Influence of Natural Hydromorphological Dynamics on Biota and Ecosystem Function, Part 1 (Riparian and Aquatic vegetation)

WP2 focuses on hydromorphological and ecological processes and interactions within river systems with a particular emphasis on naturally functioning systems. D2.1 proposes a hierarchical framework to support river managers in exploring the causes of river management problems and devising sustainable solutions. D2.2 builds on this framework by exploring published research and available data sets to more formally encompass the biota. This report (Part 1 of Deliverable 2.2) is concerned with riparian and aquatic vegetation. It encompasses the following sections: natural vegetation and hydromorphological of European rivers; a conceptual model of vegetation-hydromorphology interactions; application of the conceptual model to European rivers.

Summary of Deliverable 2.2 Part 1.

Research Objective.Riparian vegetation is not included as a biological quality element in the Water Framework Directive, and yet research conducted over the last 20 years has clearly demonstrated that riparian vegetation has a fundamental influence on the hydromorphology of rivers and their floodplains, with a geographically more widespread impact than aquatic vegetation. This report assembles evidence from published sources and available data sets to demonstrate how vegetation interacts with hydromorphology to constrain numerous aspects of river morphology and dynamics, so providing a vital component of any river management and restoration efforts.

Methods and Results.Chapter 2 proposes a conceptual model of vegetation-hydromorphology interactions (section 2.2) that provides the underpinning for the whole of chapter 3. The literature and available data sets are exploited to place the conceptual model firmly within the context of the broader ecology of riparian and aquatic vegetation (section 2.1), and to present the modelling approaches that are currently available for exploring these vegetation-hydromorphology interactions (section 2.3).

The conceptual model assumes a naturally-functioning river-floodplain system and considers three scales of influence. First, the model considers how regional physical processes place constraints on the species composition of river corridor vegetation, particularly emphasising the biogeographical zone within which the river’s catchment is located. Second, the model considers how vegetation is further constrained by longitudinal, lateral and vertical gradients in hydromorphological processes within the river corridors of a catchment, particularly by gradients of moisture availability and fluvial disturbances. Five zones of vegetation-fluvial process interaction within a river corridor are defined: perennially inundated (zone 1); fluvial disturbance dominated - predominantly coarse sediment erosion and deposition (zone 2); fluvial disturbance dominated - predominantly fine sediment deposition (zone 3); inundation dominated (zone 4); soil moisture regime dominated (zone 5). Third, a critical zone of vegetation-hydromorphology interactions is defined, which bridges zones 1 to 3, and within which vegetation heavily influences the construction of landforms (e.g. river banks, islands) at the interface between the physical-process-dominated areas of the river channel and the vegetation-dominated areas of the surrounding floodplain or hillslopes.

The model is explored in a European context in chapter 3. First riparian and aquatic species of the ‘natural vegetation’ within different biogeographical zones of Europe are assembled. Second, a traits data base is assembled for 459 aquatic and riparian plant species that are found in association with European rivers, and two trait-based typologies are devised reflecting (a) the sediment stabilisation and (b) the sediment accumulation and channel conveyance / blockage potential of the analysed species. This is a major first step in developing methods for interpreting the hydromorphological relevance of native riparian and aquatic plant species across Europe. Lastly, the applicability of the conceptual model to a sample of European rivers is tested in section 3.3. It is applied to rivers located in contrasting biogeographical zones and subject to different human pressures, highlighting for the first time how different plant species and groups act as river ecosystem engineers in different river systems.

Conclusions and Recommendations.This report presents new science concepts and analyses that clearly demonstrate the importance of vegetation as a key physical control of river form and dynamics and a crucial component of river restoration. It shows how interactions between plants and hydromorphology take on different characteristics in different biogeographical settings, leading to different spatial patterns of features and temporal dynamics within zones 1 to 5 of the river corridor, and different styles of landform development within the critical interface between fluvial processes and vegetation in zones 1 to 3. Case studies illustrate how the conceptual model provides a useful multi-scale framework for understanding and interpreting vegetation-hydromorphology interactions and so supporting sustainable river restoration design and management. However, some research gaps need to be filled to permit the work to be translated into a set of simple river management tools:

1.    The example applications of the conceptual model have synthesised pre-existing literature and field observations that were collected for many different scientific or management purpose. These provide a ‘proof of concept’ and a firm basis for recommending that new purpose-designed field research is needed to ensure the robustness and wide applicability of the model.

2.    A thorough review of available modelling tools has demonstrated that the main aspects of plant-hydromorphology interactions have received attention, although many research gaps remain. However, more importantly, most existing models address narrow aspects of these interactions. More integrated modelling approaches are needed to support river and floodplain management.

Research is needed to assemble more comprehensive native riparian and aquatic species  lists for European biogeographical zones from which a larger set of informative species traits can extend plant trait-based hydromorphological modelling.

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