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

TREE-RING RESEARCH, Vol. 75(2), 2019, pp. 73–85 DOI: http://dx.doi.org/10.3959/1536-1098-75.2.73

A 307-YEAR TREE-RING SPEI RECONSTRUCTION INDICATES MODERN DROUGHT IN WESTERN NEPAL HIMALAYAS

SANJAYA BHANDARI1,2, NARAYAN PRASAD GAIRE3,4, SANTOSH K. SHAH5, JAMES H. SPEER2*, DINESH RAJ BHUJU1,3, and UDAY KUNWAR THAPA6 1Central Department of Environmental Science, Tribhuvan University, Kirtipur, Kathmandu, Nepal 2Department of Earth and Environmental Systems, Indiana State University, Terre Haute, IN, USA 3Nepal Academy of Science and Technology, Lalitpur, Nepal 4Key Lab of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun, Mengla, Yunnan, PR China 5Birbal Sahni Institute of Palaeo Sciences, 53-University Road, Lucknow, India 6Department of Geography, Environment and Society, University of Minnesota, Minneapolis, MN, USA

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. Keywords: Nepal, SPEI, precipitation, dendroclimatology, dendrochronology, Himalaya, drought, Tsuga dumosa.

INTRODUCTION Nepal show a rapid increase in temperature and fluctuation in precipitation patterns (Shrestha and The Himalayas extends over 2400 km from Aryal 2011; Shrestha et al. 2012). The complex to- east to west and across ecologically and geopoliti- pography of the mountain range and short dura- cally significant boundaries that are highly vulner- tion of these records requires proxy reconstructions able to the impacts of the rapidly occurring cli- to truly understand climate change dynamics in this mate change (IPCC 2014). The eastern Himalayas region of the world (Cook et al. 2003; Sigdel et al. are more influenced by summer monsoon whereas 2018a). Interestingly, the diverse topography and its western region is more influenced by the west- diverse climate of the Himalayas have bestowed erlies (Zurick et al. 2005). The eastern region of it with rich natural archives of climate including the Nepal Himalayas is relatively moist compared tree rings, ice cores, and lake sediments. Among all to the western region. Available temperature data these archives, tree rings have the highest resolution since 1970 and precipitation data since 1960 in with the annual accuracy necessary for comparison of monthly and annual meteorological data (Fritts *Corresponding author: [email protected] 1976; Speer 2010). Similarly, there is a decreasing

Copyright © 2019 by the Tree-Ring Society 73 74 BHANDARI, GAIRE, SHAH, SPEER, BHUJU, and THAPA temperature gradient from north to south in the decades (Thapa et al. 2015). Gaire et al. (2017) re- Himalayas (Zurick et al. 2005) and decreasing pre- ported decreasing March-June precipitation in the cipitation from east to west across Nepal (Liang Rara National Park in recent decades. Recently- et al. 2014; Sigdel et al. 2018b). These gradients are reconstructed spring drought using tree-ring width reflected in the climate sensitivity of Himalayan tree of Picea smithiana showed intensified drought since species (Bhattacharyya and Shah 2009; Shah et al. 1980 in the central Himalayas (Panthi et al. 2017). 2014). Most of the studies in the Nepal Himalayas are Tree-ring based studies have been successfully still fragmentary and have narrow spatial coverage. carried out in the region including the western Therefore, it is necessary to increase the number of and eastern Himalayas and the Tibetan Plateau. studies in the region to capture the local to regional In the western Himalayas, temperature (Yadav et variation in hydroclimate. al. 1997, 1999; Yadav and Singh 2002; Zafar et al. Previous studies (Bhattacharyya et al. 1992; 2015), precipitation (Singh and Yadav 2005; Yadav Cook et al. 2003) have confirmed the dendrocli- et al. 2014), and stream flow (Cook et al. 2013; matic potential of Tsuga dumosa (D. Don) Eicher Shah et al. 2013; Singh and Yadav 2013) of cen- (commonly called Himalayan hemlock) in Nepal. tennial to millennial scale have been reconstructed. T. dumosa is an evergreen tree that is found in Similarly, in the eastern Himalayas, temperature humid valleys, mountains slopes, and wetter areas. (Bhattacharyya and Chaudhary 2003; Krusic et al. In the Nepal Himalayas, it occurs in the temperate 2015; Borgaonkar et al. 2018), precipitation (Sano zone, from east to west at altitudes of 2100 to 3600 et al. 2013), and stream flow (Shah et al. 2014) m a.s.l. (Devkota 2013). The present study is an have been reconstructed. Considerable progress in attempt to build a long tree-ring chronology of dendroclimatic reconstructions has also been made T. dumosa to understand its growth response to in the northern Himalayas on the Tibetan Plateau climate and to infer the past drought history in

