TRIPLEX-mortality model for simulating drought-induced tree mortality in boreal forests : model development and evaluation

Abstract:

Across the globe an increasing drought-induced tree mortality rate due to climate change is projected to generate far-reaching effects on forest ecosystems. The boreal forest, a ‘tipping element’ in the Earth’s climate system, plays a critical role in ecosystem services, structures and functions and has a high sensitivity to drought stress. Process-based models are important tools for helping ecological research, however, there is, to date, no robust model available that integrates state-of-the-art physiological mechanisms to simulate the impacts of drought-induced mortality in boreal forests. For the first time, this study has successfully incorporated two state-of-the-art mechanisms of drought-induced mortality (e.g. hydraulic failure and carbon starvation) into a new process-based TRIPLEX-mortality model. We conducted calibration and validation analysis, using data from 73 permanent sample plots (PSPs) across Canada’s boreal forests. Results show that model simulations are in good agreement with observations (R²=0.79; P<0.01; AI=0.94), suggesting a good performance of the model on simulating drought-induced mortality in boreal forests. Sensitivity analysis indicates that parameter sensitivity increases as the drought intensifies, and the shape parameter (c) for calculating percentage loss of conductivity (PLC) is the most sensitive parameter (average SI = -3.51) to simulate tree mortality. Results from sensitivity analysis of model input show that the model can capture mortality changes under different drought scenarios. Consequently, our results suggest that our model is suitable for simulating drought-induced mortality in the boreal forests and can provide new insights to improve Earth system model simulations for tree mortality and corresponding carbon dynamics under a warmer and drier world.