Resistance to pull-out of Chilean riverine species: evidence from laboratory experiments.
Vegetation growing in river bars and banks determines the morphological processes in river channels. The presence and dynamics of riverine vegetation depend on the magnitude and frequency of floods able to change morphology and uproot plants, and on the plant's resistance to pull-out. A series of pull-out experiments were carried out with the objective to determine the resistance of riverine vegetation to be uprooted and its effects on river-bed dynamics. Nine riverine species were used for the experiment: seven of them are native from Chile, and the other two are exotic and invasive in Chilean environments. A total of 200 specimens were transplanted in a substrate simulating a bar of a gravel-bed river, and after 7 months they were individually pulled-out, and the force and time needed to uproot the plant quantified. Data were analysed by means of generalised linear models (GLM) and linear regressions, and force-time curves were interpreted and related to the root morphology of the species. GLM results showed that resistance values were dependent on root and plant physical characteristics, and by the factor species. Among the species studied, the three species showing higher resistance values were native, while the less resistant was an exotic invasive species. The time needed to uproot the individuals did not seem to be influenced by plant parameters nor by the factor species. Root architecture seemed to have an effect on plant pull-out resistance and uprooting rapidity, being heart-shaped roots the ones that registered higher resistance values. Despite this, when incorporating flow strength on the findings, the exotic invasive ones seemed to be some of the more resistant to uprooting. The findings of this paper contribute to the better understanding of river vegetational and sedimentary dynamics, and are useful for parameterising the modelling of fluvial landscapes evolution and for the design of river restoration projects.