The influence of bamboo culm water content on sap flux measurements with thermal dissipation probes: observations and modeling

Abstract

Water content fluctuations in bamboo culms significantly influence sap flux measurements with thermal dissipation probes, as indicated and quantified by experimental, monitoring and model analyses. Bamboos and other plants may substantially rely on stem water storage for transpiration. Fluctuations in wood water content (θ wood) may lead to errors when estimating transpiration based on sap flux (J s) measurements with the widely used thermal dissipation probe (TDP) method. To test the effects of θ wood on J s, we conducted a culm dehydration experiment, monitored bamboos with TDPs, and implemented a steady-state thermal model. Based on the model simulation, a mathematical correction method was built. Central to the calculation of J s, and thus a major potential source of error, is the maximum temperature difference between probes (ΔT max) which is often referred to as ‘zero flow’ conditions. In the culm dehydration experiment, we observed that ΔT max decreased when θ wood increased. In long-term field monitoring, ΔT max decreased when soil moisture content increased, potentially indicating changes in θ wood and a seasonal decrease in stem water storage. The steady-state model reproduced the θ wood to ΔT max relationship of the dehydration experiment and underlined a considerable sensitivity of J s estimates to θ wood. Fluctuations in θ wood may lead to a substantial underestimation of J s, and subsequently of transpiration, in commonly applied estimation schemes. However, our model results suggest that such underestimation can be quantified and subsequently corrected for with our correction equations when key wood properties are known. Our study gives insights into the relationship between θ wood and TDP-derived J s and examines potential estimation biases.