Is ‘re-mobilisation’ nature restoration or nature destruction? A commentary

Coastal dunes are experiencing increases in vegetation cover and reduced mobility levels in many sites around the world. Ecology-led approaches to coastal dune management perceive this change as ‘undesirable’ because the increase in plant cover leads to a reduction in partially vegetated to bare sand habitats and the species depending on them. This has generated a shift in the management paradigm where the objective is to revert this trend by intervening in the landscape, with actions ranging from re-introducing grazing and mowing, to mechanical removal of dune form and vegetation (dune ‘rejuvenation’). In some cases, such as many coastal dunes in Britain, this has also led to low controls on visitor pressure and allowing/promoting human trampling as a ‘natural’ way to free up areas of bare sand. This commentary critically analyses the main principles (and terminology) underlying this relatively recent shift in management paradigm, and questions assumptions such as ‘bare sand is good’ and/or ‘mobility is natural’ in the context of dune evolutionary cycles and responses to abiotic and biotic drivers. We review the limitations and dangers of this approach and argue that it is not sustainable given the current climatic and environmental conditions, and that it can increase the risk of coastal erosion and force dune systems to deviate from adapting and changing to direct/indirect drivers. Finally, we present the benefits of a management approach that focuses on minimizing human impacts so that natural processes continue to occur.

Geomorphology

Geomorphology is the study of form and structure of sand dunes. Dunes are found in three types of landscapes: sea coasts and lakeshores, river valleys, and arid regions. Coastal dunes are formed along coasts in areas above the high water mark of sandy beaches. They occur in both the northern and southern hemi sphere from the Arctic and Antarctic to the equator, and in arid and semi-arid regions. They are very common in temperate climates but are less frequent in tropical and subtropical coasts. Dunes are also common around river mouths where the sand carried in water is deposited (Carter et al. 1990b). During floods rivers overflow their banks and deposit sand in river valleys that is subsequently dried by wind and shaped into dunes. In dry regions with less than 200 mm of precipitation per year, the weathering of sandstone and other rocks produce sand that is subject to mass movement by wind because of sparsity of vegetation. There are many similarities in processes and patterns of dune form and structure among these three systems, however each location has its own unique features. In this chapter the emphasis will be on the geomorphology of dune systems along the coasts of oceans and lakes. Coastal geomorphologists have been attempting to classify the coastal land forms but they defy a simple classification because of tremendous variability in plant taxa, sand texture, wind velocity, climate, sand supply, coastal wave energy and biotic influences including human impact. According to Carter et al. (1990b) the great variety of coastal land forms around the world is primarily related to sediment availability, climate, wave energy, wind regime and types of vegetation. Classification based on these criteria would be more useful in distinguishing between shoreline dune forms than the use of subjective terms—for example white, grey or yellow dunes—sometimes employed by plant ecologists (Tansley 1953). Cowles (1899) said ´a dune complex is a restless maze´ because the great topographic diversity depends on changes in the dune terrain from day to day, month to month, season to season and year to year.

Effects of sand supply on the morphodynamics and stratigraphy of active parabolic dunes, Bigstick Sand Hills, southwestern SaskatchewanGeological Survey of Canada Contribution 20060654.

Sand supply is a major controlling factor on parabolic dune form and stratigraphy in inland settings. In this study, aerial photographs, ground-penetrating radar (GPR), and stratigraphic analysis document the morphodynamics of an individual and compound parabolic dune in the Bigstick Sand Hills, southwestern Saskatchewan. Migration rates for the last 60 years are comparable, although the profile morphologies differ, with the individual dune having a more aerodynamic form. Stratigraphic facies are also similar in both dune types, but the overall internal architecture imaged by GPR differs considerably. Configurations of cross-strata parallel to the downwind axis represent dominant foreset development and lee-slope slipface advance of the individual dune, and impeded slipface development of the compound dune. Stratigraphy transverse to the downwind axis represents radial deposition and foreset development at the individual dune, and vertical accumulation at the compound dune. The overall difference in parabolic dune form and stratigraphy is attributed to variations in sand supply, which determine vegetation development and sedimentation processes along the crest and lee slope. Sand supplied from active blowouts upwind of the individual dune inhibits vegetation colonization on the dune, whereas an absence of sand supply upwind of the compound dune leads to high levels of vegetation cover on the dune. Once supply drops below a threshold level, vegetation cover increases, causing sediment deposition and vertical accretion, and ultimately changing dune form. Overall, this study demonstrates that local sand supply and feedback processes are critical to understanding dune development in vegetated, inland settings.