7.7.1 Drought tolerance
Tolerance of drought and the ability to regenerate in disturbed sites are two of the outstanding features of sand dune vegetation. Many of the moss and lichen species, which hold the surface of the sand dunes, have the remarkable property of being able to allow their tissues to dry out without losing viability. After prolonged periods of desiccation (e.g. lying on a herbarium sheet for 70 years!) some moss species begin to resume metabolic activity within 30 minutes of gaining access to water. One of the dangers of desiccation injury, especially when plants are exposed to sunlight, is the generation of highly destructive oxygen free radicals by transfer of energy from excited chlorophyll to oxygen. The dune moss Tortula ruraliformis when desiccated in the light has high concentrations of the antioxidants a-tocopherol and glutathione, which may contribute to its remarkable desiccation tolerance (Seel et al., 1992).
Little thought is given to the needs of sand dune plants for water, probably because most perennial dune flowering plants are either economical with water supplies, as with the sand dune grasses, or else restrict growth to seasons when water stress is not a serious problem. Dunes have a characteristic flora of winter annuals with species such as common whitlow grass (a diminutive member of the cabbage family, Erophila verna) and spring vetch (Vicia lathyroides), which survive the heat and drought of summer as seeds, then germinate in the autumn, grow over winter and flower and seed in spring and early summer. Marram grass (Ammophila arenaria) of dune tops with its hard, in-rolled leaves, appears to be the embodiment of drought resistance. Not only is transpiration reduced to a minimum, but also the deep root systems are able to access water from the lower moist layers in the dunes.
However, water is still an essential resource for dune vegetation. The transpiration needs of dune vegetation can exhaust the rainwater held in the rooting zone in a typical sand dune in four days (Salisbury, 1952). As discussed in Section 3.7.3 'Dew' the elevation of water by capillary action moves water no more than 40 cm above the water table. The average rooting depth of marram grass is in the order of 1-2 metres and the depth of the water table can be 6 metres or more below the surface of a high dune. The dune grasses are dependent therefore for their water supply in dry periods on the upward movement of water vapour, which takes place through internal condensation at night from the relatively warmer water table to the colder upper regions of the dune. Deeper-rooted plants also contribute to the water supply of the upper layers of the sand dune by a phenomenon described as hydraulic lift (Section 3.7.3).
The sand dune grasses Ammophila arenaria and Leymus mollis in common with several tropical grasses have been shown to harbour symbiotic nitrogen-fixing bacteria within their stem and rhizome tissues that may contribute to the nitrogen nutrition of the host plant. Cultivation of these sand dune grasses using surface-sterilized stem and rhizome tissue showed that these species possessed a capacity for acetylene reduction, which indicates an ability to fix atmospheric nitrogen. The stem and rhizome tissues also contained large bacteria populations which when cultured on N-free media revealed the presences of endophytic, diazotrophic bacteria (e.g. Burkholderia sp.). Evidence for a similar nitrogen-fixing association has also been detected in sea oats (Uniola paniculata) and the American beach grass (Ammophila brevigulata). It therefore seems probable that the success of these grasses on nutrient-poor sand is due at least in part to being able to compensate for the lack of nitrogen in sand dunes by fixation of atmospheric nitrogen (Dalton et al., 2004).
A most intriguing aspect of sand dune ecology is the decline in vigour of the main dune-building grasses as the dunes mature. Marram grass (Ammophila arenaria), although it can withstand and thrive with vertical sand accretions rates of up to one metre per annum, nevertheless loses vigour as the sand level stabilizes, just when it might be reasonably assumed that having survived the risks and stresses of colonizing the unstable seaward side of the dunes it might flourish on the more sheltered leeward slopes. This paradoxical loss in vigour in the seemingly more favourable environment has long intrigued ecologists (Moore, 1996) and is currently attracting particular attention in the north and mid-Atlantic coast of the United States where the American marram species (Ammophila brevigulata) is showing extensive dieback. Fresh burial of the stems of this plant have always appeared necessary for its continued vigour and a variety of explanations, including the need for new rhizome bud development and the adverse effects of soil compaction, have been discussed at various times. Both European and American marram grass have been shown to benefit from burial by sterile sand as an escape from pathogens and in particular nematode attack. Recently, however, it has been found that it is not the sterile soil per se which provides the escape from nematode attack, but the facility this sand provides for the development of fungal associations (mycorrhizal connections) which give the plants the vigour necessary to combat nema-tode infections (Little & Maun, 1996). Consequently, periodic dune destruction may be necessary in order to provide the fresh sand that is essential for the maintenance of vigour in marram grass.
Burial is a constant danger for any plant that lives on a mobile soil. One of the most extreme examples is dune migration, which engulfs and annihilates whatever vegetation over which the dunes may pass. Coastal pine forests are engulfed and destroyed by large migrating dunes (Fig. 7.41). They also show a remarkable capacity to recolonize the dunes after the migration has moved on (Fig. 7.42).
More commonly burial is an insidious and less dramatic process; it is not always fatal and is sometimes beneficial for the stability of the ecosystem. The ability of sand dune grasses to re-emerge after a short period of burial is an important factor for dune growth and stability. The different responses of the major grasses has already been referred to (Section 7.2.2) with Ammophila spp. being able to emerge vertically through a metre of sand while Leymus arenaria tends to grow out laterally, and sand couch grass (Elytrigia juncea) emerges obliquely. On the western coasts of the British Isles burial continues to be a risk in the plains that lie inland from the major
dune systems. Where the sand is rich in shell content, a typical herb-rich pasture develops which can be found from Braunton Burrows in North Devon (Willis, 1985) to the Luskentyre Banks (Fig. 7.1) in the Outer Hebrides. Where dunes are rich in shell sand the slacks usually erode to form flat, herb-rich plains sometimes referred to as machair (Scottish Gaelic, a low-lying plain; Fig. 7.43). Since pre-his-toric times the plant communities that develop on the machair have provided fertile grazing and croplands along the Atlantic seaboard of the British Isles. Among the most well known of ancient machair farmers must be St Columba (c. AD 521-597) who with his 12 disciples landed on the Hebridean island of Iona in AD 563; they founded a new monastery and farmed the machair plain, which they referred to as the Campulus occidentalis and where they both pastured their animals and sowed crops (Anderson & Anderson, 1991).
