The Effects of High Vapor Pressure Deficit on Tree Water Status in Forests

The assessment of tree health and water status is a crucial aspect of understanding and managing forests, particularly in the cases of droughts and fires. Traditionally, the evaluation of tree health has been primarily based on measurements of soil water content (SWC). However, a growing body of literature suggests that high vapor pressure deficit (VPD) can have a similar, if not more severe, impact on tree health as soil water limitations. Recent research has also indicated that tall tree forests may be more susceptible to the effects of high VPD than short tree forests. Furthermore, high VPD is being increasingly linked to the ignition potential of forest fires.

To further investigate the dual limitations of VPD and SWC, I propose to conduct a study utilizing two distinct forest ecosystems. The first forest is a sparse woodland forest rooted in bedrock, which is characteristic of central Texas. The second forest is an Ameriflux affiliated site in a temperate mesic forest rooted in sandy soil, which is characteristic of northern Michigan. In this study, wavelet coherence will be used to analyze the relationship between vapor pressure deficit (VPD) and soil water content (SWC). Tree water status will be observed by both sap flux and wood water content. We leverage novel methods for evaluating tree water storage and SWC in rocky environments. The use of wavelet coherence will help identify the extent and frequency of the effects of VPD and SWC on tree water status across different temporal scales, providing a more comprehensive understanding of the relationship between these factors and tree health.