Species-Specific Environmental Factors that Influence Sap Flow Rates of Nine Bottomland Hardwood Species

Author(s): Kassahun, Z.; Renninger, H.

Climate change models predict an increase in prolonged drought events in the southeastern United States. Due to these climate alterations, bottomland hardwood forests could experience a drastic shift in their established hydrological patterns. Individual water consumption of hardwood trees vary by species and can be influenced by environmental factors such as solar radiation, soil water availability, temperature, and atmospheric vapor pressure deficit. These environmental factors are expected to shift in intensity and availability as climate change persists. As these environmental factors shift, certain tree species could be more negatively affected over more resilient species, ultimately leading to a shift in species composition in the forest. The rate of sap flow described as the transport of water that occurs in the xylem of a tree, is indicative of a tree’s water use strategy. Sap flow rates can convey how much water a tree is using as well as how the tree copes with limited water resources. By using sap flow measurements to study the species-specific factors that influence physiological response, we can better understand how species specific water use will shift under drought conditions. Sap flow rates were measured using heat dissipation sensors on nine deciduous hardwood species found in a seasonally flooded hardwood forest. Simultaneously, temperature, relative humidity and soil moisture were measured and vapor pressure deficits were calculated. We found that cherrybark oak uses the most water during the growing season, using ~45% more water than the next highest consumer, swamp chestnut oak. Shagbark and pignut hickory use the least amount of water during the growing season, roughly 2% of cherrybark oaks’ water consumption. Sap flow rates also exhibited a linear correlation with soil moisture and vapor pressure deficit for American elm, pignut hickory, swamp chestnut oak, and willow oak in order of correlation strength. Response to changes in vapor pressure deficit were also directly linked to the soil moisture conditions for these species. These findings suggest that as drought conditions increase, leading to a decrease in soil moisture, these species will respond with a reduction in sap flow, with American elm and pignut hickory showing the greatest reduction in water use and winged elm exhibiting the least response. This information will be useful in accurately estimating forest water budgets based on future climate change predictions.

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