Temporal and spatial variation in water content within living tree stems determined by electrical resistivity tomography

Abstract

Stem water content (θ) is an important state variable in the soil-plant-atmosphere continuum (SPAC), and varies temporally and spatially in response to environmental factors and plant growth stages. However, it is difficult to measure θ distribution in living trees in a manner that is not destructive. In this study, temporal and spatial variations in θ within living tree stems were examined based on minimally destructive electrical resistivity tomography. Measurements of tree bulk electrical resistivity (ρ), stem temperature, sap flow and stem water potential were taken on an Australian native tree species, Allocasuarina verticillata. The results show that θ estimated from adjusted resistivity (ρ*) based on a reference ρ-θ relationship approach agrees well with θ estimated from predawn stem water potential. Sapwood θ gradually increased during the wet season and substantially decreased during the dry season as predawn stem water potential and sap velocity increased and decreased, respectively, and ρ*-estimated θ reveals water replenishment and loss within the sapwood during the wet and dry seasons, respectively. During the dry period, mean sapwood θ for the study trees decreased to 0.23 cm3/cm3 (January), and daily maximum sap velocity was only about 10 % of that in wet season, suggesting that the trees were suffering heavy water stress. The spatially exhaustive method to estimate stem water content within living trees proposed here provides an additional approach to investigating tree response to water stress.