Hydropeaking affects germination and establishment of riverbank vegetation

Hydropeaking, defined as frequent and rapid variation in flow in regulated
rivers with hydropower plants over a short period of time, usually
sub-daily to weekly, alters hydraulic parameters such as water levels or
flow velocity and exerts strong impacts on fluvial ecosystems. We
evaluated the effects of hydropeaking on riverbank vegetation,
specifically assessing the germination and establishment of seedlings and
cuttings of plant species representing a variation in traits. We used
seeds and seedlings and cuttings varying in size as phytometers, and
transplanted them to riverbanks both above and below dams used for
hydropower production in northern Sweden, selected to represent a gradient
in hydropeaking intensity, and along a free-flowing reach. We also
analyzed sub-daily water level variables modified by hydropeaking to
identify variables key in explaining the observed vegetation patterns. We
found that plant responses to hydropeaking varied with species, with
flood-intolerant species being the most strongly affected, as early as the
germination stage. In contrast, seeds of flood-tolerant species managed to
germinate and survive the early establishment phase, although strong
erosive processes triggered by hydropeaking eventually caused most of them
to fail. The fate of flood-intolerant species identifies germination as
the most critical life-history stage. The depth and frequency of the
inundation were the leading variables explaining plant responses, while
the duration of shallow inundation explained little of the variation. The
rise and fall rates of water levels were key explaining variation in
germination success. Based on the results, we propose restoration measures
to enhance establishment of riparian plant communities while minimizing
the impact on hydropower electricity production. Given the strong decrease
in the germination of species intolerant to prolonged flooding with
hydropeaking, planting of seedlings, preferably of large sizes, together
with restrictions in the operation of the power plant during the
establishment phase to enhance survival would be the best restoration
option. Given the high probability of plant uprooting with hydropeaking,
bank protection measures have the potential to increase riparian-plant
survival of all species, including flooding tolerant ones.