Variation in Stomatal Characteristics of <i>Bursera simaruba</i> (L.) Sarg., a Dominant Tree Species of Tropical Hardwood Hammock Forest across a Habitat Gradient in the Florida Keys

Abstract

Tree species in coastal forests may exhibit specialization or plasticity in coping with drought through changes in their stomatal morphology or activity, allowing for a balance between gas exchange and water loss in a periodically stressful environment. To examine these responses, we sought to answer two primary research questions: a) how is variation in <i> B. simaruba </i> ’s stomatal traits partitioned across hierarchical levels, <i> i.e. </i> , site, tree, and leaf; and b) is variation in stomatal traits an integrated response to physiological stress expressed across the habitat gradient of Florida Keys forests? At eight sites distributed throughout the Keys, five leaves were collected from three mature trees for stomatal analysis. Leaf carbon stable isotope ratio ( <i> δ </i> 13 C) was determined to infer the changes in water use efficiency caused by physiological stress experienced by each tree. The results showed that substantial proportions of the total variance in three traits (stomatal density, stomatal size, and <i> δ </i> 13 C) were observed at all levels, suggesting that processes operating at each scale are important in determining trait values. A significant negative correlation between stomatal density and size across scales was observed. Path model analysis showed that environmental variables, distance to ground water and ground water salinity, affect leaf <i> δ </i> 13 C indirectly, via its effects on stomatal traits, not directly to leaf <i> δ </i> 13 C. Therefore, the combination of small and densely distributed stomata seems to represent a strategy that allows <i> B. sima ruba </i> to conserve water under conditions of physiological drought induced by either higher ground water salinity or flooding stress at very low elevation.