D. van der Wal

Vegetation causes channel erosion in a tidal landscape

Vegetation is traditionally regarded to reduce the erosion of channels in both fl uvial and tidal landscapes. We present a coupled hydrodynamic, morphodynamic, and plant growth model that simulates plant colonization and channel formation on an initially bare, fl at substrate, and apply this model to a tidal landscape. The simulated landscape evolution is compared with aerial photos. Our results show that reduction of erosion by vegetation is only the local, on-site effect operating within static vegetation. Dynamic vegetation patches, which can expand or shrink, have a contrasting larger scale, off-site effect: they obstruct the fl ow, leading to flconcentration and channel erosion between laterally expanding vegetation patches. In contrast with traditional insights, our fi ndings imply that in tidal landscapes, which are colonized by denser vegetation, channels are formed with a higher channel drainage density. Hence this study demonstrates that feedbacks between vegetation, fl ow, and landform have an important control on landscape evolution.

Self‐Organization and Vegetation Collapse in Salt Marsh Ecosystems

Complexity theory predicts that local feedback processes may strongly affect the organization of ecosystems on larger spatial scales. Whether complexity leads to increased resilience and stability or to increased vulnerability and criticality remains one of the dominant questions in ecology. We present a combined theoretical and empirical study of complex dynamics in mineralogenic salt marsh ecosystems that emerge from a positive feedback between clay accumulation and plant growth. Positive feedback induces self‐organizing within the ecosystem, which buffers for the strong physical gradient that characterizes the marine‐terrestrial boundary, and improves plant growth along the gradient. However, as a consequence of these self‐organizing properties, salt marshes approach a critical state as the edge of the salt marsh and the adjacent intertidal flat becomes increasingly steep and vulnerable to wave attack. Disturbance caused, for instance, by a storm may induce a cascade of vegetation collapse and severe erosion on the cliff edge, leading to salt marsh destruction. Our study shows that on short timescales, self‐organization improves the functioning of salt marsh ecosystems. On long timescales, however, self‐organization may lead to destruction of salt marsh vegetation.

Mantle shear zones and their effect on lithosphere strength during continental breakup

Abstract The Erro-Tobbio lherzolite in the ophiolitic Voltri Massif of northwestern Italy includes several thrusted fragments of lithospheric mantle which were first exhumed to the ocean floor during Jurassic rifting and breakup, and at a later stage became emplaced in the Alpine collisional stack during Tertiary convergence between Africa and Europe. Coherent slices of these mantle rocks contain several sets of major shear zones generated during the Jurassic rift evolution. One such shear zone, several kilometres wide and formed at temperatures between 920 and 1040°C, is transected by up to 200 m wide, ultra-fine-grained hydrated mylonite zones formed at temperatures in the range 990-550°C. All these structures are cut by MORB-type gabbroic and basaltic dykes. The microstructures of the mylonite zones are interpreted to reflect progressive, reaction-related grainsize reduction plus localization of the deformation during the early stages of continental breakup. In view of experimental evidence that wet olivine rocks weaken considerably with decreasing grainsize, in response to a change from grainsize-insensitive dislocation creep to grainsize-sensitive creep mechanisms, it is proposed that shear localization and allied grainsize reduction may have resulted in a drastic decrease in strength of the upper mantle during rifting. In order to obtain an order of magnitude estimate of this rheological effect, we present a layered rheological model of the Piemonte-Ligurian lithosphere, based on the observed microstructures (i.e., grainsizes) and pressure-temperature data, and including appropriate rheological laws for grainsize-sensitive and -insensitive creep in wet olivine. The model calculations suggest strength values for the uppermost mantle up to four orders of magnitude lower than those expected for homogeneous deformation exclusively controlled by dislocation creep of dry olivine.

Shear zones in the upper mantle: A case study in an Alpine Iherzolite massif

The Erro-Tobbio upper-mantle lherzolite of northwest Italy, emplaced in the Alpine suture zone during collision of the European and Adriatic plates, has preserved a set of major shear zones formed at temperatures in the range 800-1040 °C during the period of Jurassic rifting and breakup and development of the Piemonte-Ligurian oceanic basin. These structures clearly demonstrate localization of the deformation in the Piemonte-Ligurian upper mantle during lithosphere stretching and breakup.

Immunomodulatory and anti-inflammatory activity of Picrorhiza scrophulariiflora.

Extracts of the rhizomes of Picrorhiza scrophulariiflora Pennell (Scrophulariaceae) were investigated for their in vitro and in vivo immunomodulatory properties. Diethyl ether extracts showed potent inhibitory activity towards the classical pathway of the complement system, the respiratory burst of activated polymorphonuclear leukocytes, and mitogen-induced proliferation of T-lymphocytes. Furthermore, such extracts showed anti-inflammatory activity towards carrageenan-induced paw edema. No effects were observed in experimentally induced arthritis in mice.

