David Bart

Phragmites australis invasion and expansion in tidal wetlands: Interactions among salinity, sulfide, and hydrology

Through their physiological effects on ion, oxygen, and carbon balance, respectively, salinity, sulfide, and prolonged flooding combine to constrain the invasion and spread ofPhragmites in tidal wetlands. Initial sites of vigorous invasion by seed germination and growth from rhizome fragments appear limited to sections of marsh where salinity is <10‰, sulfide concentrations are less than 0.1 mM, and flooding frequency is less than 10%. In polyhaline tidal wetlands the invasion sites include the upland fringe and some high marsh creek banks. The zones of potential invasion tend to be larger in marshes occupying lower-salinity portions of estuaries and in marshes that have been altered hydrologically. Owing to clonal integration and a positive feedback loop of growth-induced modification of edaphic soil conditions, however, a greater total area of wetland is susceptible toPhragmites expansion away from sites of establishment. Mature clones have been reported growing in different marshes with salinity up to 45‰, sulfide concentration up to 1.75 mM, and flooding frequency up to 100%. ForPhragmites establishment and expansion in tidal marshes, windows of opportunity open with microtopographic enhancement of subsurface drainage patterns, marsh-wide depression of flooding and salinity regimes, and variation in sea level driven by global warming and lunar nodal cycles. To avoidPhragmites monocultures, tidal wetland creation, restoration, and management must be considered within the context of these different scales of plant-environment interaction.

The role of large rhizome dispersal and low salinity windows in the establishment of common reed,Phragmites australis, in salt marshes: New links to human activities

In spite of its long history,Phragmites australis’ (Cav.) Trin ex Stuedel invasion in tidal marshes defies explanation. Initial establishment in these systems is particularly perplexing, because seedlings and rhizome fragments do not perform well in poorly drained saline environments. We tested the possibility that dispersal and burial of large rhizomes, periods of low salinity, and localized, well-drained areas facilitate initial establishment in brackish marshes. In a greenhouse we exposed large and small rhizomes to two drainage treatments: mimics of poorly-drained, high marsh interiors and mimics of well-drained, mosquito ditch banks. In well-drained treatments we exposed rhizomes to one of three salinity treatments: fresh, natural salinity regime of an invaded brackish water marsh, and a 2-wk freshwater window followed by a natural salinity regime. Small rhizone fragments did not emerge in saline treatments or treatments with high water tables, while emergence was spotty in well-drained freshwater treatments. Large rhizomes emerged only in well-drained, treatments. For large rhizomes, growth, survival, and clonal spread decreased when exposed to the natural salinity regime, but improved with exposure to the 2-wk freshwater window. These results suggest that dispersal and burial of larger rhizomes, well-drained features, and low salinity windows following dispersal improve the chances of successful establishment. These results help explain case-specific historical links between establishment and such human activities as hydrological alterations, construction activities, and lowered salinity.

Human facilitation of Phragmites australis invasions in tidal marshes: a review and synthesis.

Efforts to manage or prevent Phragmites australis invasion in salt and brackish marshes are complicated by the lack of a general causal role for specific human activities. The pattern of invasion within a marsh differs among sites, and each may have different causal histories. A review of the literature finds three establishment/invasion patterns: (1) from stands established on ditch- or creek-bank levees toward interior portions of high marshes, (2) from stands along upland borders toward high marsh interiors, and (3) centroid spread from high marsh stands established in ostensibly random locations. Each invasion pattern seems to have different anthropogenic precursors, therefore preventing generalizations about the role of any one human activity in all sites. However, historical and experimental evidence suggests that regardless of invasion pattern, establishment is much more likely at sites where rhizomes are buried in well-drained, low salinity marsh areas. Any human activity that buries large rhizomes, increases drainage, or lowers salinity increases chances of establishing invasive clones. To integrate these patterns and improve our understanding of the rapid spread of Phragmites, recent evidence has been synthesized into a dichotomous flow chart which poses questions about current site conditions and the potential for proposed activities to change site conditions that may facilitate invasion. This simple framework could help managers assess susceptibility and take preventative measures in coastal marshes before invasion occurs or before removal becomes very expensive.

Environmental constraints on early establishment of Phragmites Australis in salt marshes

Finding environmental constraints on the establishment in salt marshes of Phragmites australis may help elucidate human activities that facilitated its invasion. We tested the effects of rhizome burial, salinity, anoxia, and sulfides on emergence, survival, growth, biomass production, and spread. In greenhouse and field experiments, rhizome burial facilitated initial emergence in well-drained soils. Rhizome emergence was prohibited in poorly drained treatments, regardless of salinity or sulfide concentrations. Emergence in well-drained treatments was not affected by salinity or sulfides, but survival, growth, and biomass storage of the culms and rhizomes were diminished in salt treatments. Combined with other studies, these results indicate that Phragmites invasion is a multi-stage process, with emergence constrained by poor drainage and survival constrained by lack of burial opportunities and salinity. These conditions constrain early stages of the invasion only, as later stages of the invasion can spread into anoxic and high salinity areas. These results also suggest that the process of invasion is facilitated by different human activities at different stages. Emergence is facilitated by soil disturbance, rhizome burial, and altered drainage. Survival through the first season can be facilitated through activities that lower porewater salinity.

