Plant Functional Traits of Carapa guianensis, a Widespread Tropical Tree, Adapted to Local Climate Conditions at Two Elevations in Costa Rica

Tropical rainforests are among the most biodiverse ecosystems in the world. However, plant species occurrence are highly dependent on localized topography, elevation, and climate. Convergent evolution has been studied extensively to identify key plant traits that vary along those gradients, which contributes to a greater understanding of how species adapt to climate variation. This project investigated leaf physiological and anatomical trait variation within a select tropical tree species, Carapa guianensis Aubl, which thrives in a wide range of elevations and climates within Central and South America. At two sites in Costa Rica, four leaves were sampled on each of three sample trees of C. guianensis growing at approximately 500 m (Texas A&M University Soltis Center) and 900 m elevation (Pocosol Biological Preserve), in which we measured leaf net photosynthesis and stomatal densities. We further assessed the relationship of net photosynthesis across a range of light intensities to determine the photosynthetic optimums and minimums of trees occurring at contrasting elevations. We found that stomatal density was higher at increasing elevations, possibly as a means to compensate for frequent cloud coverage and differences in climate (air temperature and solar radiation). Furthermore, C. guianensis trees located in lower elevations had higher net photosynthesis rates (P_gmax) than trees sampled at higher elevations, after accounting for differences in leaf temperature. These findings suggest that lower elevations with higher light intensity can significantly drive critical plant processes. Hence, further experiments are needed, not only to explore plant functional traits in species adapted to occur across wide ranges of elevations, but climatic changes that drive them, especially in our current global scenario in which tropical forests are expected to have higher temperatures and longer dry periods.