Impacts of salt marsh plants on tidal channel initiation and inheritance

At the transition between mudflat and salt marsh, vegetation is traditionally regarded as a sustaining factor for previously incised mudflat channels, able to conserve the channel network via bank stabilization following plant colonization (i.e., vegetation-stabilized channel inheritance). This is in contrast to recent studies revealing vegetation as the main driver of tidal channel emergence through vegetation-induced channel erosion. We present a coupled hydrodynamic morphodynamic plant growth model to simulate plant expansion and channel formation by our model species (Spartina alterniflora) during a mudflat-salt marsh transition with various initial bathymetries (flat, shoal dense, shoal sparse, and deep dense channels). This simulated landscape development is then compared to remote sensing images of the Yangtze estuary, China, and the Scheldt estuary in Netherlands. Our results propose the existence of a threshold in preexisting mudflat channel depth, which favors either vegetation-stabilized channel inheritance or vegetation-induced channel erosion processes. The increase in depth of preexisting mudflat channels favors flow routing through them, consequently leaving less flow and momentum remaining for vegetation-induced channel erosion processes. This threshold channel depth will be influenced by field specific parameters such as hydrodynamics (tidal range and flow), sediment characteristics, and plant species. Hence, our study shows that the balance between vegetation-stabilized channel inheritance and vegetation-induced channel erosion depends on ecosystem properties.

Ecological evaluation of an experimental beneficial use scheme for dredged sediment disposal in shallow tidal waters

An experiment was performed to test an alternative dredging strategy for the Westerschelde estuary. Clean sand dredged from the navigation channel was disposed seawards of an eroding intertidal flat in order to modify morphology and hydrodynamics, improving the multi-channel system with ecologically productive shallow water habitat. Five years of intensive monitoring revealed that part of the disposed sediment moved slowly towards the flat, increasing the very shallow subtidal and intertidal area, as planned. The sand in the impact zone became gradually finer after disposal, possibly due to reduced current velocities. Nevertheless, no changes in macrobenthic biomass, density, species richness and composition were detected in the subtidal zone, also demonstrating rapid macrobenthic recovery. In the intertidal zone, no ecological effects could be revealed superimposed on trends associated with long-term sediment fining. Thus, despite morphological success and absence of detected negative ecological impacts of the experiment, new beneficial habitat was not created.

Oblique fabrics in porphyroclastic Alpine-type peridotites: a shear-sense indicator for upper mantle flow

Abstract A detailed structural study in Alpine-type peridotites of the ultramafic Voltri Massif in NW Italy reveals the existence of oblique fabrics in porphyroclastic tectonites. The dominant foliation of these tectonites is defined by stretched pyroxenes and aligned grains of spinel, which formed as a result of high-temperature flow in a km-scale shear zone bounded by a wall-rock of granular, virtually undeformed spinel-lherzolites. The oblique fabrics in these tectonites are characterized by elongate olivine grain shapes and a marked grain boundary alignment oriented at an angle of up to 40° to the tectonite foliation. In addition, kink-type subgrain boundaries are frequently parallel to (100), and show a preferred orientation at large angles to the tectonite foliation. Olivine lattice preferred orientation patterns in tectonites with this oblique grain shape fabric show bimodal distributions of [100] and [001]. The oblique fabrics and bimodal orientation distributions are inferred to result from the formation of high-angle kink-like grain boundaries followed by deformation-induced grain boundary migration at the expense of unfavourably oriented grains. The sense of shear derived from the oblique fabrics is consistent with the sense of shear derived from the lattice preferred orientation patterns, asymmetric pyroxene porphyroclast systems and the map-scale geometry of the shear zone in which these microstructures occur. It is therefore suggested that these oblique grain shape fabrics represent a reliable kinematic indicator for high-temperature flow in upper mantle shear zones.

Flow Velocity and Morphology of a Submerged Patch of the Aquatic Species Veronica anagallis - aquatica L.

The interaction between macrophytes and hydrodynamic conditions is an important feature in many aquatic ecosystems. Submerged macrophytes can form monospecific patches that interact with the flow and alter current velocity; within the same vegetation patch, plants are exposed to different levels of hydrodynamic stress. Due to the high morphological variability of aquatic plants, we expect different architectural and morphological traits to emerge for individuals located at different positions within the same patch. In this study, we have measured the flow velocity around a patch of Veronica anagallis-aquatica in submerged conditions and measured the morphological traits of individuals along a gradient of exposure to flow velocity within the patch. Results show that the more exposed individuals present smaller sizes than the sheltered ones, lower relative allocation to stems, higher allocation to roots and reduced water content in roots and stems. The knowledge obtained helps to clarify the role of morphological adaptations to flow stress in the context of plant-flow interactions.