Sand Dune Ecology
Sand dune ecology
Dunes are mounds of sand that have been piled by the action of winds. The sand is usually composed of bits of minerals that have been eroded from rocks, picked up by water or winds, and then re-deposited somewhere else. Typically, the sand is deposited behind some object that is a barrier to the movement of air currents, which causes the windspeed to slow suddenly so that the load of sand particles can no longer be contained against the force of gravity, and it falls to the ground. If there is a suitable source of sand, dunes may deposit along the edges of oceans, large lakes, rivers, and even in inland locations.
The mineralogical composition of sand in dunes varies greatly from place to place. The most common minerals are quartzitic (this is a silica sand), but other minerals may also be mixed in, and they may dominate the composition of the sand in some places. The White Sands National Monument in New Mexico, for example, is characterized by a sand of gypsum (calcium sulfate). The grain size of sands varies depending on the strength of the winds that delivered the sand to the location where the dune has formed—the greater the typical windspeed, the larger the sand grains can be.
Once deposited to the surface, unconsolidated sand tends to be moved about and shifted by the winds. In fact, dunes tend to "migrate" in the direction of the prevailing winds, with sand being picked up at the windward side of the dune, and deposited on the leeward side. Because of the mechanical instability of their surface young dunes initially provide rather precarious habitats for the establishment of vegetation. However, if plants do manage to establish a foothold on a dune, the progressive development of vegetation over time will help to stabilize the surface, allowing an ecosystem to develop through the process of succession .
Some of the most extensive dune systems in the world occur in deserts, including parts of the Sahara Desert of northern Africa, the Gobi Desert of eastern Asia, and the Great Western Desert of the United States and Mexico. Because of the aridity of those environments only a very limited amount of ecological development is possible. In places with a more moderate climate , however, sand dunes can be colonized by vegetation and succession can proceed to the development of tall, mixed-species forests. This type of succession is typical of many dune systems along the coasts of the oceans and many large lakes and rivers of North America.
Sandy soils are generally free-draining, so that even in regions with abundant rainfall sand-dune habitats can be quite arid . The dry soil conditions are made worse by the frequently windy conditions in dune habitats, a factor that speeds the evaporation of water from plant foliage and the soil surface. Droughty conditions are especially important on the higher parts of dunes; in low places the water table may come to the surface, creating locally moist or wet conditions, which are sometimes referred to as dune slacks.
In addition, most sands contain few nutrients, so they provide rather infertile conditions for plant growth. Sandy soils are also lacking in organic matter, which prevents them from holding much water or nutrients for later uptake by plants. In fact, some sand-dune habitats can be referred to as ombrotrophic, or "fed from the clouds," which means that atmospheric deposition is the principal source of nutrient inputs in support of plant productivity. In addition, the low levels of calcium concentrations make sandy soils highly vulnerable to acidification , a condition that can be stressful to some species of plants.
Vegetation of sand dunes
The species of plants that grow on sand dunes vary depending on the climate, the stability of the dunes, soil chemistry, and biogeography . Dunes of the deserts of southern California, southwestern Arizona, and nearby Mexico are sparsely vegetated with a mixture of perennial grasses, such as Pleuraphis rigida, perennial herbs such as the sand sagebrush (Artemisia filifolia ), and species of shrubs, including ephedra (Ephedra trifurca ), daleas (Dalea spp.), Colorado desert buckwheat (Eriogonum deserticola ), and mesquite (Propsis spp. ). During occasional rainy periods these sandy deserts can bloom prolifically with showy annual herbs, such as species of Allionia, Boerhavia, Euphorbia, Oenothera, and Pectis. Areas of gypsum-rich sand have other species that are specific to that soil type, including yucca (Yucca elata ), ephedra (Ephedra torreyana ), onion blanket (Gaillardia multiceps ), and grasses such as Bouteloua breviseta.
In more humid climatic regimes of North America, younger sand-dune systems are initially colonized by species of dune-grasses, such as American beach grass (Ammophila breviligulata ), sea oats (Uniola paniculata ), sand rye (Elymus mollis ), and beach grass (Calamovilfa longifolia ). As the dunes stabilize, other species invade the community. These may form a prairie with interspersed shrub- and tree-dominated nuclei, which may eventually coalesce to form continuous forest (see the next section on dune succession). Some dune-rich areas of the east coast of the United States support forests of pines (Pinus spp.), oaks (Quercus spp.), and magnolia (Magnolia virginiana ). Older dunes in the Great Lakes region tend to be covered by mixed stands of oaks, pines, and tulip-tree (Liriodendron tulipifera ). The inland sand hills of northern Alberta are commonly occupied by stands of jack pine (Pinus banksiana ).
Primary succession on sand dunes
Succession on recently formed sand dunes is a type of primary succession. This is because the sandy substrate that is newly exposed for ecological development has no preexisting biological capability to initiate the succession. Succession can not begin until microbes , plants, and animals have invaded the site from existing ecosystems somewhere else. (Secondary succession occurs after a disturbance, but one that has allowed some of the original biota to survive, so they can play a prominent role in ecological recovery.)
Some classic studies of primary succession have involved sand-dune ecosystems. In North America, the most famous studies were conducted on systems of dune ridges at several places on Lake Michigan and Lake Huron. Dune ridges are actively developing in those places to this day. This occurs as sand is piled up onto the beach by wave action, to be moved inland by the prevailing on-shore winds and mostly deposited on the dune closest to the edge of the lake, on its leeward, or building side. This process of dune-building is an ancient phenomenon, and it has been actively occurring in the region since deglaciation, more than twelve thousand years ago.
The dune building is, however, superimposed upon another geological process—the slow rebounding in elevation of the regional ground surface, a phenomenon that is known as isostasy. This slow, uplifting process is still occurring in response to the long-ago melting of the continental glaciers that once covered virtually all of northern North America; when the glaciers were present, their enormous weight actually depressed crustal bedrocks beneath them into Earth's plastic mantle. The surface has been rebounding since deglaciation released the crust from the weight of the gigantic glaciers, at a fairly steady rate of about 1 yd (1 m) per century. Because of isostatic rebound, ancient shorelines of the Great Lakes are now higher in elevation than they used to be, and in some flatter areas well-defined dune systems formed on ancient shores can be found miles inland of the present shoreline of the lakes. These circumstances mean that dunes located further inland are older than dunes occurring closer to the modern lake shores. Because the ages of the various dunes can be dated, ecologists are able to study a series of dunes of various age and their associated communities. This so-called chronosequence technique allows the ecologists to reconstruct the basic elements of the successional process.
The first plants to colonize the embryonic dunes are annual and biennial plants of shoreline habitats, such as the American searocket (Cakile edentula ), Russian thistle (Salsola kali ), orache (Atriplex patula ), and a small euphorb (Eurphorbia polygonifolia ). Although buffeted by severe weather, these plants help in the initial stages of dune stabilization. The next invaders of the site are several species of dune grasses, particularly Ammophila breviligulata and Calamovilfa longifolia. The extensive roots and rhizomes of these grasses are important in binding the sandy substrate, and in stabilizing the lakeward dune ridges and allowing them to grow larger. Once these grasses take hold, some other foredune plants can establish and begin to grow, including the beach pea (Lathyrus maritimus ) and evening primrose (Oenothera biennis ). With time, a tall grass prairie develops, dominated by various species of grasses and broad-leaved herbs. The prairie becomes invaded by shade-intolerant shrub and tree species, which form forest nuclei. Eventually a climax forest develops that is dominated by oaks, pines, and tulip-tree.
[Bill Freedman Ph.D. ]
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