Soil and vegetation development of coastal dunes

Where coastal dunes receive sufficient rainfall to support vegetation and are free draining, they commonly exhibit the soil and vegetation features linked to environmental gradients from foredune to hind dune. The incipient foredune supports low grasses, herbs and succulents, and some low shrubs are added to this assemblage on the foredune. Larger shrubs are able to flourish in the swale, protected by the foredune, although they may be replaced by grasses and dwarf shrub forms on the next seaward facing slope. Further landward across the dunes, plant forms usually become larger and more densely spaced: thus grasses with occasional shrubs on the foredune are replaced by dense low shrubland, then tall shrubland and finally dense, low, eucalypt woodland on some hinddunes. This landward change of structure may also be accompanied by increasing species diversity. For example, at Waitpinga Dunes south of Adelaide, the simplicity of half a dozen or so species in one kind of niche on the foredune contrasts with the complexity of more than 50 species within a variety of niches a hundred metres landward.

The most precarious environment in the dunes is at the incipient foredune. Here plants must survive burial and blasting by blown sand, high salt levels from blown spray, periodic high surface sand temperatures, periods of drought, and low to zero nitrogen and humus levels. Sometimes the drift or strand line of seaweed or seagrass may provide shelter, a source of nutrients and promote water retention in the underlying sand, giving pioneer plants a vital toehold. Plants such as Sea Rocket (Cakile maritima), Spinifex (Spinifex hirsutus) and Pigface (Carpobrotus glaucescens) have the capacity to rapidly establish themselves on the backshore: in the struggle to stabilise the wind-blown sand these plants are the front-line troops, the specialists. They cope with the harsh location through a variety of adaptations: high seed production (Sea Rocket), the development of an extensive and dense root network to overcome the water and nutrient deficiency (Spinifex), succulence (Pigface), and leaf-curling to overcome high evaporation rates. All these plants are salt tolerant, although not halophytic.

Behind the foredune the environmental stresses are reduced. There is some protection against wind blasting for low shrubs such as the Coast Daisy-bush (Olearia axillaris), and a variety of other plants obtain a footing on the partially stabilised sand, such as the Knobby Club Rush (Isolepis nodosa), and in damper hollows rushes and sedges (e.g. Lepidosperma gladiatum) may be found. Only slightly less salt tolerant than the Coast Daisy-bush is the Coast Wattle (Acacia longifolia var. sophorae) and the Native Currant-bush or Coast Beard-heath (Leucopogon parviflorus).

Further landward, the environmental gradients continue to change, and biodiversity increases as the environmental stress reduces. Biomass increases and more humus is found in the soil, wind speeds and salt levels decline, and nitrogen-fixing bacteria enrich the soil. Where these older dunes are quartzose and free-draining, podsol development is advanced, often with a marked pan or humate layer.

 
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