Population dynamics of the endangered shrub Myricaria germanica in a regulated Alpine river is influenced by active channel width and distance to check dams

Despite its high conservation and indicator value, there is a lack of information on the long-term dynamics of Myricaria germanica (L.) Desv. (German tamarisk), and of factors affecting its population density and viability. Here we studied the dynamics of adult and juvenile M. germanica at 19 sites during a seven year period (2009-2015) along a 30 km long stretch of an Italian Eastern Alpine river, by assessing a set of anthropogenic and geomorphological factors. Adult (239-571 individuals) and juvenile (62-292 individuals) population numbers showed no significant difference between years, and a remarkably uneven distribution among sites. Yet, several remnant populations have declined or disappeared in recent years and a few populations have increased. We found a positive effect of the width of the active channel on adult and juveniles plants. This means that anthropogenic river narrowing is a leading cause of the decline of the species along river banks. For juveniles, abundance also decreased with distance from downstream check dams. We conclude that in the regulated river studied, the conservation of M. germanica appears to be possible not only as a result of natural flow dynamics, but also at an artificial sediment storage area with a semi-natural river dynamic that maintains favourable riverine habitats. Our results are useful in a decision-making framework for the conservation of M. germanica, for river restoration and for flood protection in alpine valleys.