Evaluating a new hydraulic implementation in LPJ-GUESS for three sites in north Europe

Abstract

Drought is projected to increase in frequency and intensity and impacts trees with increased water stress and increased mortality rate. Water stresses can cause hydraulic failure-related mortality or carbon starvation due to tree species having different strategies to deal with water stresses. LPJ-GUESS-HYD (Hydraulic implementation of a new plant hydraulics scheme in LPJ-GUESS) was developed to include strategies plants are taking to deal with drought. The new model is an enhanced version of LPJ-GUESS, introducing parametrizations of hydraulic mechanisms in plants. LPJ-GUESS (Lund-Potsdam-Jena general ecosystem simulator) is a state-of-the-art ecosystem model, which combines a Dynamic Global Vegetation Model (DGVM) with a more detailed representation of vegetation dynamics, to simulate vegetation at a regional scale.
The aim of this study was therefore to evaluate a new version of the dynamic global vegetation model LPJ-GUESS-HYD with an upgraded hydraulic implementation by testing the model results’ accuracy to observed data and the behaviour of mostly hydraulic parameters, never tested in Sweden. A parameter calibration was done to improve the hydraulic model representing GPP (gross primary production) and ET (evapotranspiration). After two calibrations the model was improved with a final mean RRMSE of around 40% aggregated using all sites and model outputs (ET and GPP). The hydraulic model underestimated ET while the standard version represented ET fluxes better, with a lower site mean RRMSE compared to observations. The hydraulic model predicted GPP better than ET, even if the model tends to overestimate the carbon fluxes for HTM and NOR. A One At a Time (OAT) sensitivity test of LPJ-GUESS-HYD was done to evaluate which parameters cause the highest variability in model outputs representing GPP and ET fluxes. The yearly mean changes during the simulation period (2010-2019) of carbon and water fluxes showed high sensitivity to isohydricity (λ), optimal forcing pressure to maintain canopy conductance (ΔΨmax), maximum sapwood conductivity (Ksmax), water potential representing 50% loss of conductance (Ψ50) and the ratio of intercellular to ambient CO2 (λmax). Cavitation slope (d) (how fast air bubbles form in the tree xylem and prevent water from being pulled upward) and the proportion of resistance located below ground and above ground (b) showed negligible sensitivity for all sites. Recommendations for further studies are varied and range from testing parameter interactions to including additional parameters in an improved sensitivity test and calibration. It is crucial to further evaluate the hydraulic model to improve the water fluxes predictions and to go beyond the scope of the current observations to strengthen climate change prediction, including how ecosystems react to increase drought conditions in future scenarios.

Popular Abstract

The forest plays a major role in mitigating the ongoing climate change but is negatively affected by drought caused by climate change. For example, the trees will be affected by closing their stomata openings and reducing their carbon dioxide uptake. In the future, droughts are expected to become increasingly common, and therefore new ecosystem models are needed to predict how forests' carbon and water flows will be affected.
Modelling different ecosystem flows is an effective tool for finding out how a changing climate affects carbon and water flows. To deal with more common droughts in future climate a new upgraded model was built trying to include different strategies tree species have. This study evaluated a new hydraulic version of LPJ-GUESS (Lund-Potsdam-General ecosystem simulator) ability to represent the amount of carbon fixed in photosynthesis, GPP (Gross primary production) and the amount of water vapour the forest emits, ET (Evapotranspiration). The new hydraulic model never tested before in Sweden, is compared with a standard version of LPJ-GUESS by studying the results of ET and GPP. Three different forests in Sweden (Hyltemossa, Norunda and Svartberget) were chosen based on where carbon and water flows are currently measured. By comparing model values of ET and GPP and observation values over a period from 2014-2019, the new model could be calibrated with gradual changes in the model's built-in hydraulic parameters. By gradually changing the parameters one at a time and observing the change in ET and GPP, a sensitivity analysis could be done for the new model. Sensitivity analysis was important to find out which of the parameters affected ET and GPP to gain a deeper understanding of the model.
The new hydraulic model's representation of ET was strongly underrepresented compared to the observations for all three locations in Sweden, especially for Svartberget. After calibration was done, the new hydraulic model's representation of ET could be improved by increasing the ET values and being closer to the observation values. For GPP, model values were closer to observational values even before calibration. The standard version of the model represented GPP and ET generally closer to the observations. The sensitivity study resulted in five out of seven tested parameters being sensitive, which meant a noticeable change in ET and GPP with changed parameter values. For example, the parameter for isohydricity, the tendency to close the leaves' stomata early in drought or to continue to keep them open as far as possible, was sensitive to GPP and ET. In other words, allowing the trees to continue their photosynthesis to a greater or lesser extent under dry soil conditions caused changes in GPP and ET. Isohydricity is an important parameter for understanding how different tree species react in water-stressed situations and therefore gradually changed isohydricity resulted in changed GPP and ET was important for understanding the model. An example of a parameter with low sensitivity was the cavitation slope, the parameter that indicates how quickly air bubbles form in the trunk, which prevents water from being pulled up and in some cases causes the tree to die. When the cavitation slope was changed one at a time cavitation was not present, and no effect could be seen on ET and GPP.

• author: Holmquist, Carl
• supervisor: Thomas Pugh; Annemarie Eckes-Shephard
• organization: Dept of Physical Geography and Ecosystem Science
• course: NGEM01 20221
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Physical Geography and Ecosystem analysis, LPJ-GUESS, drought, sensitivity analysis, evapotranspiration, gross primary production, hydraulic parameters, boreal forest.
• publication/series: Student thesis series
• report number: 629
• language: English
• id: 9136934
• date added to LUP: 2023-09-08 14:03:15
• date last changed: 2023-09-08 14:03:15