Optimization of canopy conductance models from concurrent measurements of sap flow and stem water potential on Drooping Sheoak in South Australia

Abstract

Canopy conductance (g c ) is a critical component in hydrological modeling for transpiration estimate. It is often formulated as functions of environmental variables. These functions are climate and vegetation specific. Thus, it is important to determine the appropriate functions in g c models and corresponding parameter values for a specific environment. In this study, sap flow, stem water potential, and microclimatic variables were measured for three Drooping Sheoak (Allocasuarina verticillata) trees in year 2011, 2012, and 2014. Canopy conductance was calculated from the inversed Penman-Monteith (PM) equation, which was then used to examine 36 g c models that comprise different response functions. Parameters were optimized using the DiffeRential Evolution Adaptive Metropolis (DREAM) model based on a training data set in 2012. Use of proper predawn stem water potential function, vapor pressure deficit function, and temperature function improves model performance significantly, while no pronounced difference is observed between models that differ in solar radiation functions. The best model gives a correlation coefficient of 0.97, and root-mean-square error of 0.0006 m/s in comparison to the PM-calculated g c . The optimized temperature function shows different characteristics from its counterparts in other similar studies. This is likely due to strong interdependence between air temperature and vapor pressure deficit in the study area or Sheoak tree physiology. Supported by the measurements and optimization results, we suggest that the effects of air temperature and vapor pressure deficit on canopy conductance should be represented together.

Effects of terracing on soil water and canopy transpiration of Pinus tabulaeformis in the Loess Plateau of China

“…Studies have shown that the effectiveness of stomatal conductance induced by VPD fluctuation could result in the variation of transpiration rate (Addington et al, 2004;Igarashi et al, 2015), and a decline in canopy conductance with increasing VPD is an indicator of physiological restrictions to transpiration (Chang et al, 2014a;Shen et al, 2015). This occurred to avoid any highly negative values of leaf water potential and the subsequent xylem cavitation (Addington et al, 2004;Wang et al, 2014b). When soil conditions are severely stressed or in a prolonged period of VPD tension, it is inevitable that the varying degrees of embolisms can be caused by runaway cavitation (Vergeynst et al, 2015), which could trigger a chain reaction, such as reducing water transport, and stomatal closure (Pataki et al, 2000).…”

Modeling the environmental controls on tree water use at different temporal scales

“…Therefore, as discussed in previous work (Wang et al, 2014;Yang et al, 2013) stem 147 water potential was used as an indicator of root-zone soil water availability. We 148 conducted measurements on four Drooping Sheoak (Allocasuarina verticillata) trees over 149 different time periods in 2011, 2012 and 2014.…”

“…The discussion in this study is based on 150 one tree with continuous measurements in January to April and October to December in 151 2012. Data from the other three trees covered shorter periods, and were mainly for 152 consistency check on results of canopy conductance modelling among trees in a previous 153 work (Wang et al, 2014), and not included in this study.…”

“…Equations (3-6) are the selected stress functions for the four variables based on a 195 previous study (Wang et al, 2014). …”

Assessing the environmental controls on Scots pine transpiration and the implications for water partitioning in a boreal headwater catchment

“…Whilst such studies are 30 common in semi-arid regions, they are limited in boreal zones, therefore, our findings are a 31 valuable contribution to understanding plant-water relations in a changing environment. 32 T is regulated by stomata behavior constrained by both atmospheric conditions and soil water 49 availability (Whitehead, 1998;Buckley et al, 2003;Wang et al, 2014). The mechanisms have 50 been extensively explored at multiple scales from stomata and leaf to whole tree and ecosystem 51 levels (Jarvis and McNaughton, 1986;Schulze et al, 1994;Bovard et al, 2005).…”