along the elevation gradient, with higher surface elevations supporting greater densities of annual species. We explored the generation of this pattern by evaluating how different life history stages of annual species are related to elevation and to each other. In 2006, we counted seedlings emerging from the seed bank under optimal greenhouse conditions, as well as seedlings and mature stems of annual species in 38 plots located at different elevations in a tidal freshwater marsh near Alexandria, Virginia, USA. Annual species seedling and mature stem density increased with elevation of the marsh surface, but seed density did not change with elevation. Seeds of annual species germinated in saturated rather than flooded conditions (4.50 ±0.54 versus 1.66 ±0.26 species/ greenhouse container ±SE). Germination and survival from seedling to mature stem affected density of annual species across an elevational gradient, but the relative importance of either process differed by species. Amaranthus cannabinus was common and frequent in the seed bank (68% of all plots), and seed density increased with elevation, but seedlings and mature stems were infrequent at any elevation (21% and 5%, respectively), suggesting that germination is limiting its recruitment to standing vegetation. In contrast, Bidens laevis was uncommon in the seed bank (18% of all plots), but was frequently observed as seedlings and mature stems (55% and 34%, respectively). We therefore conclude that B. laevis recruitment was limited by its ephemeral seed bank. Impatiens capensis density did not appear to be limited by germination or survival to maturity as density of the species was high at all life history stages. Rather, its strong positive relationship with elevation at all life stages shows that I. capensis is limited by water inundation, particularly in spring when seeds require oxygen to germinate. Overall, we show that annual species increase in density at higher elevations in a tidal freshwater marsh, but that recruitment of each species may be limited by different intrinsic and extrinsic processes.

Does genetic diversity of restored sites differ from natural sites? A comparison of Vallisneria americana (Hydrocharitaceae) populations within the Chesapeake Bay

The goal of ecological restoration is to re-establish self-sustaining ecosystems that will resist future perturbation without additional human input. We focus here on the re-establishment of submersed aquatic macrophyte beds in the restoration of the Chesapeake Bay estuary. Degraded environmental conditions are often to blame for poor bed establishment, but genetic factors could also be contributing to low survival. We quantified the effect of restoration practices on genetic diversity in the submersed aquatic plant species Vallisneria americana Michx. (Hydrocharitaceae) in the Chesapeake Bay. In 2007, we collected 440 shoots from 8 restored/natural site pairs and 4 restoration stock repositories, and genotyped those individuals at 10 microsatellite loci. Restoration practices do not appear to negatively impact genetic diversity, and basic measures of genetic diversity within restored sites overlap with natural sites. However, small population size of restored sites, significant inbreeding coefficients within 3 sites, and low overlap of allele composition among sites provide cause for concern. These problems are relatively minor, and we propose several corrections that would alleviate them altogether. Managers should be encouraged by our findings as well as the current state of the genetic diversity within V. americana restoration efforts.

The structure of population genetic diversity in Vallisneria americana in the Chesapeake Bay: implications for restoration

Submersed aquatic macrophyte beds provide important ecosystem services, yet their distribution and extent has declined worldwide in aquatic ecosystems. Effective restoration of these habitats will require, among other factors, reintroduction of genetically diverse source material that can withstand short- and long-term environmental fluctuations in environmental conditions. We examined patterns of genetic diversity in Vallisneriaamericana because it is a cosmopolitan freshwater submersed aquatic macrophyte and is commonly used for restoring freshwater habitats. We sampled 26 naturally occurring populations of V. americana in the Chesapeake Bay estuary and its tributaries and found that the majority of populations have high genotypic diversity and are not highly inbred. Fourteen of the populations had high allelic and genotypic diversity and could serve as source sites for restoration material. However, substantial geographic structuring of genetic diversity suggests that caution should be used in moving propagules to locations distant from their source. In particular, we suggest that propagules at least be limited within four primary geographic areas that correspond to freshwater tidal and non-tidal, oligohaline, and seasonally mesohaline areas of the Chesapeake Bay.

COEXISTENCE OF TYPHA ANGUSTIFOLIA AND IMPATIENS CAPENSIS IN A TIDAL FRESHWATER MARSH

Several salt marsh plant species can oxygenate soils through their aerenchymous tissue and thereby facilitate the growth of neighboring species. Such positive interactions remain poorly understood for tidal freshwater marshes, yet may explain why species adverse to submerged roots are able to thrive in these marshes. Field observations showed a positive association between Typha angustifolia L. and Impatiens capensis Meerb., the two co-occuring dominant species in our study system, suggesting either that T. angustifolia facilitates I. capensis or that both species respond to environmental gradients the same way without interacting directly. We tested the facilitation hypothesis in a greenhouse experiment that measured the growth response of I. capensis with and without T. angustifolia as a neighbor species. Both species affected reduction-oxidation potential of the substrate, but facilitation of I. capensis was not observed. Typha angustifolia depleted soil nutrients in experimental mesocosms and negatively affected belowground root growth but not aboveground biomass of I. capensis; however, nutrient replenishment by tidal flushing may decrease the likelihood for competition in natural systems. Our greenhouse study suggests that abiotic conditions, dispersal events, or competitive biotic interactions, and not facilitation, are predominantly influencing the distribution of these two species in tidal freshwater marshes. Indeed, field measurements demonstrate that both species are positively associated with marsh surface elevation, which is related to frequency and length of water inundation of the marsh surface.

