Estimating groundwater evapotranspiration by a subtropical pine plantation using diurnal water table fluctuations: implications from night-time water use

Exotic pine plantations have replaced large areas of the native forests for timber production in the subtropical coastal Australia. To evaluate potential impacts of changes in vegetation on local groundwater discharge, we estimated groundwater evapotranspiration (ET) by the pine plantation using diurnal water table fluctuations for the dry season of 2012 from August 1st to December 31st. The modified White method was used to estimate the ET, considering the night-time water use by pine trees (T). Depth-dependent specific yields were also determined both experimentally and numerically for estimation of ET. Night-time water use by pine trees was comprehensively investigated using a combination of groundwater level, sap flow, tree growth, specific yield, soil matric potential and climatic variables measurements. Results reveal a constant average transpiration flux of 0.02 mm h at the plot scale from 23:00 to 05:00 during the study period, which verified the presence of night-time water use. The total ET for the period investigated was 259.0 mm with an accumulated T of 64.5 mm, resulting in an error of 25% on accumulated evapotranspiration from the groundwater if night-time water use was neglected. The results indicate that the development of commercial pine plantations may result in groundwater losses in these areas. It is also recommended that any future application of diurnal water table fluctuation based methods investigate the validity of the zero night-time water use assumption prior to use

Effects of earlywood and latewood on sap flux density-based transpiration estimates in conifers

Heat-based sap flux density (SFD) methods have been widely used to estimate the water use by conifers, but complexities arise due to the heterogeneous nature of conifer sapwood with annual rings of earlywood (EW) and latewood (LW), which differ in water- and heat-conducting properties. Laboratory-based controlled flow experiments using freshly cut stem segments from 11 pine trees were undertaken to evaluate the potential impact of hydraulic architecture of conifer sapwood on tree water use estimates from the Heat Ratio Method (HRM) and Heat Field Deformation (HFD) method, by considering different scenarios regarding the hydraulic conductivity and thermal diffusivity of EW and LW. The results show that the actual water flux was systematically underestimated in Scenario 1 (assuming only EW was water-conductive but thermal diffusivity was mean of EW and LW) and Scenario 3 (assuming equal hydraulic conductivity of LW and EW but thermal diffusivity was that of only EW). However, the mean sap flux densities obtained from 11 sample trees after correction by the LW/EW ratios were pretty close to the gravimetrical flow. Assuming equal hydraulic conductivity of LW and EW and mean thermal diffusivity of EW and LW led to either overestimation or underestimation of water use by individual trees, but the mean tree-scale water use was unbiased when including all this variance in the study system. The observed heterogeneous radial SFD variability from the HFD measurements was closely linked with patterns of successive EW and LW, especially in the central parts of the sapwood where higher SFD values were generally observed. The decreasing SFD patterns towards the cambium and heartwood were partially attributed to the decrease in moisture content, tracheid diameter and the increase in wood density of EW and LW compared with the central sapwood. The results indicated that the LW/EW ratio in stems where sap flow probes have been inserted can be measured a posteriori to correct HRM-based sap flow measurements. The sap flux is recommended to be radially corrected using the SFD patterns from HFD sensors measured at the same location of the HRM measurements in the same tree

Xylem hydraulic properties in subtropical coniferous trees influence radial patterns of sap flow: implications for whole tree transpiration estimates using sap flow sensors

Key message: A high spatial resolution dataset of sap flux density in subtropical conifers is used to assess the minimum number and location of sap flow sensors required to monitor tree transpiration accurately. Abstract: Tree transpiration is commonly estimated by methods based on in situ sap flux density (SFD) measurements, where the upscaling of SFD from point measurements to the individual tree has been identified as the main source of error. The literature indicates that the variation in SFD with radial position across a tree stem section can exhibit a wide range of patterns. Adequate capture of the SFD profile may require a large number of point measurements, which is likely to be prohibited. Thus, it is of value to develop protocols, which rationalize the number of point measurements, while retaining a satisfactory precision in the tree SFD estimates. This study investigates cross-sectional SFD variability within a tree and successively for six individual trees within a stand of Pinus elliottii var. elliottii\ua0×\ua0caribaea var. hondurensis (PEE\ua0×\ua0PCH). The stand is part of a plantation in subtropical coastal Australia. SFD is estimated using the Heat Field Deformation method simultaneously for four cardinal directions with measurements at six depths from the cambium. This yields a reference value of single tree SFD based on the twenty-four point measurements. Large variability of SFD is observed with measurement depth, cardinal direction and selected tree. We suggest that this is linked to the occurrence of successive narrow early and latewood rings with contrasting-specific hydraulic conductivities and wood water contents. Thus, an accurate placement of sensors within each ring is difficult to achieve in the field with the sensor footprint covering several rings of both early and latewood. Based on the reference dataset, we identified both an “ideal” setup and an “optimal” setup in terms of cost effectiveness and accuracy. Our study shows the need of using a systematic protocol to optimize the number of sensors to be used as a trade-off between precision and cost. It includes a preliminary assessment of the SFD variability at a high spatial resolution, and only then based on this, an appropriate placement of sensors for the long-term monitoring