Vegetation and Mobile Ground as Illustrated by Suaeda Fruticosa on Shingle

Abstract

The materials of a shingle beach froniting the open sea are liable to two distinct,effects from the play of waves and currents. There is the alongshore effect or longitudinal drift of the shingle between tide marks on the seaward face depending on the co-operation of wave agitation and currents; it is to control this movement of the materials that groynes are contrived. In contrast to this there is the eff'ect produced when onshore gales accompany very high tides and waves break over the crest. Under these circunmstances shingle from the front is carried above ordinary tidal limits often in great quantities, and in the case of beaches like, spits and bars which stand away from the lanld these materials are propelled down the lee slope as the sea water runs off. In this way the salt-marsh, estuary or lagoon which commonly separates the beach from the land suffers gradual encroachment. This transverse or landward drift of the beach is of course an intermittent phenomenon, the brief and occasional periods of active advancee being separated as a rule by long intervals of rest. Whilst the alongshore drift is of no particular interest to the botanist-a rolling stone gathers no moss-the crest and back of the beach on the contrary enjoy intervals of rel)ose whereby the establishment of a vegetation is permitted. We propose in the present article to consider in some detail the circumstances of establishment and maintenance of a vegetation on an intermittently active beach -employing Suaeda fruticosa as our type -and, further, to consider the effect of the presence of plants upon the landward movement of the shingle. Finally, in the light of our observations, an attempt is rnade to deduce the probable results that may be expected to follow the systematic plantiing or afforestation of mobile beaches. On those shingle beaches upon which it is found, Suaeda fruticosa fromi its stature and density of occurrence is undoubtedly the most important plant; at the same time there are a number of other common forms such as Silene maritirna, Arenaria peploides, Crambe maritima, Rurnex trigranulatus, Lathyrus maritimus, Solanum Dulcamara and Glaucium luteum, all of which in their several ways cope successfully with the habitat and would fully repay detailed study.