An improved IBIS model for simulating NPP dynamics in alpine mountain ecosystems: A case study in the eastern Qilian Mountains, northeastern Tibetan Plateau

Abstract

The Integrated Biosphere Simulator (IBIS) model is an effective means of simulating potential vegetation dynamics driven by climate change. However, this model neglects certain key processes and the influences of terrain and soil thickness in alpine mountain ecosystems, which leads to a poor accuracy of net primary production (NPP) simulation. Taking the eastern Qilian Mountains as a case, we revised the IBIS model by integrating terrain influences, water distribution processes, and vegetation and soil characteristics. Terrain effects on solar radiation and precipitation intensity were executed using a hill-shade value and a terrain correction factor, respectively. Then, the re-infiltration process of surface runoff within a pixel was integrated into water distribution sub-module. In the revised model (IBISi), soil hydraulic conductivity was dynamically updated based on the saturated hydraulic conductivity of soil texture and soil-pore water content, instead of a constant value. Inverse migration of soil water due to surface evaporation was also integrated to simulate its impacts on soil moisture. Considering spatial heterogeneity of soil thickness in alpine mountains, the IBISi adopts a thickness-based soil layer structure instead of a 4-meter one. The leaf area index base in herbaceous areas was optimized by a germination coefficient. A measured CO2 concentration series was utilized as an alternative to the calculated values by model prediction. Meanwhile, the key parameters of alpine mountain ecosystems were set based on field survey from the case study area. The NPP values simulated by the IBISi model were compared with the ones measured by remote sensing in areas with relatively less human activities. The results indicated that the IBISi model had a high reliability and accuracy. Thus, it is a good option for simulating NPP dynamics in alpine mountain ecosystems under climate change, and could also provide a scientific basis for assessing human activity impacts at the regional scale.