Marginal plants, particularly trees, play a crucial role in sustaining the biodiversity of Europe's big river systems, according to a workshop organised by the European Science Foundation (ESF).
This finding provides important clues for protecting Europe's rivers against human development and climate change, which are tampering with existing ecosystems and changing the physical and biological forces acting upon them.
Aquatic and riparian (growing along the banks and on islands) plants are critical for building and sustaining habitats for colonisation by other species and in the chemical and biochemical processes that keep rivers and their ecosystems healthy.
This is according to Professor Angela Gurnell, convenor of the ESF workshop and director of the Centre for Environmental Assessment, Management and Policy at King's College, London.
Gurnell described some plant species as 'ecosystem engineers', marshalling habitat development and maintenance.
Ecosystem engineering by plants operates at different spatial scales and in different ways along rivers from source to mouth.
The vegetation is part of the habitat it supports and so is vulnerable to the same forces, with the potential for destroying whole ecosystems when certain thresholds are breached, for example as a result of a slight change in climate or river flows.
Gurnell said: 'Vegetation - physical process interactions are highly complex and are subject to distinct thresholds across which massive shifts in system-conditions can occur.
'Threshold crossing can be driven by both physical and biological processes and is particularly susceptible to changes in climate, river flow and channel management.' The ESF workshop focused on Alpine systems because most of Europe's largest rivers, including the Rhine, Rhone and Danube, have their source in the Alps.
Alpine rivers receive a significant part of their flow from snow and ice melt and so are sensitive to climate change, but these rivers also embrace ecosystems and conditions that are found in many other European rivers.
The ESF workshop took into account vegetation's role as a guardian of habitats and in modulating water flow and sediment movements.
A full understanding of river habitats therefore requires these effects to be incorporated into models and to predict response to forcing factors such as climate change.
Gurnell added: 'Complex river channel patterns, including a wide variety of vegetated and unvegetated landforms, induce complex flow patterns at the surface and subsurface, driving a range of hydraulic 'patches' which change their hydraulic properties and also connect and disconnect at different flow stages.
'It is crucial to develop models that represent this hydraulic patchiness and its dynamics under changing river levels, whether through detailed numerical approaches or more aggregated statistical approaches, because these make it possible to define the range of hydraulic conditions available to aquatic organisms within different river settings.' Rivers are connected systems, because water, sediment and organisms move between upstream and downstream reaches and because the faster-flowing and deeper middle of the river is linked to the edges where the water may move quite slowly, as well as to flood plains during flood events.
An important aspect of river modelling therefore lies in defining the major associations between physical patterns of flow, sediment and landforms, how organisms and ecosystems relate to them and how both may change when threshold conditions are reached.
Achieving this relies on synergy between numerical models and experiments or observations in the laboratory and in the field.
A major objective is to apply this work to develop tools that can help identify the best ways of managing rivers.
Gurnell added: 'Colleagues in mainland Europe have been developing ideas of channel-widening with managers, whereby the river is given more space to adjust its morphology (structure) in a dynamic way within reaches where space can be made available for this.' The idea is to reconnect river ecosystems with the banks and even floodplains in cases where space is available, but past management has severed such links.
If sustainable benefits to the river ecosystem are to be achieved, it is also important that river flows can sustain the widening and that this process is applied at different sites along rivers to maintain upstream to downstream connections between affected sites.
The ESF workshop, 'Linkages and Feedbacks in Highly Dynamic Alpine Fluvial Systems' was held in Cornino, Italy, 30 June - July 2008.
