Zeima Kassahun and Heidi Renninger Results and Discussion Environmental factors that influence physiological functioning of eight bottomland hardwood species Zeima Kassahun and Heidi Renninger College of Forest Resources, Department of Forestry, Mississippi State University Abstract Methods and Materials With increases in temperature and prolonged drought events, southern hardwood forests are expected to experience a shift in forest productivity and competition. As environmental conditions shift, certain tree species may experience an increase in productivity or could be negatively affected, leading to a shift in species composition, water use, and carbon uptake. The goal of this research is to use sap flow measurements, photosynthetic rates, and leaf characteristics to study species-specific responses to environmental drivers. Sap flow of eight hardwood species, as well as soil moisture (SM) and vapor pressure deficit (VPD), were measured continuously over the course of 2 years, which included a severe drought. Results suggest that sap flow rates of cherrybark oak, willow oak, swamp chestnut oak, water oak, American elm, and winged elm respond differently to changes in VPD under low or ample SM conditions. Green ash and shagbark hickory did not differ in response to VPD, suggesting these species are more drought tolerant. Leaf gas-exchange data showed that green ash exhibited the lowest water-use efficiency. Regarding leaf phenology, willow oak, cherrybark oak were the first to experience leaf abscission during the 2016 drought. In 2017, all species, excluding willow oak, dropped ~80% of the canopy within a 2-week time span. Yearly water consumption per m2 canopy leaf area was significantly lower in 2016 than in 2017 for all study species. Taken together, these responses can be used to improve land surface models and identify species in this forest type that will be most successful under changing environmental conditions. The study site is located in the Mississippi State University John W. Starr Memorial Forest. Relative humidity and temperature were measured throughout the growing season using a Rotronic HC2S3 temperature and humidity probe. Values were used to calculate vapor pressure deficit. Soil moisture content was measured at three locations with time domain reflectometry soil volumetric water content sensors. Sap flow measurements were collected using Granier heat dissipation sensors. Photosynthetic data were measured with the LI-COR 6400 Photosynthesis System Epidermal peels were conducted on leaves in order to analyze stomatal characteristics under a microscope. Leaves were collected for phenology measurements using 10 leaf litter traps distributed across the site. Collections occurred every 2 weeks. Figure 3. Li-COR 6400 Portable Photosynthesis System. Figure 2. Soil moisture sensor measuring the top 30 cm Figure 4. Sap flow sensors installed into sapwood of tree. Results and Discussion Introduction Study Justification: Global climate change models predict severe global droughts, increases in VPD, and a longer growing season. This could shift forest competition to favor species that can sustain high photosynthetic rates under water stress and negatively impact more sensitive species. These results can improve estimates of forest water budgets, projections of species distribution, and stand carbon sequestration rates. Study Objective: To use sap flow, photosynthetic rates, and leaf phenology to identify species-specific response to the environment. Figure 6. Progression of litterfall per species in 2016 and 2017. Leaf abscission began earlier and progressed gradually in 2016, while 2017 experienced a rapid abscission period, with more than 80% of the canopy lost in a 2 week period. Total yearly water use per leaf area was significantly lower in 2016 when compared to 2017 for all species. Green ash consumed the most water in both years, further suggesting this species does not restrict water-consumption. Alternatively, the oaks had the lowest transpiration rates as well as the highest canopy leaf area, resulting in low yearly total water consumption. Figure 5. Sap flow response to lnVPD plotted as two separate functions under low (17-30%) and high (35-47.5%) SM. Significant differences (marked with *) in slope terms between SM conditions suggest the influence of soil moisture on tree sap flow sensitivity to VPD. Sensitivity of sap flow to VPD is significantly affected by changes in soil moisture for the oaks and the elms Suggests these species have a conservative water use strategy under low soil water availability. Sensitivity of sap flow to VPD was not significantly affected by soil moisture conditions for green ash and shagbark hickory. Other studies found that green ash is anisohydric, meaning it does not alter water uptake even under limited water resources. Our findings were similar to that of others, suggesting green ash may prefer wetter sites while cherrybark oak, willow oak, and swamp chestnut oak may prefer drier sites ( ( Figure 7. Total water use (kg/year-1) per unit leaf area (m2) for each species in 2016 (drought year) and 2017 (average year). Figure 1. Standing water on bottomland study site. Soil saturation usually starts in winter and continues until mid-spring. Due to a drought in the fall of 2016, flooding was delayed until late January. Summary: Sap flow rates of shagbark hickory and green ash did not vary in response to VPD under drought conditions. Green ash’s water-use strategy was corroborated by its high photosynthetic and transpiration rates and low water-use efficiency. Stomatal densities were significantly larger in the oaks when compared to green ash, suggesting smaller densities aid in drought-tolerance. Our findings suggest that under climate change projections, these drought-tolerant species could have a competitive advantage as global water scarcity persists. Table 1. Photosynthetic measurements averaged by species. Green ash (* marks significant difference) exhibited the highest photosynthetic rates and the lowest intrinsic water-use efficiency (iWUE). While water-conservative species such as water oak, cherrybark oak, and winged elm had the highest iWUE. Hypotheses Hypotheses: Drought tolerant species will exhibit similar slope terms in the relationship between sap flow and VPD under low and high SM. Species that exhibit lower sap flow per unit VPD increase under low SM conditions (drought sensitive) will drop their leaves earlier in the growing season. Yearly water use will be significantly lower in 2016 than in 2017 due to restricted water availability. Acknowledgements Forest and Wildlife Research Center, Mississippi State University, Dr. Brent Frey, Dr. Courtney Siegert, Dr. David Evans, Nicole Hornslein, Justin Yow