STRAND
Strand here is used for the narrow littoral marine zone including beach, foredune, and remaining sandy habitat up to the edge of stabilized dune or inland vegetation. Much of this zone is under the potentially harmful influences of salt spray, shifting and abrasive sand, and, nearest shoreline, severe storm surf and high tides. Strand occurs from tropical to temperate, even arctic, latitudes and along coastlines of continents and islands. At first glance, certain physical properties of these sandy environments are relatively simple, repeated, although not uniformly, from site to site, therefore it is not surprising that the same plant forms, even a few of the same species, appear on beaches around the world.
Physical Properties of Strand.
- Daily and annual solar radiation of the unobstructed beach environment can be relatively high, especially in the tropical, subtropical, and warm temperate zones. Full sun on a cloudless day can yield very high instantaneous values of intercepted light, because white sand has high albedo and, therefore, a substantial amount of solar radiation that strikes the white sand is reflected from the surface. On the other hand, coastal fog and overcast skies may also occur daily or seasonally, so that during much of the daytime leaves may be exposed to lower solar radiation than they would intercept in a cloudless, inland habitat at the same latitude.
- Mean annual air temperature (also annual range) varies greatly along a gradient of tropical to arctic sites, but typically each is less extreme than for comparable inland sites from the same latitudes, because coastal temperature highs and lows (most sites have little or no freezing) are moderated by maritime conditions. Temperature at the surface of reflective, dry sand greatly exceeds air temperature during full-sun conditions, being significantly higher for leaves resting on the sand surface than several inches above the sand.
- Daily land and sea breezes are typical for many beaches. Some strand sites receive calm to mild winds year-round, some regularly receive strong to moderate winds, and many are strongly affected by winds of violent, catastrophic storms. Wind reduces boundary layer thickness (i.e., unstirred air) next to leaf and sand, thereby increasing evapotranspiration while also decreasing leaf and sand temperature, if winds are strong enough. Strand may have markedly different wind microhabitats on windward and leeward sides of dunes and established plants.
- In addition to the influence of high tides, strand plants obtain moisture (freshwater) from precipitation and fog drip (intercepted by shoots) and may as well benefit from moisture condensation (dew) on the shoot and sand surface. To be physiologically significant, the sand must be sufficiently wetted in the region used by roots.
- Salt spray is a minute aerosol formed above heavy surf, generated when bubbles of saltwater break. Salt spray is therefore highest at water's edge and decreases dramatically inland. The vegetation on the ocean-facing edge consequently receives the highest concentrations of salts, but these levels are very weak. Vertical surfaces have much higher interception rates of salt spray than horizontal ones; linear surfaces higher than broad ones; structures high above the sand intercept substantially more than structures close to the sand.
- The outer edge of vegetation will experience the greatest effects of physical buffeting and salinity from soaking high tides and swells during storm surges.
- Dry surface sand is mobile. Plants can be damaged by sand blast, roots are sometimes exposed, and shoots frequently are buried by shifting sand, pushed by strong winds. To move sand grains, wind speed must be at least four meters per second.
- Beach sand has a low capacitance to retain water and is nutrient-poor, with little organic matter. Surface sand, which experiences rapid wet-dry episodes, is a stressful environment for plant roots. Deep sands may remain moist, receiving and storing water while the surface sand stays dry much of the time. Roots and rhizomes can penetrate sand rapidly, in comparison with growth within heavy soils having clay.
Zonation.
Ecologists have often recognized, and sometimes attempted to name and characterize, zonation of the shoreline sandy habitats for land plants. For convenience, two zones are used here (following Barbour and coworkers): (1) leading edge of the beach, and (2) inner beach and foredune (embryo dune) up to start of stabilized dunes. Obviously, the zones are not sharply defined, because some species appear in both subcommunities.
Plants successful in the leading edge must be able to tolerate higher salt spray and impacts of storm or high sea-produced saltwater overwash. Diversity of species is, for this reason, much lower within the leading edge.
There are many subtle differences in strand vegetation produced on windward versus leeward sides of dunes. A particular plant species may create a microhabitat that either favors or discourages other species from growing nearby.
General Characteristics and Adaptations.
- Most strand species are perennials. Typical monocotyledons (ignoring here geophytes) have rhizomes, from which fibrous roots arise at the nodes. Typical perennial dicotyledons have a deep taproot, generally reaching several meters to wet sand at the water table. Annuals tend to have more shallow, fibrous roots.
- Examples of perennials with fibrous roots
- Ammophila arenaria, a perennial C3 grass
- Calystegia soldanella, a nonsucculent perennial dicotyledon
- Carpobrotus chilensis, a leaf succulent perennial
- The most salt-tolerant species tend to be those on the leading edge of vegetation. For example, Cakile maritima can tolerate being wetted by relatively high concentrations of seawater, and Abronia maritima is also extremely tolerate of salinity. Nonetheless, Ambrosia chamissonis occurs on the leading edge but is relatively salt-sensitive.
