Ammophila arenaria and Its Activity

פורסם: July 24th, 2011 | עודכן: 14/01/15

Seeds of various plants that are transported by wind arrive on the slip-face of the dunes and remain there. Wind velocity on these slopes is almost zero and “resting” seeds become covered by sliding sand, so that by the time winter comes, the sand contains a variety of seeds at various depths. The most successful of all is [Ammophila arenaria] (Fig. 4.1.20). Following several consecutive rainy days when the wet grains do not move, many seedlings begin to sprout (Fig. 4.1.21). When the sand dries up it returns to a state in which strong winds, above a certain threshold, move the sand grains and this may lead to the exposure of a few of the seedlings (Fig. 4.1.22).

Ammophila seedlings are resistant to the exposure of parts of their roots, but exposure of the root tip of a plant will kill it and if the plant has no additional roots anchored in the sand it may die. In the site where seedlings survive, a change in the land-morphology takes place (Fig. 4.1.23). The sand among the Ammophila plants displays ripples in its surface. Ripples morphology and the appearance of the nearby sand surface may be explained by the size of the sand grains. The winds moving the dune’s sand grains take out the small particles into the aerosols. The dune surface now contains only fine, coarse and larger grains. Lighter winds move the fine sand, leaving the larger grains. Stronger winds move the larger, yellow grains and pile them at the top of the ripples (Fig. 4.1.24/1) that are perpendicular to the direction of the wind that created them. The yellow particles are fragments of hard sandstone that have been added to the sand at the bottom of the sea.

They protect the sand of the ripple from further wind erosion. The original sand plain is white (4.1.24/2), and most of its grains are small but there are large black particles which are not moved by the winds. Young Ammophila plants (Fig. 4.1.20, 4.1.21, 4.1.23) do not influence sand mobility and the direction of the ripples among them is not distorted by the plants. While maturing, the plant decreases the sand mobility around it and inside its tuft. Individual No.1 in Fig. 4.1.25 made a small elongated sand mound around it. Individual No. 2 in the same figure is older and its sand mound is larger and taller. The direction of the plant extension is influenced by the rhizomes’ development (Fig. 4.1.26); they enable vegetative reproduction without rain events such as those that caused germination. Over the years, the Ammophila tuft or bulk spreads and the efficiency of decreasing the velocity of passing wind is shown (as in Fig. 4.1.27) by the formation of higher mounds above the dune surface. It also minimizes the development of ripples inside the tuft (compare Figs. 4.1.20 and 4.1.27).

In scientific literature these biogenic mounds are termed “nebka.” Ammophila does not suffer from the stationary sand that covers it, but even benefits from it. In dunes stabilized by this plant many individuals die as a result of the aging of the conducting tissues. Sand hills develop in areas with long-term prevalence of Ammophila (Figs. 4.1.29, 4.1.30). In sites where sand removal from the roots is intensive, parts of the tuft die. Hence the bulk may be built up on the lee-ward side (Fig. 4.1.31) and disintegrate on the windward side. Ammophila arenaria is also resistant to sea water spray. Its leaves fold and form a cylinder with the shiny underside of the leaf facing outward. Salty water drops brought from the sea by the wind slide along the leaf to the ground. The salts are leached by rain water and cause no harm.

Fig.4.1.20: Ammophila arenaria. Ripples in the sand around the plant pass through the tuft. On the left top the middle part of a leaf with a ligule of two long lobes.

Fig.4.1.21: Seedlings of A. arenaria in the mobile sand around an old tuft.

Fig.4.1.22: A seedling of A. arenaria with exposed roots after sand was removed through wind erosion. The seedling keeps living as long as the root tips are still in the soil.

Fig.4.1.23: Young plants of A. arenaria which have a small influence on sand mobility and on ripples formation.

Fig.4.1.24: A close-up to sand area with ripples. 1. A ripple with coarse sand grains which moved (“creeping”) by strong winds and protect now the sand below them. 2. Fine sand with large black particles that can hardly move.

Fig.4.1.25: Sand accumulates in A. arenaria tufts. 1. A young tuft with a small sized sand mound. 2. An older tuft with a larger sand mound.

Fig.4.1.26: Three rhizomes of A. arenaria assisting the expending growth of the plant.

Fig.4.1.27: A close-up to a sand mound (nebka) built by A. arenaria through growth above the sand covering it. There are no ripples among the stems testifying the efficiency of it as a wind break.

Fig.4.1.28: A compound nebka of A. arenaria. The sea-ward slope is steeper than the opposite slope due to wind erosion.

Fig.4.1.29: A compound sand hill built through the activity of A. arenaria.

Fig.4.1.30: A tall sand hill built through A. arenaria activity in a beach, which receives more rain than Israeli beaches, in Newbourgh, Scotland.

Fig.4.1.31: Following wind erosion stems which were covered by sand and produced stem-borne roots.

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