Georgios Xenakis

Climate change is expected to alter precipitation patterns, making droughts more frequent in some areas, which will expose trees to more severe water deficits which could result in the catastrophic collapse of their water transport system and, eventually, mortality. To inform forest management and tree species suitability to increase forest resilience requires a robust method for understanding the time, duration and impact of water deficits. In this study, I discuss how an application of Electric Circuit Theory (ECT) is a reliable and robust way to detect water deficit in the Soil-Plant-Atmosphere Continuum (SPAC). Based on an electric closed-circuit analogy of the SPAC, I define its "hydraulic efficiency", and when there is a water deficit. Using tree sap flow and evapotranspiration from eddy covariance, I demonstrate the application of the theory and the use of hydraulic efficiency to quantify how long trees and forest ecosystems spend under water deficit conditions. Calculations of ECT-based hydraulic efficiency showed that individual trees of an upland Sitka spruce plantation in N. England spent up to 80% of a monitoring period during the growing season at water deficit in a non-drought year, depending on their location within the stand. The results also showed that before a drought in 2018, the spruce plantation was between 3 and 6% of the time under water deficit, increasing to 31% in the year of the drought and dropping to 16% two years post-drought. Furthermore, using a global database of sap flow measurements, I provide an approach which suggests a way to compare hydraulic efficiency of species in different biomes. This dataset showed 6 out of 10 tropical rainforest species were under water deficit, compared to 5 out of 36 temperate forest species. I discuss how ECT is similar to other supply/loss theories describing tree water relations and conclude that combined with modern technology, it can provide a continuous system for monitoring water deficit.