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 fl ow concentration 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.

Abstract. Biologically mediated modifications of the abiotic environment, also called ecosystem engineering, can significantly affect a broad range of ecosystems. Nevertheless, remarkably little work has focused on the costs and benefits that ecosystem engineers obtain from traits that underlie their ecosystem engineering capacity. We addressed this topic by comparing two autogenic engineers, which vary in the degree in which they affect their abiotic environment via their physical structure. That is, we compared two plant species from the intertidal coastal zone (Spartina anglica and Zostera noltii), whose shoots are exposed to similar currents and waves, but differ in the extent that they modify their environment via reduction of hydrodynamic energy. Our results indicate that there can be trade-offs related to the traits that underlies autogenic ecosystem engineering capacity. Dissipation of hydrodynamic forces from waves was roughly a factor of three higher in vegetation with stiff leaves compared to those with flexible leaves. Drag was highest and most sensitive to hydrodynamic forces in stiff vegetation that does not bend with the flow. Thus, shoot stiffness determines both the capacity to reduce hydrodynamic energy (i.e., proxy for ecosystem engineering capacity) and the drag that needs to be resisted (i.e., proxy for associated costs). Our study underlines the importance of insight in the trade-offs involved in ecosystem engineering as a first step toward understanding the adaptive nature of ecosystem engineering.

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[1] Feedback between vegetation growth, water flow, and landform is important for the biogeomorphic evolution of many landscapes, such as tidal marshes, alluvial rivers, and hillslopes. While experimental studies often focus on flow reduction within static homogeneous vegetation, we concentrate on flow acceleration around and between dynamically growing vegetation patches that colonize an initially bare landscape, with specific application to Spartina anglica, a pioneer of intertidal flats. Spartina patches were placed in a large-scale flow facility of 16 × 26 m, simulating the growth of two vegetation patches by increasing the patch diameter (D = 1-3 m) and decreasing the interpatch distance (d = 2.3-0 m). We quantified that the amount of flow acceleration next to vegetation patches, and the distance from the patch where maximum flow acceleration occurs, increases with increasing patch size. In between the patches, the accelerated flow pattern started to interact as soon as D/d ≥ 0.43-0.67. As the patches grew further, the flow acceleration increased until D/d ≥ 6.67-10, from which the flow acceleration between the patches was suppressed, and the two patches started to act as one. These findings are in accordance with theory on flow around and between nonpermeable structures; however, the threshold D/d values found here for permeable vegetation patches are higher than those for nonpermeable structures. The reported flow interactions with dynamic vegetation patches will be essential to further understanding of the larger-scale biogeomorphic evolution of landscapes formed by flowing water, such as tidal flats, floodplain rivers, and hillslopes.

Abstract. Many ecosystems are greatly affected by ecosystem engineering, such as coastal salt marshes, where macrophytes trap sediment by reducing hydrodynamic energy. Nevertheless, little is known about the costs and benefits that are imposed on engineering species by the traits that underlie their ecosystem engineering capacity. We addressed this topic by comparing ecosystem engineering efficiency defined as the benefit-cost ratio per unit of biomass investment for two species from the intertidal habitat: the stiff grass Spartina anglica and the flexible grass Puccinellia maritima. These species were selected for their ability to modify their habitat by trapping large quantities of sediment despite their contrasting growth form. On a biomass basis, dissipation of hydrodynamic energy from waves (a proxy for benefits associated with ecosystem engineering capability as it relates to the sediment trapping capability) was strikingly similar for both salt marsh species, indicating that both species are equally effective in modifying their habitat. The drag forces per unit biomass (a proxy for costs associated with ecosystem engineering ability as it relates to the requirements on tissue construction and shoot anchoring to prevent breaking and/or washing away) were slightly higher in the species with flexible shoots. As a result, stiff Spartina vegetation had slightly higher ecosystem engineering efficiency, due to lower engineering costs rather than to a higher engineering effect. Thus, Spartina is a slightly more efficient rather than a more effective ecosystem engineer. Ecosystem engineering efficiency was found to be a species-specific characteristic, independent of vegetation density and relatively constant in space. Analyzing ecosystem engineering by quantifying trade-offs offers a useful way toward developing a better understanding of different engineering strategies.