| dc.description.abstract | With a high species diversity, novel ecosystem functioning, and distinct topographic features, tropical forests are home to vegetation that is highly specialized and adapted to its environment. While this has resulted in many tropical species being confined to limited microclimatic conditions and elevations, certain phenotypically plastic species are able to survive across differing environmental gradients. To investigate this adaptation and plasticity, as well as, its implications for tropical species response to climate change, we assessed differences in leaf physiology and anatomy of Carapa guianensis Aubl and Otoba novogranatensis Moldenke, two shade tolerant plants found across a wide range of elevations within the Alajuela province of Costa Rica. Utilizing a portable leaf gas exchange system, A/Ci curves were generated for three replicate trees of both species at 600 m (Texas A&M University Soltis Center) and 820 m elevation (Pocosol Biological Preserve). Because trees were growing under varying light conditions, canopy cover of each individual was determined using the Leaf Area Index (LAI) measurements from hemispherical photos. Stomata density was determined for C. guianensis at both sites using stomata impressions. A/Ci curves were fitted using a revised Sharkey model to determine maximum carboxylation rate of Rubisco (Vcmax), rate of electron transport (J), the rate of use of triose phosphates (TPU), daytime respiration (Rd), and mesophyll conductance (gm). Results from this study depict stark differences in the photosynthetic capacity between our study plants with C. guianensis having significantly different Vcmax and Rd across sites while O. novogranatensis’s response did not differ between sites. C. guianensis also had significantly lower stomatal density at higher elevations, associated with shaded conditions, despite canopy cover being similar at both sites. This suggests that lower light intensity dictated primarily by clouds and fog, led to the observed differences between elevations. Another common theme within tropical forests is the presence of tree fall gaps that occur and afford suppressed trees the opportunity for more light. While important drivers of species diversity within these systems, the effects of such small-scale disturbances on water use distribution among understory, mid-story, and dominant trees has not been explored in detail. To address this, we conducted a study exploring stand-level response to the death of a large dominant tree, Mortoniodendron anisophyllum Standl. & Steyerm (DBH > 220 cm; Height ~ 40 m). This study was conducted across four suppressed, four mid-story, and two dominant trees within a 50 × 50 m pre-montane tropical forest plot at the Texas A&M Soltis Center for Research and Education located in the Alajuela Provence, Costa Rica. We compared the proportion of water use by suppressed, mid-story, and dominant trees before (2014) and after the tree gap was created (2019) using thermal dissipation sap flow sensors. From our results, we found that, water usage of remaining trees in the gap had increased across all canopy levels; with suppressed trees, water usage now averaged proportionally closer to rates observed in dominant and mid-story trees. With increasing global temperatures and shifts in rainfall patterns increasing the likelihood of tree mortality coupled with the high environmental variability within tropical forests, there is a greater need to enhance our fundamental understanding of tropical forest response to these phenomenon in order to better understand and predict possible changes in forest composition that may arise from climate change. | |
