Salt Spray Distribution and Its Impact on Vegetation Zonation on Coastal Dunes: a Review

Salt spray mainly originates from the bursting of bubbles in breaking waves and is often considered as one of the dominant factors contributing to vegetation zonation in coastal dunes. In this paper, the literature on salt spray distribution and impact on dune plants are reviewed. Salt spray distribution is greatly affected by wave energy, wind conditions, distance from the coast, topography, vegetation, precipitation, and sand/soil properties. The amount of salt accumulation and trapping efficiency of the vegetative canopy are largely dependent on the plant characteristics such as architecture and leaf morphology. Salt concentrations in sand mainly vary with soil texture. Salt spray has negative impacts on plant growth and can cause water stress, promote tissue necrosis and leaf loss, reduce stomatal conductance, water use efficiency, photosynthesis, affect assimilates, or hormone supply to the growing organs. Damage to plants is increased by sand and wind abrasion and insect damage on leaves. High humidity, dew formation, light drizzle, and fog increase the rates of salt uptake by plant leaves. Plant seedlings and reproductive organs are more significantly affected by salt spray compared with mature plants and plant leaves respectively. Salt spray can also provide nutrition to plants particularly in coastal dunes with lower soil nutrition and salt accumulation rates. Species near the sea often show phenological, morphological, and physiological adaptations to salt spray including dormant times/seasons, low heights, compacted and asymmetrical canopies, unique leaf morphologies and/or orientations, dense hairs, rigid cuticles, and closed stomata. Surfactants produced from human activities can aggravate the damage on plants at average salt spray levels. Methods to trap salt vary considerably in the literature, and results from different studies may not be comparable due to trap type, placement, and landscape position. Greenhouse versus field experiments and sampling may not always be compatible. The relations between salt spray distribution and vegetation zonation will be further complicated under climate change and human activities.