A 307-Year Tree-Ring Spei Reconstruction Indicates Modern Drought in Western Nepal Himalayas

Abstract

Western Nepal has experienced a severe drought in the past two decades, but observation records across Nepal are too short to place the recent drought in a longer context to understand the full range of natural variability in the climate system. In the present study we have collected tree core samples of Tsuga dumosa from two sites, Chhetti and Ranghadi, in the Api Nampa Conservation Area of the western Nepal Himalayas to understand drought variation for the past three centuries. We have developed a 357-year (AD 1657-2013) tree-ring chronology. The tree growth-climate response analysis revealed a stronger positive correlation with spring (March-May) standardized precipitation evapotranspiration index (SPEI01) (r = 0.523, p < 0.01) than precipitation (r = 0.459, p < 0.01), self-calibrating Palmer drought severity index (scPDSI) (r = 0.250, p < 0.01), or temperature (r = -0.486, p < 0.01). Stronger positive correlation with SPEI01 indicates moisture availability is the limiting factor for the growth of this species on these sites. Based on this growth-climate response we reconstructed spring SPEI from AD 1707 to 2013 for the region. The reconstruction showed several dry and wet episodes indicating no persistent climate trend within the past three centuries. The current drought is one of the four most severe in our 307-year record.