Tree rings reveal recent intensified spring drought in the central Himalaya, Nepal
Introduction
The high mountains of Himalayas in south Asia are warming at an alarming rate of 0.6 °C per decade, which is considerably higher than the global average (Shrestha et al., 1999, Shrestha et al., 2012, IPCC, 2013). Increasing sea surface temperatures (SSTs) concurred with global warming and resulted in unusual El Niño–Southern Oscillation (ENSO) extreme events in the late 20th century (Li et al., 2013). Repeated ENSO events have contributed to mega-drought events over Asia due to failure of south Asian summer monsoon (SASM) (Cook et al., 2010, Sano et al., 2012). An overall weakening trend of monsoon precipitation have been revealed on global scale (Wang and Ding, 2006, Zhou et al., 2008, Li et al., 2010) as well as in the Himalayan region (Davtalab et al., 2015). The climate conditions in the central and western Himalaya is dominated by the SASM during summer, and the mid-latitude north-westerly disturbances during winter and spring. Increasing drought in the recent decades has been observed in the central Himalaya due to decreasing boreal winter and spring precipitation (Wang et al., 2013, Cannon et al., 2015, Gaire et al., 2017), and summer precipitation (Sano et al., 2012, Xu et al., 2017). However, the heterogeneous behavior of drought variability in the central Himalayan region and its teleconnection to the global climate variability needs to be investigated in a long-term perspective.
Drought records are of utmost important in the ecologically sensitive regions of the Himalaya (Yadav, 2013). The drought combined with increasing temperature not only alters regional hydrological regimes, but also triggers forest mortality and vegetation die-off (Martínez-Vilalta and Lloret, 2016). Better understanding about the underlying causes of historical droughts can provide deeper insight on possible effects of climate change and vulnerability of human societies to future droughts (Cook et al., 2010). The paucity of long-spanned and spatial representative instrumental records in the Himalayas requires the study of climate history from high-resolution proxy data like tree-rings (Cook et al., 2003, Cook et al., 2010, Sano et al., 2012). Tree-ring data have been used to reconstruct historic droughts and precipitation in the western Himalayan regions (Singh et al., 2009, Yadav, 2013, Yadav et al., 2014, Yadava et al., 2016). However, dendroclimatic reconstructions for the central Himalaya are rather scarce (Cook et al., 2003, Sano et al., 2012, Thapa et al., 2015, Gaire et al., 2017). Cook et al. (2003) established the most comprehensive tree-ring network from 46-sites throughout Nepal and reconstructed February–June and October–February Kathmandu temperature based on multi-species tree-ring chronologies. Sano et al. (2012) reconstructed a summer drought index based on tree-ring δ18O isotope variations from Abies spectabilis and found increasing aridity due to a weakening of SASM over the central Himalaya since past two centuries. Thapa et al. (2015) reconstructed spring temperature from Picea smithiana in the western part of the central Himalaya, Nepal and revealed increasing spring temperature since the 1980s. Gaire et al. (2017) reconstructed tree-ring based March–June precipitation and found decreasing spring precipitation in the north-western part of the Nepal since mid-1970s. Other studies disclosed the importance of spring moisture availability on tree growth (Dawadi et al., 2013, Liang et al., 2014) and highlighted the opportunity to reconstruct historic spring drought events in the central Himalaya (Liang et al., 2014).
Spring moisture availability plays a crucial role for vegetation growth, agricultural production, forest productivity and ecosystem functions. Therefore, study on long-term perspective of spring drought variability and its teleconnection to the ocean-atmospheric circulation is a relevant issue. In this study, we developed a series of new tree-ring width chronologies of Himalayan spruce (Picea smithiana) in the central Himalaya, north-western Nepal and we aimed: 1) to identify the dominant climatic factors limiting radial growth of high-elevation Himalayan spruce, 2) to determine historical drought variations in the central Himalaya, and 3) to evaluate the teleconnections of spring drought variability with the global climatic drivers.
Section snippets
Study area
The study area is located at Rara National Park (RNP) and adjacent forests in the north-western part of the central Himalaya, Nepal (Fig. 1, Table 1). Regional climate is warm and wet during summer, but rather cold and dry during winter, and warm and dry during spring (Putkonen, 2004, Ichiyanagi et al., 2007). Monsoonal air masses originating from Indian Ocean flow north to the Himalayan Mountains and cause summer monsoon rainfall from June to September (Barry, 2008). In contrast, boreal winter
Characteristics of chronology statistics
The three local site-chronologies showed significant inter-site correlations (average r = 0.480) for the common period 1850–2013 (Table 2), indicating a common growth signal under the common environmental forcing (Fig. S1). This is confirmed by the results of principal component analysis (PCA), when PC1 explained 65% of the common variance of the three site chronologies over the common period 1850–2013 (Table S1). Moreover, the three local site-chronologies showed similar tree-growth responses to
Discussion
We found that radial growth of Himalayan spruce were positively (negatively) related to precipitation (temperature) during the spring season. Further correlations with scPDSI confirmed that the spring-moisture availability was a crucial tree-growth limiting factor in the central Himalaya (Fig. 4). The spring drought stress influences the onset of xylogenesis and xylem cell production and finally controls tree-growth of high elevation forests (Bräuning and Grießinger, 2006, Fan et al., 2008,
Conclusions
We presented a new tree-ring chronology of Picea smithiana extending back to 1498 CE, which provides evidence for tree-ring based spring drought variability over the past 289 years (1725–2013 CE) in the central Himalaya. Several mild-to-severe drought episodes occurred in 1730s–1770s, 1801–1804, 1807–1828, 1848–1876, 1892–1898, 1900–1912, 1915–1927, 1930s, 1940s–1960s and 1980s–2013. The dry and wet episodes were generally consistent with previous studies from Himalaya and vicinities. Drought
Acknowledgements
We would like to acknowledge the Department of National Park and Wildlife conservation Nepal and Rara National Park for providing permission to collect tree-ring increment cores. Department of Plant Resources and Ministry of Forest and Soil Conservation, Nepal provided the permission for exporting tree-ring samples from Nepal. Meteorological data were obtained from Department of Hydrology and Meteorology, Nepal. We are grateful to Ram R Yadav, Jayendra Singh, Masaki Sano, and Narayan Gaire for
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