The establishment of riparian pioneer vegetation on river bedforms depends on how seedling roots develop and anchor efficiently in the low-cohesive sediment in relation to morphodynamic processes. After germination or vegetative reproduction on river bars or islands, juvenile plants are often exposed to mortality by uprooting caused by floods. We distinguish two main types of root erosion by flow. As Type I root erosion, we defined a flow induced drag mechanism,which causes a nearly instantaneous uprooting of mainly very young vegetation with not fully developed root systems by pullout drag exceeding root resistance. Type II root erosion arises as a combination of bedform erosion resulting in a decreased anchoring resistance of the roots and subsequent Type I uprooting. This second type applies to later stages of root development and is a delayed process induced by erosion of morphodynamic origin (Edmaier et al. 2011). We conducted laboratory experiments in order to test the validity of the two uprooting mechanisms. First, we performed static pullout experiments with 1550 seedlings of Avena sativa and Medicago sativa grown in low-cohesive sediment to quantify the distribution of anchorage forces of seedlings for different sediment size and moisture conditions as well as for varying root structure. We measured uprooting force and root parameters. While the maximum uprooting force was found to increase linearly with total root length and be mainly dependent on the length of the main root, uprooting work followed a power law and has to be related to the whole root system. Thus, for young plants, secondary roots are responsible for the ability to withstand environmental disturbances in terms of duration rather than magnitude (Edmaier et al. in press). This distinction between primary and secondary roots can be of crucial importance for seedlings of riparian species germinating on river bars and islands where floods are a main cause of mortality. In addition we tested the root strength of Avena sativa seedlings and compared root breaking and pullout forces.We found that increasing sediment saturation and sediment size both favour root pullout.
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