Initial Recruitment Mechanism of Riparian
Vegetation onto Bare Bar in Sand Bed Riverby

ZHOU Yuexia
Department of Civil and Environmental Engineering
Nagoya University

Supervised byProfessor TODA Yuji

Submitted in partial fulfillment of the requirements for the degree ofDOCTOR OF ENGINEERING

September 2019

ABSTRACT
It is important to identify the recruitment zone of riparian vegetation and the mechanism ofvegetation recruitment since the recruitment of riparian vegetation may promote vegetation(forest) expansion, which can reduce the flood passage capacity and alter ecological balances.
This dissertation focuses on the initial recruitment of riparian vegetation onto bare bar.
Three processes of the phenology of initial recruitment of vegetation, i.e., seed dispersal, seedgermination and seedling settlement, were considered in this study. The mechanism of initial
recruitment of riparian vegetation was clarified by a case-studied river, Suzuka River, which
is a sand bed river and located at Mie prefecture, Japan. The dissertation consists of four mainparts as following.
The distribution of accumulated seeds and the methods of seed dispersal were studied first.
Generally, the relative large flood and shoreline area were considered in the previous studies
and they were considered in this study too. As a new proposal of this study, different flow
regimes and ground surface undulation such as dunes at the upland area of bare bar were also
considered. The field investigation of accumulated seeds distribution in the upper soil layer ofthe bare bar under different flow regimes was conducted first. Then the possible influencing
factors on seeds distribution and the possible methods of seed dispersal were analyzed. Theinvestigation results showed that moderate floods were the most favorable condition for seeddispersal and seed accumulation. Hydrochory and wind dispersal seem to be the dominant
methods of seed dispersal to the shoreline and the upland area of bare bar, including flat areaand dune of bare bar, respectively.
Second, the characteristic of the distribution of initial recruitment, such as initial
recruitment zone location and coverage rate of vegetation, was studied. The land coverIcondition and river morphology were investigated by using UAV monitoring method. The
distribution characteristics of initial recruitment zone and coverage rate of vegetation wereanalyzed by referring the high resolution of the UAV field survey results in ArcGIS. The field
investigation results showed that the initial recruitment zone concentrated mainly along theshoreline, which has close relationship with the relative elevation, and the downstream side ofdune of upland area of bare bar, which was a new discovery in this research. The coveragerate of riparian vegetation presented the decreasing trend from the internal boundary to theexternal boundary of the initial recruitment zone along the shoreline.
Third, the influencing factors on the distribution of the initial recruitment of riparianvegetation were analyzed by referring the field survey data and 2-D flow simulation results.
The analyzed results showed that the physical environment of the bare bar, such as watercontent, was more significant than the seed density for the initial recruitment of riparianvegetation. The greater surface roughness, e.g., vegetation litter and gravel on river bed
surface, may promote the initial recruitment of vegetation more. The location of the initial
recruitment zone was determined by the annual maximum flood and the inundation frequency
during the seed dispersal period.
Finally,aninitialrecruitment

modelwasproposedfromtheviewpointofhydro-morphology. The established recruitment model was calibrated by using the fieldsurvey results, and then it was validated in the object river, Suzuka River. The internal andexternal boundary of the initial recruitment zone was determined by considering theinundation frequency and flood magnitude during the seed dispersal period, respectively. The
coverage rate of vegetation was calculated as the function of inundation frequency and thesouring force of spring flood. The simulation results showed that the proposed initial
recruitment model can well predict the initial recruitment zone and approximately represent
the coverage rate of vegetation, and this means the inundation frequency and spring flood aredominant factors for the initial recruitment of riparian vegetation.
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