Salt spray as a microenvironmental factor in the distribution of beach plants at point reyes, California.

Measurements of salt spray were made weekly for 15 spring weeks with a grid of 50 traps in a 32 m longx24m wide plot of beach vegetation along the central California coast. The trap centers were 14 cm above the ground, the traps could swivel to continuously face the wind, and trap area (16 cm2) was small in order to minimally affect wind flow. Regressions or analyses of variance of wind speed and direction, rainfall, tide level, plot topography, and plant distribution were performed against salt spray. The period was lengthy enough to include a wide range of climatic conditions and the time of maximum plant growth and flowering. An ecologically and statistically significant salt spray gradient, which correlated with distance back from, and elevation above, mean tide line, was revealed. Overall, the front-most, lowest traps received 4-5 times the spray that the back-most, highest traps received. Weeks with moderate wind and spray (7-14 kph, in contrast to weeks with lower or higher wind speed) resulted in even steeper gradients, up to 20+-fold. The highest trap reading for any one week was 70 mg salt deposited dm-2 of trap surface day-1. Plant distribution correlated with salts spray and there is some evidence that it is a functional relationship. Attempts were made to extrapolate from the overall mean trap load of 13 mg dm-2 day-1 down to the leaf surface just below the traps, and to data from salt spray studies with larger traps on the Atlantic Coast. It appears that the salt spray load just above the beach plant canopy ranges from less than 1 to nearly 200 mg dm-2 vertical trap surface day-1, depending on trap height, distance back from tide line, and wind speed. The plant canopy and sand surface receive an order of magnitude less spray than that received by vertical traps.