Integrating local ecological knowledge and manipulative experiments to find the causes of environmental change

In spite of the increasing use of local ecological knowledge (LEK) in various environmental studies, few methodological innovations exist to facilitate new uses. This is especially true in studies of environmental change, where standard methods of comparing LEK with scientific knowledge do little to bolster causal claims. Here, I report on a novel approach that uses LEK to formulate manipulative experiments. This interdisciplinary approach exploits LEKs rich historical observations, while employing experimental approaches to disentangle competing hypotheses and to identify underlying mechanisms. This integrated approach produces historically based, mechanistically sound explanations that can be more useful than either LEK or experimental studies alone. I illustrate this approach with recent studies of common reed (Phragmites australis) invasion in salt hay farms.

Altered Plant-community Composition and Edaphic Features Associated with Plowing in Southern Wisconsin Fens

The potential for land-use legacies to alter current wetland macrophyte communities and edaphic conditions are underexplored. Here we present results of a study describing associations among these factors in southern Wisconsin fens. Plant-community composition, hydrology, and nutrient availability data were recorded from 1) fens that had been plowed, 2) fens that have never been plowed, and 3) fens with plowed and never-plowed areas. Plowed fens had: 1) lower total species richness, 2) higher invasive species richness, 2) lower native graminoid richness, 3) lower shrub richness, 4) lower root-zone volumetric water content, and 5) higher available N and P then never-plowed fens. Most of these same relationships held when comparing plowed and never-plowed areas of the same fen. Non-metric Multidimensional Scaling revealed that the most important community gradient separates graminoid- and fen-specialist-rich communities from those dominated by invasive species. This gradient also separated plowed from never plowed plots. Accordingly, we believe that a history of plowing has fundamentally altered biotic and, potentially, abiotic conditions, with real consequences for wetland management and restoration.

Historical Land Use and Plant-Community Variability in a Wisconsin Calcareous Fen

Abstract In 2008 a calcareous mound fen not listed in the state Natural Heritage Inventory was discovered in south-central Wisconsin. Air photo interpretations and interviews with land users indicated two distinct major historical alterations to this fen: (1) drainage tiling without widespread soil disturbance and (2) plowing with or without direct drainage. Nonmetric multidimensional scaling ordination of survey plots revealed that an assemblage of calciphiles, fen indicators, and wetland generalists persisted in the mounds center, while invasive herbaceous and woody vegetation reached highest cover along the disturbed flanks. Cluster analyses revealed three distinct communities: (1) a fen meadow in the center of the fen, (2) invading shrub-carr in tiled areas, and (3) a weedy/invasive herbaceous community where soil disturbance was widespread. This study lends support to past assertions that drainage can lead to shrub-carr invasion and suggests that widespread soil disturbance can lead to domination by herbaceous invasive plants on fen flanks.

The influence of legacy impacted seed banks on vegetation recovery in a post-agricultural fen complex

Agricultural legacies shape soil seed banks, which in turn alter subsequent vegetation recovery. However, the role of legacy-impacted seed banks in shaping wetland-vegetation recovery is not well studied, and hydrologic context may alter impacts of seed bank legacies on extant vegetation. Here we compare seed banks and extant vegetation of four plowed fens to an on-site reference fen to determine (i) if plowing legacies have altered seed banks, (ii) if 13-year post-abandonment vegetation differs from the reference fen, and (iii) whether extant vegetation composition is different than that of seed banks in both reference and plowed fens. The relative abundance of key native graminoid and fen specialist species was greatly reduced in plowed-fen seed banks compared to the reference fen, while weedy/invasive species present in the post-agricultural fens’ seed banks were absent from the reference fen. The depletion of normally dominant native graminoids from seed banks was reflected in extant vegetation. However, shallow water tables predicted shifts in extant vegetation from weedy-mesic annuals to areas dominated by either native clonal forbs or Phalaris arundinacea. In general, plowed plant communities had decreases relative abundance of weedy-mesic species and increased relative abundance clonal forbs or clonal-invasive graminoids compared to seed banks. Our results suggest that while depletion of native graminoids from seed banks plays a part in vegetation recovery, the abundance of weedy-mesic species was not reflected in the recovering vegetation in wetter areas.