Vegetative dispersal in riparian vegetation: Processes and impacts in braided rivers

Background/Question/Methods

Braided rivers are susceptible to significant ecological change resulting from plant species invasions. The Platte River (Nebraska, USA) is a classic example, having experienced a 75% loss  of pre-development channel area from the interaction between altered hydrology and vegetative encroachment. This occurred in 3 stages: 1) widespread colonization by Populus deltoides (cottonwood) through the early 1960’s, 2) a lag period of ~40 years, during which channel area was stable, and 3) invasion by a European genotype of Phragmites australis, which accelerated channel narrowing. Attempts to reverse channel constriction through chemical control and mechanical removal of Phragmites have been hindered by repeated reinvasion, though the causes of this are poorly understood. We propose that the dispersal of vegetative propagules by Phragmites plays a substantial role in the reinvasion process, and that the detachment of vegetative propagules may be promoted by some management actions. Our work seeks to determine the mechanisms that govern vegetative dispersal by Phragmites and the physical and biological variables that may be used to predict its rate in the stages of propagule detachment, transport, stranding, and establishment.

Results/Conclusions

Here we report results for transport, stranding, and establishment. Propagule transport by Phragmites  was measured using in-channel drift nets. These data indicate a positive influence by upstream Phragmites abundance and local discharge on propagule transport (p<0.001, r^2=0.55).  Stranding patterns were examined through channel surveys and artificial releases of propagules in an experimentally flooded study reach. Results show that propagule stranding patterns are dominated by the influence of discharge, which is inversely proportional to propagule stranding frequency (p<0.001). We also find evidence that propagule morphology and the roughness of stranding sites contributes significantly to the spatial distribution of stranded propagules. Propagule establishment was investigated in a 2-step controlled experiment. Four seasonally-harvested groups of propagules from Platte Phagmites populations were stored in temperature-controlled water tanks for time periods ranging from 0 to 30 days, followed by a 1 year grow-out period. Our results show that while establishment rates for propagules experiencing 0 days in simulated dispersal are >60%, establishment decreases rapidly with time between propagule detachment and stranding. Though seasonal variation in establishment probability is not significant, early-season propagules produced significantly more biomass than late season propagules. These studies provide a robust foundation for an enhanced predictive model of vegetative dispersal and riparian plant dynamics, with applications in the prevention of reinvasion and the design of effective invasive plant management strategies.