Noninvasive Analysis of Tree Stems by Electrical Resistivity Tomography: Unraveling the Effects of Temperature, Water Status, and Electrode Installation

Abstract

The increasing demand for tree and forest health monitoring due to ongoing climate change requires new future-oriented and nondestructive measurement techniques. Electrical resistivity (ER) tomography represents a promising and innovative approach, as it allows insights into living trees based on ER levels and ER cross-sectional distribution patterns of stems. However, it is poorly understood how external factors, such as temperature, tree water status, and electrode installation affect ER tomograms. In this study, ER measurements were carried out on three angiosperms (Betula pendula, Fagus sylvatica, Populus nigra) and three conifers (Larix decidua, Picea abies, Pinus cembra) exposed to temperatures between −10 and 30°C and to continuous dehydration down to −6.3 MPa in a laboratory experiment. Additionally, effects of removal of peripheral tissues (periderm, phloem, cambium) and electrode installation were tested. Temperature changes above the freezing point did not affect ER distribution patterns but average ER levels, which increased exponentially and about 2.5-fold from 30 to 0°C in all species. In contrast, freezing of stems caused a pronounced raise of ER, especially in peripheral areas. With progressive tree dehydration, average ER increased in all species except in B. pendula, and measured resistivities in the peripheral stem areas of both angiosperms and conifers were clearly linearly related to the tree water status. Removal of the periderm resulted in a slight decrease of high ER peaks. Installation of electrodes for a short period of 32–72 h before conducting the tomography caused small distortions in tomograms. Distortions became serious after long-term installation for several months, while mean ER was only slightly affected. The present study confirms that ER tomography of tree stems is sensitive to temperature and water status. Results help to improve ER tomogram interpretation and suggest that ER analyses may be suitable to nondestructively determinate the hydraulic status of trees. They thus provide a solid basis for further technological developments to enable presymptomatic detection of physiological stress in standing trees.

Insights into trunks of Pinus cembra L.: analyses of hydraulics via electrical resistivity tomography

“…In spring 2013, a cold period (May) caused another increase in ER mean , and accordingly ER mean and air/xylem temperature were negatively correlated (see “ Results ”). In laboratory tests, Ganthaler et al ( 2019 ) demonstrated that temperature directly affects the measured ER and reported for six species, including P. cembra , an about 2.5-fold increase in ER mean between 30 and 0 °C. In the present study, this increase was even higher, and we suppose that this indicates two additional processes: First, conifers at higher elevation suffer from frost drought during winter, which induces low water potentials and respective low water contents (e.g.…”

“…Second, the repeated use of nail probes installed in September 2012 might have affected ER mean . Ganthaler et al ( 2019 ) pointed out that resin production and wound reactions may reduce the electric connection to trunk tissues over time and, in consequence, affect tomography measurements. More ER tomograms of conifer species with high resin content, such as P. cembra , are required to estimate the relevance of wound reactions.…”

“…were selected. In September 2012, nail probes were installed around the trunk circumference at 24 measuring points (MP), 40 cm above the ground (Al Hagrey 2007 ; Bär et al 2019a ; Ganthaler et al 2019 ). Nails (length 5 cm) were installed around the trunk circumference with equal distance to each other (counterclockwise numbering with north orientation of MP 1), until contact to the sapwood was established.…”

“…Recently, Bär et al ( 2019a , b ) reported wood moisture content of conifers to be the main factor influencing ER patterns, which thus accurately display sapwood areas. Ganthaler et al ( 2019 ) showed that average ER values of conifer trunks are correlated with shoot water potential and trunk temperature. While ER distribution patterns do not change above the freezing point, average ER values decrease exponentially from 0 to 30 °C (Ganthaler et al 2019 ; see also Luo et al 2019 ).…”

“…Ganthaler et al ( 2019 ) showed that average ER values of conifer trunks are correlated with shoot water potential and trunk temperature. While ER distribution patterns do not change above the freezing point, average ER values decrease exponentially from 0 to 30 °C (Ganthaler et al 2019 ; see also Luo et al 2019 ). Thus, temperature effects should be negligible as long as small changes in temperature occur (e.g.…”