Ecosystem disturbance is a common occurrence in forest systems and is considered a necessity for proper ecosystem function and diversity to be maintained. Currently, small-scale tree fall gaps are the most common form of forest disturbance in tropical forests, which are known to create light gaps that promote rapid growth of understory plants. However, 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), 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 understory, mid-story, and dominant trees before (2014) and after the tree gap was created (2019) using thermal dissipation sap flow sensors. Additionally, Leaf Area Index (LAI) and gap fraction measurements were conducted using hemispherical photography to observe changes in canopy structure between both time periods.
Results/Conclusions
Preliminary results depict a stark increase in the amount of water used by suppressed trees in relation to dominant trees after the gap was created, and a more modest increase relative to mid-story trees as well. In 2014, mid-story and dominant trees used at least 20% more water than suppressed trees on average, but that was reduced to a difference of only 14% and 3%, respectively in 2019 after the gap was created. Comparisons in canopy showed no significant differences in LAI, however gap fraction was significantly higher in 2019, meaning that more visible sky was present. These preliminary data capture how drastically suppressed trees can increase their water usage under increased light availability while competing with more developed trees. With increasing global temperatures and shifts in rainfall patterns increasing the likelihood of tree mortality within the tropics, there is an increasing need for studies assessing tropical forest response to disturbances of this magnitude in order to better understand and predict possible changes in forest composition that may arise from mortality induced by climate change.