TransP - a novel sensor for continuous in-situ measurement of transpiration and conductivity at the leaf level

Accurate parametrization and validation of SVAT- or evapotranspiration-models requires robust estimates of transpiration and conductivity on the level of individual leaves. Such estimates are commonly made from measurements with mobile gas exchange systems, which allow precise measurements of leaf transpiration. However, this method has some decisive practical (expensive and labor intensive, both constraining feasible number of replicates) as well as methodological limitations (destruction of the leaf boundary layer). In order to validate the FO3REST model – which estimates the phytotoxic ozone uptake of forest stands – a sensor was required that continuously measures leaf transpiration and conductivity with a high number of replicates. Within the valORTree project, which was carried out from 2019-2021 in the climate chambers of the TUMmesa ecotron facility (Jákli et al. 2021), a novel, low-cost leaf sensor ("TransP") was developed that enables continuous in-situ determination of transpiration and conductivity for the important forest tree species beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) H. Karst.) over the entire growing season. The sensor records different temperatures in the leaf/needle environment and was calibrated against the gravimetrically determined transpiration rate (r² = 0.74 for beech; r² = 0.84 for spruce). Measurement inaccuracies can be compensated for by using many of the inexpensive sensors in parallel. The sensor output was validated against measurements using Li-6400 and Li-6800 gas exchange systems (Licor, USA). Differences in the outputs of the two methods could be explained by the fact that the Licor systems measures transpiration based on stomatal conductance, whereas TransP includes the in-situ boundary layer resistance. So far, the sensor has been applied under low-wind conditions in indoor applications and is currently further developed for application in the field.

However, we clearly show that measuring transpiration of beech leaves and spruce needles with the TransP sensor provides robust data. Since TransP operation is minimally invasive and the leaf boundary layer is preserved during measurements, it is assumed that the sensor provides a realistic representation of the in-situ transpiration of individual leaves/needles. In addition, the high temporal resolution of the measurements provides the ability to accurately integrate transpiration over the entire period of the measurement.

 

Reference

Jákli, B., Meier, R., Gelhardt, U., Bliss, M., Grünhage, L., & Baumgarten, M. (2021). Regionalized dynamic climate series for ecological climate impact research in modern controlled environment facilities. Ecology and evolution, 11(23), 17364-17380.