Spatial patterns of plant litter in a tidal freshwater marsh and implications for marsh persistence

The maintenance of marsh platform elevation under conditions of sea level rise is dependent on mineral sediment supply to marsh surfaces and conversion of above- and belowground plant biomass to soil organic material. These physical and biological processes interact within the tidal zone, resulting in elevation-dependent processes contributing to marsh accretion. Here, we explore spatial pattern in a variable related to aboveground biomass, plant litter, to reveal its role in the maintenance of marsh surfaces. Plant litter persisting through the dormant season represents the more recalcitrant portion of plant biomass, and as such has an extended period of influence on ecosystem processes. We conducted a field and remote sensing analysis of plant litter height, aboveground biomass, vertical cover, and stem density (collectively termed plant litter structure) at a tidal freshwater marsh located within the Potomac River estuary, USA. LiDAR and field observations show that plant litter structure becomes more prominent with increasing elevation. Spatial patterns in litter structure exhibit stability from year to year and correlate with patterns in soil organic matter content, revealed by measuring the loss on ignition of surface sediments. The amount of mineral material embedded within plant litter decreases with increasing elevation, representing an important tradeoff with litter structure. Therefore, at low elevations where litter structure is short and sparse, the role of plant litter is to capture sediment; at high elevations where litter structure is tall and dense, aboveground litter contributes organic matter to soil development. This organic matter contribution has the potential to eclipse that of belowground biomass as the root:shoot ratio of dominant species at high elevations is low compared to that of dominant species at low elevations. Because of these tradeoffs in mineral and organic matter incorporation into soil across elevation gradients, the rate of marsh surface elevation change is remarkably consistent across elevation. Because of the role of plant litter in marsh ecosystem processes, monitoring and assessment of these dynamic geomorphic marsh landscapes might be streamlined through the measurement of plant litter structure, either via LiDAR technologies or field observation.

Development of 11 polymorphic microsatellite markers in a macrophyte of conservation concern, Vallisneria americana Michaux (Hydrocharitaceae)

Vallisneria americana Michaux (wild celery) is currently a target of submersed aquatic vegetation restoration efforts in the Chesapeake Bay watershed. To aid these efforts, we have developed 11 polymorphic microsatellite markers to assess the distribution and degree of genetic diversity in both restored and naturally occurring populations in the Chesapeake Bay. In 59 individuals from two populations, we detected two to 10 total alleles per locus. Observed heterozygosity ranged from 0.125 to 0.929, and two loci exhibited significant deviations from Hardy–Weinberg equilibrium in at least one of the populations assayed.

Modeled Tradeoffs between Developed Land Protection and Tidal Habitat Maintenance during Rising Sea Levels

Tidal habitats host a diversity of species and provide hydrological services such as shoreline protection and nutrient attenuation. Accretion of sediment and biomass enables tidal marshes and swamps to grow vertically, providing a degree of resilience to rising sea levels. Even if accelerating sea level rise overcomes this vertical resilience, tidal habitats have the potential to migrate inland as they continue to occupy land that falls within the new tide range elevations. The existence of developed land inland of tidal habitats, however, may prevent this migration as efforts are often made to dyke and protect developments. To test the importance of inland migration to maintaining tidal habitat abundance under a range of potential rates of sea level rise, we developed a spatially explicit elevation tracking and habitat switching model, dubbed the Marsh Accretion and Inundation Model (MAIM), which incorporates elevation-dependent net land surface elevation gain functions. We applied the model to the metropolitan Washington, DC region, finding that the abundance of small National Park Service units and other public open space along the tidal Potomac River system provides a refuge to which tidal habitats may retreat to maintain total habitat area even under moderate sea level rise scenarios (0.7 m and 1.1 m rise by 2100). Under a severe sea level rise scenario associated with ice sheet collapse (1.7 m by 2100) habitat area is maintained only if no development is protected from rising water. If all existing development is protected, then 5%, 10%, and 40% of the total tidal habitat area is lost by 2100 for the three sea level rise scenarios tested.

Twenty-First Century Climate Change and Submerged Aquatic Vegetation in a Temperate Estuary: The Case of Chesapeake Bay

ABSTRACT Introduction: The Chesapeake Bay was once renowned for expansive meadows of submerged aquatic vegetation (SAV). However, only 10% of the original meadows survive. Future restoration efforts will be complicated by accelerating climate change, including physiological stressors such as a predicted mean temperature increase of 2–6°C and a 50–160% increase in CO2 concentrations. Outcomes: As the Chesapeake Bay begins to exhibit characteristics of a subtropical estuary, summer heat waves will become more frequent and severe. Warming alone would eventually eliminate eelgrass (Zostera marina) from the region. It will favor native heat-tolerant species such as widgeon grass (Ruppia maritima) while facilitating colonization by non-native seagrasses (e.g., Halodule spp.). Intensifying human activity will also fuel coastal zone acidification and the resulting high CO2/low pH conditions may benefit SAV via a “CO2 fertilization effect.” Discussion: Acidification is known to offset the effects of thermal stress and may have similar effects in estuaries, assuming water clarity is sufficient to support CO2-stimulated photosynthesis and plants are not overgrown by epiphytes. However, coastal zone acidification is variable, driven mostly by local biological processes that may or may not always counterbalance the effects of regional warming. This precarious equipoise between two forces – thermal stress and acidification – will be critically important because it may ultimately determine the fate of cool-water plants such as Zostera marina in the Chesapeake Bay. Conclusion: The combined impacts of warming, coastal zone acidification, water clarity, and overgrowth of competing algae will determine the fate of SAV communities in rapidly changing temperate estuaries.