Climate change is expected to cause more frequent droughts in many parts of the world, and Central Texas is no exception. A better understanding of trees’ water availability and hydraulic responses during periods of water stress will help us predict possible range shifts and changes in dominance among tree species. At the eastern edge of the Edwards Plateau, fire suppression has allowed junipers (Juniperus ashei J.Buchholz) to increase in abundance relative to escarpment live oaks (Quercus fusiformis Small) within their shared native range. But could an increase in drought frequency change the relative abundances? To predict this, we might first ask the question: which species is better able to maintain transpiration during dry periods? We use sap flow measurements over two summers to investigate changes in transpiration rates for junipers and oaks during the times of greatest water stress, seeking to understand which species is better able to maintain its transpiration. Additionally, we measure predawn water potential measurements before, during, and after a dry period in order to understand which species had greater water access throughout the dry-down.
Results/Conclusions
While oaks and junipers both decrease transpiration rates during extended dry periods, we find oaks are able to maintain a greater percentage of peak transpiration than junipers. Oaks also maintain higher absolute transpiration velocities as the dry periods progress. Additionally, we find that oaks’ predawn water potential (PWP) is higher (less negative) than junipers’ during a dry period. Taken together, these findings could be due to oaks’ deeper roots reaching water sources junipers cannot access (input-related), to junipers closing stomata to a greater extent to protect sensitive xylem (output-related), or to junipers experiencing greater injury to xylem in the form of cavitation, causing lower sap flow rates (flow-related). Laboratory work suggests junipers’ xylem is much less sensitive to cavitation than that of oaks (Johnson et al. 2018), and even though junipers’ PWP measurements are lower than oaks’, they are well above the P50 level for junipers, making the output and flow-related explanations unlikely. We conclude that the most likely reason that oaks are better able to maintain transpiration during dry periods is greater water access via deeper roots, and that in a future scenario of more-frequent and longer droughts, live oaks may begin to outcompete junipers in Central Texas.