Associations between the morphology and biomechanical properties of Sparganium erectum: Implications for survival and ecosystem engineering

Aquatic plants are able to alter their morphology under stressful hydrodynamic conditions to improve survival and growth; adjustments can include increases in below ground ‘anchoring’ biomass, or escape strategies such as extending the internodal length of rhizomes. Whilst research has demonstrated how macrophytes respond morphologically to stressful hydraulic conditions, there is little evidence of how these changes, particularly in below ground size and structure, influence the forces required to damage and dislodge aquatic plants. The aim of our study was to further establish the links between mechanical force (i.e. the forces required to uproot plants or cause stem breakage) and plant morphology in the common and widely distributed emergent species Sparganium erectum. Mechanical force was applied using a pulling device that consisted of a winch mounted on a metal frame that sat on the river bank, and from which a cable, with attached load cell, was connected to the stem of a S. erectum plant. We found strong associations between the below ground structures of the plant and the force at which it experiences failure. Below ground plant organs were important in determining the overall resilience of this species, with root length showing the strongest association with its ability to resist uprooting, whilst rhizomes had the longest seasonal influence on the force required to uproot the plant. Biomass measurements of below ground organs revealed rapid increases in root biomass early in the season, in contrast to the rhizome and corm biomass, which peaked at the end of the growth season. We argue that these results are demonstrative of a number of biomechanical and morphological traits that contribute to the pervasiveness of the species, its management challenges, and its function as an ecosystem engineer, whereby it alters the physical structure of river channels by deflecting flow and building large sediment accumulations. These traits include the presence of a mechanical fuse, rapid growth of roots early in the season and reproductive structures at the end of the season, which probably have a reinforcing effect on the surrounding sediment, and the high resistance of the species to uprooting, stem and rhizome breakage.

► Individuals with longer roots and more rhizomes were more resistant to uprooting.
► The force needed to uproot S. erectum far exceeds that of previously studied aquatic species.
► S. erectum has a mechanical fuse; it sacrifices stem biomass under stress.
► Root biomass peaks in spring and increases plant stability.
► Rhizome and corm biomass peak in autumn, potentially enhancing survival and propagation.