Floods for riparian biodiversity – Flood pulses to maintain riparian biodiversity in regulated rivers
financed by Swedish centre for sustainable hydropower
One of the main effects of river regulation is that the frequency and magnitude of flood pulses are reduced, leading to invasion of terrestrial species into riparian zones. Specialised riparian species are replaced by widespread species, leading to loss of species, vegetation types and ecosystem functions. A remedy to this problem is the implementation of flood pulses mimicking natural floods. In this project we plan to find out how long and how often banks need to be flooded to keep invading spruce plants away and encourage the establishment of deciduous tree species such as oak.
One of the main effects of river regulation world-wide is that following onset of regulation, the frequency and magnitude of flood pulses are reduced, leading to invasion of terrestrial species into riparian zones. This means that specialised riparian species tolerant to flooding are replaced by widespread species, leading to loss of species, vegetation types and ecosystem functions. A remedy to this problem is the implementation of flood pulses mimicking natural floods, but the magnitude and frequency of inundation needed is poorly understood.
This problem is acute along the lower Dalälven in Sweden, where unique riparian deciduous hardwood forests with oaks are being invaded and replaced by Norway spruce following regulation. To date, spruce invasion is kept at bay by logging and girdling, a practice that does not bring the multiple benefits of flood pulses (disturbance, seeding, soil fertilization etc.) in addition to spruce control. Annual floods lasting at least a month have been suggested as a rehabilitation measure in Dalälven, but this would be in conflict with hydropower production. We argue that the frequency and duration of inundation needed to stop invasion of spruce and other terrestrial species and to benefit riparian plant species are poorly known. However, in free-flowing rivers such as Vindelälven, species-rich riparian vegetation is maintained by inundation considerably shorter than those suggested for Dalälven.
In this project we aim to identify the magnitude and duration of floods needed to avoid invasion of terrestrial plant species into riparian zones in regulated rivers, helping to develop environmental flow measures with relevance for many regulated rivers in Sweden and world-wide. Specifically, this knowledge will be used to project the inundation duration needed to maintain riparian forests with deciduous hardwood trees along the regulated Dalälven river. This is done by performing (1) a laboratory experiment, (2) a field transplant experiment, and (3) a survey of tree seedling occurrence in relation to inundation duration. This information is used to (4) project the area of rehabilitated riparian forest along Dalälven in the Bredforsen reach at different magnitudes and frequencies of flow, based on a hydrological model of the reach. The lab and field experiments along with the survey give complementary information on inundation tolerance under different conditions, used as input to the models projecting the flows and water-levels needed to provide environmental benefits.
1. Quantify the magnitude and duration of flows needed to maintain riparian forest communities and stop invasion of terrestrial plants, notably Norway spruce.
2. Establish the tolerance of Norway spruce seedlings to inundation and submergence depending on plant size, oxygenation, soil type, and biotic interactions with other species in a controlled laboratory and a field transplantation experiment.
3. Estimate the relationship between inundation duration and frequency of occurrence of seedlings of tree species, including Norway spruce and deciduous hardwood species such as oaks in a survey of riparian zones.
4. Project the area of riparian vegetation belts as a function of magnitude and duration of inundation (attained by flow release or a combination of rock ramps and flow release) along a section of Dalälven, providing a basis for environmental flow schemes to maintain riparian plant diversity.
5. Provide recommendations on the duration and frequency of inundation (high water levels and flows) needed to maintain and restore riparian forests dominated by oaks and other deciduous hardwood trees along regulated rivers.