Importance of calibration in determining forest stand transpiration using the thermal dissipation method

Abstract

Forest transpiration (Tr) is the major hydrologic flux from terrestrial ecosystems; it dominates the catchment water budget and land surface energy balance and can be used as an index of exchange efficiency between vegetation and the atmosphere. The thermal dissipation method (TDM) is used to obtain the xylem sap flux density (Fd) of individual tree stems; it is among the most powerful tools for estimating forest stand Tr (Tr_stand) values. Individual tree Tr values can be upscaled to Tr_stand as a product of stand total sapwood area (AS_stand) and stand mean Fd (JS). In this study, we performed laboratory experiments using Japanese cedar (Cryptomeria japonica D. Don) to examine errors in Tr_stand values generated by mis-estimation of AS_stand and/or JS. We found that simple color identification of the stem hydroactive zone in sapwood and heartwood had little effect on accurate estimation of AS_stand values. Then we examined the effects of mis-estimation of JS, which is primarily the result of Fd measurement failures. Considering that uncertainties in heat-tracer-derived Fd values using the TDM have been identified in numerous previous studies, we applied calibration to improve the accuracy of TDM-Fd derivation using freshly cut sapwood segments of sample Japanese cedar stems and negative pressure as a driving force to determine Fd within the segments experimentally. The calibration experiment revealed that the calibrated conversion equation for heat-tracer-derived Fd values resulted in values that differed significantly from those determined using the original TDM equation and significant underestimation of Tr_stand. The Fd correction increased the annual Tr_stand value from 229 to 593 mm year−1, while the corrected value was not significantly different from the Tr_stand value obtained from catchment-scale water budget observations. The novel TDM calibration equation improved TDM-based Tr_stand estimates dramatically, highlighting the importance of this study's calibration method.