Tibetan Railway Induced Slight Carbon Loss: Photosynthetic Capacity Decrease Offsets Growing Season Length Increase in Grasslands

Abstract

The Tibetan Railway has introduced pressures on the fragile grassland ecosystems of the Tibetan Plateau. However, the impact of the railway on the carbon sequestration remains unclear, as existing studies primarily focus on in-situ vegetation observations. In this study, we employed the satellite-based Normalized Difference Greenness Index (NDGI) as a proxy for Gross Primary Productivity (GPP) to generate regional GPP datasets along the railway corridor. We extracted the start and end of the growing season (SOS, EOS) and maximum daily GPP (GPPmax) from the GPP data and quantified the extent and intensity of the railway's disturbance on these indicators. We further employed the Statistical Model of Integrated Phenology and Physiology (SMIPP) to translate these disturbances into annual cumulative GPP (GPPann). Results show that the Tibetan Railway significantly influences phenology (SOS and EOS) and physiology (GPPmax) for grasslands within 50 meters, with an earlier SOS (advance rate: 0.1086 d m-1), delayed EOS (delay rate: 0.0646 d m-1), and a reduced GPPmax (decline rate: 0.0069 gC m-2 d-1 m-1) as the distance to the railway gets closer. The advancement of SOS and the delay of EOS contributed gains of 28.82 MgC y⁻¹ and 104.26 MgC y⁻¹, respectively, while the reduction in GPPmax accounted for a loss of 2952.79 MgC y⁻¹. The trade-off between the railway’s influences on phenology and physiology, lead to a net loss of 2819.71 MgC y⁻¹ in GPPann. This study reveals Tibetan Railway's impact on grassland carbon cycling, offering insights for grassland conservation and sustainable transportation infrastructure projects.

Keywords: Tibetan Plateau, Tibetan Railway, Carbon Sequestration, Phenology, GPPmax, SMIPP Model