Codominant tree species showcase opposing hydraulic capacitance-reliance in Central Texas

Abstract. Understanding the stress-responses of the tree species that comprise ecosystems is important for making predictions of how those ecosystems will respond to future stress events, e.g., drought. Past work has shown differential reliance on hydraulic capacitance, water released from internal storage, to maintain daily transpiration and gas-exchange with the atmosphere. In this study, we used wavelet coherence analysis to quantify the relationship between water supply (soil moisture), atmospheric water demand (VPD), and tree-water measurements (wood water content and sap flow) for two codominant tree species in a Central Texas woodland, escarpment live oak (Quercus fusiformis) and Ashe juniper (Juniperus ashei), over one and a half years including a period of extreme drought. Wavelet coherence provides a measure of correlation between the periodicity of signals at different frequencies or periods. The goal of our study was to analyze how the study trees' responses to environmental moisture conditions changed over time, especially during drought conditions. Our results revealed that, for both species, vapor pressure deficit and sap flow were strongly correlated at the 24-hour period, even during severe drought. Wood water content, especially for Q. fusiformis, was strongly correlated with soil moisture at daily to weeks-long periods. Our results also showed that vapor pressure deficit had a strong, and sustained, correlation with Q. fusiformis wood water content at the 24-hour period as drought severity increased, while J. ashei wood water content did not have this same relationship to vapor pressure deficit. Our results suggested that both species continue daily transpiration during severe drought, and that Q. fusiformis is more reliant on capacitance than J. ashei to maintain daily transpiration leading up to, and during, severe drought conditions. This difference in drought response strategy has important implications for each species' survival through shifting hydroclimatic conditions.