Engineers or opportunists? Examining the role that Australian native plants play in constructing fluvial landforms in subtropical river corridors

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American Geophysical Union (AGU)
Sediment deposition around even-aged stands of riparian trees has been observed in many fluvial systems leading to the theory that most rheophytic species have biologically evolved to trap suspended sediment (SS), and construct fluvial landforms. That is, they are ecosystem engineers. However, few studies have attempted to show the causal relationship between vegetation by calculating the hydraulic impact of vegetation on flow and SS deposition. Furthermore, many of these examples of ecosystem engineering occur at locations in the river where SS deposition is already expected. Here we assess the question: does vegetation really influence deposition to a great extent, or is it more likely exploiting an area where deposition would have occurred anyway? We assess the effect of riparian vegetation on SS deposition in South East Queensland, Australia, where recent floods have scoured out much of the inner channel, and triggered recruitment of even aged cohorts of Casuarina cunninghamiana and Meleleuca bracteata. Using hydraulic theory from recent flume studies we explore whether these vegetation patches can induce deposition and creating new fine sediment deposits in the river corridor? We used Terrestrial LiDAR scanning (TLS) to estimate the physical effect of even-aged vegetation patches on SS deposition. In addition, we measured diameter at breast height and stem densities at even aged stands on the Brisbane, North Pine, and Mary River corridors in SEQld, while constraining patch ages via historic aerial imagery, and unconventional dendrochronology. By applying cutting edge hydraulic theory to our TLS data, we generate a process based estimate of the sediment trapping capability of these vegetation patches as they mature. We find that these patches will only trap fine sediment when channel velocities are less than 0.1 m/s, far lower velocities than experienced through any of the floods, limiting ecosystem engineering potential. These patches must first take advantage of the geomorphology in order to influence it. This methodology can be used elsewhere to determine where the “hotspots for ecosystem engineering” occur in a river corridor, and the relative effectiveness of different species of vegetation in constructing landforms and reducing SS yields. © 2019 American Geophysical Union
Australia, Queensland, Plants, Geomorphology, Sediments, Rivers
Garber, J., Rutherfurd, I., Ghisalberti, M., Zawadzki, A., Hua, Q., & Gadd, P. (2019). Engineers or opportunists? Examining the role that Australian native plants play in constructing fluvial landforms in subtropical river corridors. Paper presented at the AGU Fall Meeting, San Francisco, California, December 9-13, 2019. Retrieved from: