Physiological Characteristics of Coastal Dune Pioneer Species from the Eastern Cape, South Africa, in Relation to Stress and Disturbance

A characteristic of coastal dune systems is that species diversity and total plant biomass is less than that of the adjoining inland areas, and furthermore, there is generally pronounced zonation of the species present on the dunes (Clements 1916; Lubke 1983).Why are the foredunes inhabited by only few species, and dominated by even fewer?

There are obviously a number of factors controlling plant distribution and productivity, but for the foredunes in particular, adaptations to resource stress and/or disturbance are likely to be very important (Barbour 1992). The resources required by plants for growth include light, water and nutrients, and stress could be a consequence of either deficiencies or excesses of these resources. Adaptations to acquire resources present in low amounts, or to resist those present in excess, may also play a role in competition between species. Disturbance is the process by which part or all of a plant is damaged or destroyed, generally by physical processes.Disturbance is particularly high in coastal foredunes and includes wind, salt-spray, occasional inundation by seawater and sand movement.The latter can lead to burial, exposure, or physical damage from sand-blasting. Salt spray could be argued to be an environmental stress, but because it can lead to direct physical damage, we prefer to consider it a disturbance. However the distinction is unimportant in the discussion that follows. Further complications arise because of adaptations to a combination of stresses and/or disturbance, and because plant response may vary with stage in the life cycle (particularly seedlings in comparison with established plants).