The attraction of these peripheral storm-exposed areas on the western side of the island, as opposed to the more stable and sheltered areas on the east, was due to the combined benefits for field manure of fresh windblown shell sand and plentiful supplies of kelp and tangle (Laminaria digitata and L. hyperborea) washed up on the beaches exposed to the Atlantic storms. The securing of seaware for manuring the light but easily cultivated soils of the machairs appears to have been long practised in the Hebrides. Prayers and rituals have been traditionally carried out at the edge of the sea to ensure a bountiful harvest of kelp for manuring the machair.
The fertility of the machair and the extent of its herb-rich pasture depend on fresh depositions of calcareous sand that is constantly blown across these level plains. Although such deposits are beneficial they also carry the risk of being excessive. For the plants growing on the machair, as on other sand dune systems, burial has both risks and disadvantages particularly for species that do not have substantial reserves in rhizomes and other perennating organs. Many machair species can recover from sand burial if it is not too deep. However, there are frequently periods of active sand deposition that buries plants beyond their limits of recovery, as in the stratification that is often seen in the buried soils horizons when the machair erodes. Experimental investigations of the physiological effects of burial on machair vegetation have shown that they possess an ability to maintain a potential for photosynthesis. They are therefore able to resume photosynthetic activity rapidly on emergence, which is an important adaptation as survival is dependent on being able to replenish carbohydrate reserves before the next burial event (Kent etal, 2005).
Typical machair develops through a growth, stabilization and degeneration cycle over a long period. Continued growth with fresh calcareous sand, blown inland from the dunes, continues until the surface rises so high above the water table that drought becomes a stress, and degeneration ensues. When the soil surface falls far enough for the water table to be available, colonization resumes and the growth cycle starts again (Dickinson & Randall, 1979). This slow renewal cycle has maintained the fertility of this type of pasture for thousands of years. Unfortunately much of the biodiversity of these coastal dune pastures is being lost due to the subsidized improvement of coastal grazings; the natural cycle of erosion and renewal is being suppressed due to the application of fertilizers, herbicides, and electric fencing. Even though in many cases the machair may not have received direct applications of fertilizer, it is sufficient for cattle that have been previously fed on nutrient-rich herbage to be allowed on to the area to transport enough minerals to stimulate the growth of a few rapidly growing species to the eventual exclusion of the less competitive plants.
Machair and dune slacks are highly dependent on reserves of fresh water below the soil surface. Fresh water flows over the denser salt water to provide the dunes and slacks in summer with a buried freshwater resource that unlike water added by sprinklers is protected from surface evaporation. However, all too frequently conflicting interests such as forestry and agriculture, which are intolerant of winter flooding in neighbouring areas, insist on drainage measures, which reduce the water reserves for the summer. When the water table falls to more than a metre below the surface in summer, distinct ecological changes take place in the slacks, with many of the moisture-demanding herbaceous species being replaced by grasses. Thus, in common with the dune system, removal of water from areas to landward can be deleterious, as reduced winter flooding and increased summer drought both contribute to a loss in biodiversity.
In an ecologically ideal situation, dunes are not just a line of sandy, grass-covered hillocks between the coastal road or golf course and the sea, but occur as a series of ridges interspersed by level plains usually described as dune slacks (Fig. 7.44). The word slack (cf. slake, to allay thirst) implies a tendency for these plains to be flooded, particularly when the water table rises in spring. In well-developed coastal systems there can even be a successional series of dune-slack communities. These begin with slacks still exposed to intermittent sea flooding which have a characteristic salt-loving flora with sea milkwort (Glaux maritima; Fig. 7.45) and sea plantain (Plantago maritima). These are replaced by a freshwater, nutrient-poor slack where creeping willow (Salix repens) is usually a dominant feature. As dunes generally block the flow of water to the sea, the slacks furthest from the sea have higher water tables and longer periods of flooding than those near the sea. To landward, the slacks gradually develop into marshes where flooding is of longer duration and a greater supply of nutrients becomes available in the floodwater. In addition to the varied habitats of the different dunes and slacks, there can also develop long bands of alder from seeds that float to the edge of the flood line. Similar bands of birch may also intersperse the dunes and colonize some of the drier slacks.
Despite the fact that all these communities are supported by a uniform oligotrophic (nutrient poor)
sandy soil, it is quite remarkable that different patterns of inundation, desiccation and exposure are sufficient to create an extraordinarily diverse series of habitats which contribute collectively to the overall resilience of the area to disturbance and erosion. In the unmanaged state, trees come and go as natural flooding of slacks together with areas of windblow and fire denude the tree cover. Unfortunately, these boundary zones are frequently neglected, as management plans for nature reserves tend to focus on preserving areas as one particular type of habitat unless the reserve is very large.
Dune slacks are remarkable for the diversity of the plant communities. Unfortunately, in recent years the features which maintained this biodiversity, the oligotrophic soil and the seasonally varying flooding regime, with the summer water table usually little more than a metre below the surface, are disappearing from many dune slacks. Eutrophication of the freshwater input to the slacks is promoting the advance of dominant grasses, and local drainage activities and boreholes for watering golf courses, together with climatic warming, are making summer drought more frequent.
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