Flow interaction with dynamic vegetation patches: Implications for biogeomorphic evolution of a tidal landscape

Abstract

[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.

Multiple Stable States and Catastrophic Shifts in Coastal Wetlands: Progress, Challenges, and Opportunities in Validating Theory Using Remote Sensing and Other Methods

“…Within patches of pioneer plants (e.g., Spartina anglica tussocks), tidal currents are reduced and sediment accumulates, raising the plant in the tidal range and resulting in a positive feedback on plant growth. At the same time, the tidal currents accelerate and sediment erodes around the vegetation patches, leading to a negative feedback on plant growth between patches ( Figure 3B) [145,148,165,[177][178][179]. Both feedbacks are density-dependent and exhibit threshold behaviors, i.e., feedbacks only appear to start to influence system dynamics once a threshold plant shoot density is exceeded, and the intensity of the feedbacks increases with increasing shoot density [166].…”

“…The self-organized patches may be induced by scale-dependent feedbacks, i.e., a local positive feedback between the ecosystem state and environmental conditions within the patches, and a long-distance negative feedback around or in between the patches [121]. In the coastal zone, this self-organization manifests at the patch scale, e.g., where positive biomass and topographic growth within pioneer vegetation clusters or mudflat diatom patches is juxtaposed with adjacent erosion [145][146][147][148]. It also manifests at the ecosystem scale as the juxtaposition of upland, salt marsh, patchy pioneer salt marsh vegetation, seagrasses, and mudflat in the intertidal zone [149].…”

“…Several experimental studies have attempted to create alternative stable states in laboratory flume microcosms of salt marsh pioneer plants [148,165,166]. Higher stem density reduces surface water velocities and reduces the Reynolds number, promoting net deposition despite some scouring on the upstream side of stems [167][168][169].…”

Patches in a side-by-side configuration: A description of the flow and deposition fields

“…Vandenbruwaene et al (2011) considered the change in flow distribution close to a pair of vegetation patches, to understand under what conditions adjacent patches would merge together, rather than remain separated by a channel. Their velocity measurements were taken adjacent to and in between patches of different diameters (D) and different separation distances (∆).…”

“…As such, recent attention has been focused on the study of finite patches of vegetation, both in the lab and in the field (Cotton et al 2006, Bouma et al 2009, Zong and Nepf 2011. The interaction between neighboring patches has also been considered (Vandenbruwaene et al 2011). Rietkerk and Van de Koppel (2008) explained the process called spatial self-organization, in which large-scale, ordered spatial patterns occur because of feedbacks between small-scale landscape elements.…”