No abstract

Interdependence among disturbance events, ecosystem properties, and biological invasions often make causal relationships difficult to discern. For example, Phragmites australis invasion in mid‐Atlantic salt marshes is often associated with disturbances that create well‐drained features as well as with low sulfide concentrations, but explanations of these associations have been elusive. We tested experimentally: 1) that disturbances increasing wetland drainage facilitate Phragmites invasion by altering sulfide concentrations and salinity; 2) that translocation allows plants to spread beyond drainage areas; and 3) that plants can then lower edaphic stress through pressure ventilation of the rhizosphere and promote further expansion. At the invasion front, treatments of 1) severing rhizomes to halt translocation and 2) combined severing with clipping dead culms to limit ventilation of the rhizosphere killed most culms, but did not affect pore water chemistry. In already invaded areas, severing and clipping reduced culm height and panicle production, severing alone and in combination with clipping also raised sulfide and ammonium concentrations in the root zone. There were no treatment effects on plant performance or pore water chemistry along mosquito ditches, where sulfide concentrations were negligible. Small‐scale hydrological alterations such as ditches appear to provide suitable sites for the establishment of Phragmites because soils are well‐drained and are low in free sulfides. Subsequent expansion into more hostile areas occurs through translocation, with well‐drained areas acting as sources for essential substances. Once established, the plant increases rhizosphere oxygenation and lowers sulfide concentrations.

In order to understand the role microbial communities play in mediating ecosystem response to disturbances it is essential to address the methodological and conceptual gap that exists between micro-and macro-scale perspectives in ecology. While there is little doubt microorganisms play a central role in ecosystem functioning and therefore in ecosystem response to global change-induced disturbance, our ability to investigate the exact nature of that role is limited by disciplinary and methodological differences among microbial and ecosystem ecologists. In this paper we present results from an interdisciplinary graduate-level seminar class focused on this topic. Through the medium of case studies in global change ecology (soil respiration, nitrogen cycling, plant species invasion and land use/cover change) we highlight differences in our respective approach to ecology and give examples where disciplinary perspective influences our interpretation of the system under study. Finally, we suggest a model for integrating perspectives that may lead to greater interdisciplinary collaboration and enhanced conceptual and mechanistic modeling of ecosystem response to disturbance.

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.