Tropical hardwood hammock forests are found on coastal ridges in the Florida Keys. Trees in these forests depend on groundwater as their main source of water. Trees at lower elevations have more access to the ground water due to their proximity to the water table, while trees at higher elevations are far away from water table and go through drought stress. However, trees at very low elevations frequently flood, and if the ground water table is brackish or saline, they may have to cope with salt stress as well as flooding. We examined how stomatal density and size in one of the dominant species (Bursera simaruba) in these forests co-vary with elevation and ground water salinity. Eight study sites that vary in elevation and ground water salinity were chosen across the Florida Keys. In each site, three mature trees were randomly selected and five leaves per tree were collected for stomatal analysis. Leaf imprints were made and stomatal length, width, and density were measured in digital images under a compound microscope. Leaf carbon stable isotope ratio (δ13C) of each tree was determined to infer the underlying variation in physiological stress of each tree.
Stomatal density varied widely among sites. The lowest mean stomatal density was found in Lower Keys sites with fresh ground water, while sites with elevated ground water salinity had the highest stomatal density. Across the elevation gradient, the stomatal density pattern was U-shaped: slightly greater at very low elevations, decreasing slowly toward mid-elevations, then increasing more sharply at higher elevations. A positive correlation between stomata density and ground water salinity was found. Furthermore, a significant positive correlation between δ13C and stomata density indicates that stomatal density may be associated with water use efficiency. Observed significant negative correlation between stomatal density and size may be associated with leaf area development under stressful environment. Overall, the results showed that B. simaruba exhibits a degree of stomatal trait plasticity in response to environmental changes.