Declining trees growth and vegetation productivity resulting from decreasing soil water contents induced by tunnels excavation in karst mountain areas

Abstract

Tunnel construction in karst areas greatly impacts the local hydrological conditions. The drawdown of groundwater levels and the loss of soil water storage due to tunneling limit the available water resources on the karst land surface, and this may affect the function of plants. Here, we evaluated the responses of the radial growth rates of two major tree species (Masson pine and Camphor laurel) to a tunneling induced water stress event in a typical karst trough valley. Meanwhile, to discuss the overall influence of tunnel excavation on plant growth in all vegetation cover types, the dynamics of vegetation productivity around the tunnel were evaluated using remote sensing indices, the Normalized Difference Vegetation Index (NDVI), and Net Primary Productivity (NPP). Furthermore, we estimated the spatial–temporal variations of NDVI and NPP in other tunneling regions of Southwest China. The results showed that the soil moisture contents (SMC) around the Zhongliang Mountain Railway Line 6 (ZMRL6) tunnel had dramatically declined −5.06%∼−10.53% within the areas < 1500 m. After the excavation of ZMRL6 tunnel, the basal area increment (BAI) for the tree-ring widths of Masson pine and Camphor laurel within 1500 m of the tunnel central reduced by 94.8%∼135.8% and 116.6%∼139.9%, respectively, while the trees beyond this distance range were less influenced. Meanwhile, the remote sensing indices showed that tunnel excavation can reduce 3.3%∼7.6% of the NDVI and 15.7%∼17.4% of the NPP within 1500 m of the ZMRL6 tunnel. A similar distance range of such declines in NDVI and NPP was also found in other tunneling regions throughout the karst mountain areas of Southwest China. The findings in this study suggest that tunnel excavation may cause severe ecosystem degradation within 1500 m of the tunnel center, which should be considered in ecosystem management and conservation strategies in karst mountain areas.