Sexual productivity and spring intramarsh distribution of a key salt-marsh microbial secondary producer

Phaeosphaeria spartinicola is known to be an important fungal (ascomycetous) secondary producer in the smooth-cordgrass (Spartina alterniflora) decomposition system of western Atlantic salt marshes, yet its degree of predominance among the ascomycete assemblages of salt marshes and the concentration of its sexual reproductive structures (ascomata) have been largely unknown. During May–June, we measured by direct microscopy the percent occupancy of leaf abaxial area and concentrations of ascomata in leaf blades of smooth cordgrass at three elevations in three drainage systems within the marshes of Sapelo Island, Georgia, United States. We also measured in water-saturation chambers the rates at which the ascomata expelled ascopoores (sexual propagules) out of decaying leaves from marsh sites containing or not containing shredder gastropods.P. spartinicola ascomata were found at averages of 36% to 93% of grid-circles (3-mm radius) on decaying leaf blades, with lower values at points directly adjacent to the leaf sheath, on leaves at earlier stages of decay, and at elevational subsites where shredder snails were more active. Marsh elevation had no effect of its own on percent occupation. No other species of ascomycetes were found at overall avarage frequencies greater than 3%. Average concentration of ascomata along the intervascular rows where they were located was 1 ascoma per 0.5 mm row (～1000 cm^?2 abaxial leaf surface, translating to production of 1.6×10⁷ ascomata m^?2 intermediate-height marsh per standing crop of living stems). The fraction of total fungal production allocated to ascomata is speculatively and crudely estimated at about 9%, without taking into account potential loss to invertebrate shredders. At sites with abundances of snails >-50 m^?2 peaks of ascospore expulsion (about 50–75 spores cm^?2 leaf h^?1, 3–5× the overall average rate) observed at snail-free sites were absent. Our measured rate of ascospore expulsion (averaged over snail-free and high-snail sites, and possibly an underestimate) translated to 6.5×10⁴ spores m^?2 marsh h^?1 for times of freshwater saturation of leaves, and one-third that value for times of saltwater wetting.     

Application of multi-temporal differential interferometry to slope instability detection in urban/peri-urban areas

We present two case studies regarding the application of Synthetic Aperture Radar (SAR) Persistent Scatterers Interferometry (PSI) techniques to landslide-prone slopes situated in the municipal territories of Caramanico Terme and Volturino (Italy). The analysis of satellite SAR data with PSI techniques poses often problems on sites where, due to the scarcity of human artefacts and the presence of vegetation cover, density of coherent points (PS) is low (< 10 per km²). Moreover, the steep and rough topography typical of landslide-prone areas hamper the interferometric pre-processing, making more difficult the joint estimation of displacements and of DEM errors. Under these conditions the significance of temporal interferometric phase trends can be uncertain and conservative assumptions, necessary to ensure low false detection probabilities, need to be coupled with innovative processing strategies to increase the detection efficiency of PS objects. Here, the SPINUA (Stable Point Interferometry over Un-urbanised Areas) processing technique is applied together with an alternative PS Candidate (PSC) selection procedure based on the use of pixels classified as urban.

The cases of Caramanico and Volturino are representative, respectively, of harsh and favourable conditions for PSI applications. The results from Caramanico show clusters of PS exhibiting similar line-of-sight (LOS) deformation behaviour in the period 1995–2000. The locations of moving PS often coincide with distressed buildings and appear consistent with the areal distribution of recent and past landslide activity. The temporal displacement trends, however, are characterised by very low annual average velocities (from 3 to 7 mm/y) and it is uncertain to what extent the PS data reflect true slope movements, local deformations (e.g. settlement of engineering structures) or both.

Thanks to the more favourable conditions, the application of the standard SPINUA approach in the Volturino area was sufficient to obtain suitable densities of PS, as well as spatially and temporally consistent displacement results for a period 1992–2000. In particular, a group of moving PS was identified in a peri-urban area, known for the past and recent slope stability problems. The slowly moving PS (from 3 to 5 mm/y) fall in a location that, unlike the remaining part of the town, is characterised by the presence of many distressed buildings and structures. Although the site information confirms the reliability of PS data, in the absence of ground monitoring and detailed records of landslide movements, it is difficult to identify the main mechanism of the detected deformations. In general, in geologically and topographically complex urban/peri-urban settings, the significance of very low-velocity PSI surface displacements should always be considered together with in situ geotechnical controls and ground monitoring data.