- On strand of temperate shorelines, nearly all species are C3 plants, the commonest exception being Atriplex, whereas in subtropical and tropical latitudes, C4 grasses generally comprise a large portion of the biomass. There appears to be no local zonation of these two types of plants. Crassulacean acid metabolism (CAM) is extremely uncommon, except where species of the ice plant family (Aizoaceae) are present.
- Strand plants generally do not show morphological or anatomical features that you would find in dry habitats (i.e., no xeromorphic designs). Although sand can be extremely reflective and hot, relatively few species have leaves with high reflectance (silvery). [In deserts silvery reflectance is an adaptation to reduce leaf heat load by filtering out the heating wavelengths of infrared light.] Nobody has determined what function reflective hairs may play in any beach species, e.g., Ambrosia chamissonis, but plant hairs can also have functions related to water condensation on leaves. Instead, beach leaves are apparently designed for taking advantage of the high-light environment. Species that have been measured have good photosynthetic rates and saturate at half full sun. Leaves of beach plants have stomates on both surface (amphistomatic) to permit high rates of carbon dioxide uptake. C3 leaves have palisade mesophyll on both sides (isolateral organization), and, consequently, high internal cell surface for diffusion of carbon dioxide to the chloroplasts (high Ames/A). Stomates generally are not sunken, and the wax of the cuticle is not excessively thickened.
- Along many beaches, roots are present in sand that is not truly haline, having only 0.1 to 0.3% salt, even after storm surges. Plants that grow in this habitat therefore are not true halophytes, although many have some salt tolerance. Other beach habitats may have salt concentrations in sand exceeding 4%.
- Common dicotyledons at the leading edge are sprawling or prostrate with either comparatively broad leaves or fleshy ("succulent") leaves. Fleshiness is more common on the leading edge, because chloride ions (from salt spray), which enter leaves through breaks and lesions, cause cell enlargement (hypertrophy) of new leaves. [Cell volumes doubles or triples due to formation of a larger vacuole within each cell.]
- Examples:
- Ipomoea pes-caprae, a common resident on tropical beaches
- Cakile maritima, a thick-leaved annual, widespread on temperate beaches
- Calystegia soldanella, a thin-leaved perennial of East and West Coast U.S. beaches; its rhizome creeps about ten centimeters below the sand surface.
- Carbobrotus edulis, a leaf succulent that was introduced to California from South Africa to stabilize sandy hillsides; Carpobrotus chilensis also fills this niche
- Species inhabiting the unstabilized sand are likely to become buried during strong winds. A number of the pioneers on this shifting habitat are capable of very rapid growth in response to burial, and a number of species actually grow longer shoots when buried than when they are protected from burial.
- Germination of many beach plants appears to occur during a season when the sand is wetted with freshwater from precipitation and condensation, because seawater kills embryos and young seedlings. This critical stage in the life cycle of a plant may be another important factor in determining whether species become established in the leading edge of strand vegetation, because that environment may be too toxic for seedlings of most species.
On some beaches you will find strand vegetation in very close proximity to salt marsh, and some halophytic groups may occur in strand vegetation. Nonetheless, strand vegetation is basically a terrestrial, not a wetland habitat, and strand is not a populated with halophytic plants. Water used by the plant is only weakly saline, and salt spray affects mainly the fleshiness of the leaves.
Woody plants, such as trees, shrubs, and woody climbers, on beaches tend to occur far away from the water and near or only on stabilized dunes. In the dry tropics and subtropics, commonly thorn scrub may be the terrestrial vegetation adjacent to strand. Along wetter tropical coastlines of the Pacific Basin, the back dunes are commonly lined with coconut palms (Cocos nucifera), other palms, Pandanus, or Barringtonia. In the West Indies, sea-grape (Coccoloba uvifera and sea-bean (Entada) are common trees behind strand. Many of the trees that colonize behind strand have fruits or seeds that float on seawater without suffering death of the embryo within the seed. Seawater flotation is the proposed mechanism how some strand plants arrived as colonizers on distant isolated new islands, such as the Hawaiian Islands in the middle of the Pacific Ocean. On the Hawaiian Islands, possible immigrants via seawater flotation were the native species of cotton (Gossypium) and coral tree (Erythrina sandwicensis).
Some authors have suggested that strand has important similarities to the desert environment. There are some remarkably dry strand habitats, and some beaches are adjacent to fog desert. Uncommonly, species found on strand can occur in inland desert habitats, e.g., Croton californicus of California dunes and the Mojave Desert, but typically beach species are not native to inland types of vegetation. Probably one reason for shared genera between beach vegetation and deserts is because in both places there may be sandy, saline, or alkaline habitats. Desert plants also have leaves that are designed to maximize photosynthesis under high light environments, but beach plants are not modified desert plants, or vice versa.
Important Species of Beach and Dune Plants.
Species that occur in strand habitats are, by and large, specialized for life on sand dunes, and the majority do not appear in other vegetation types. The following are commonly observed strand plants of the world.