(Bräuning 2001; Liu et al. 2006; Gou et al. 2007; the Api-Nampa Conservation Area. This study Huang and Zhang 2007; Liang et al. 2009; Zhang will help to strengthen the tree-ring chronology et al. 2014). These regional climate reconstructions network (Gaire et al. 2013; Thapa et al. 2017) of have shown spatiotemporal variations in tempera- Nepal. By comparing with other studies in the ture and precipitation, which are consistent with Nepal Himalayas, the response of multiple species regional, continental, and global patterns (Cook to climate from a region will help to understand et al. 2010; PAGES 2k Consortium 2013; Shi et al. climate signals in the Nepal Himalayas, which can 2015). capture influences of heterogeneous and complex Compared to other regions, very few dendro- geographies. climatic reconstructions are present in the Nepal Himalayas (Cook et al. 2003; Sano et al. 2005, METHODS 2012; Thapa et al. 2015; Gaire et al. 2017; Pan- Site Description and Sample Collection thi et al. 2017; Gaire et al. 2019). Cook et al. (2003) reconstructed temperature (February-June Tree cores of T. dumosa were collected and October-February) for all of Nepal using 32 from two forest patches, Chhetti (29°47.364N tree-ring chronologies. This study does not cap- and 81°00.845E) and Ranghadi (29°47.195Nand ture the regional and local climatic heterogeneity 80°58.890E) of Api Nampa Conservation Area in that exists because of the highly diverse topogra- the far western Nepal Himalayas (Figure 1). We phy of the nation. Sano et al. (2012) have reported cored the trees at breast height using an increment increasing aridity in the western Nepal Himalayas borer with normally two cores per tree (Fritts 1976; based on oxygen stable isotope analysis of Abies Speer 2010), but very steep slopes sometimes re- spectabilis. A reconstructed spring temperature us- quired that we could only take a single core per ing ring-width measurements of Picea smithiana tree. All together, 89 cores were collected from 49 revealed that there was no continuous tempera- trees. Samples were collected at 2700−2800 m a.s.l. ture change in the western Nepal Himalayas since on north-facing slopes in May 2014. T. dumosa was the mid-16th Century until warming in the recent found in the mixed forest of Abies spectabilis, Acer Drought Reconstruction in Western Nepal 75

Figure 1. Map showing location of sampling sites (Chhetti and Ranghadi) in Api-Nampa Conservation Area (ANCA) and meteorological stations (Darchula and Jumla in Nepal and Mukteshwar in India). sp., Betula utilis, Rhododendron sp., Taxus baccata, the signal-free standardization process (Melvin and

and Quercus semecarpifolia. Briffa 2008). This relatively stiff spline along with the signal-free technique improves the preservation Chronology Development of low-frequency signal in tree-ring chronologies. The collected tree core samples were brought The EPS value of 0.85 was taken as a threshold to to the lab and were air dried at room tempera- determine the adequacy of sample depth and corre- ture. After 2-3 days, samples were fixed on wooden sponding reliability of the chronology length in the core mounts with water-soluble glue with the cross- study (Wigley et al. 1984). sectional view facing up. Then the samples were sanded and polished with progressively finer grits Climatic Data of sandpaper (ANSI 120, 220, 320, 420, and 600 We collected climatic data from three me- grit, which range from 105 to 13µm) until the tree- teorological stations. The closest meteorological ring boundaries were clearly visible under a stereo- station is Darchula (29°51N, 80°34E, 1097 m zoom microscope with 10-40X magnification (Orvis a.s.l.), which is ca. 40 km west of the study site. The and Grissino-Mayer 2002). Ring widths were mea- available precipitation and temperature data for the sured using a LINTAB-5 measurement system at- station covered 26 and 40 years, respectively. The tached to a PC having the TSAPwin computer pro- next closest stations are Jumla (29°17N, 82°10E, gram (Rinn 1996). The ring-width series were cross- 2300 m a.s.l.) and Mukteshwar (29°28N, 79°39E, dated graphically and statistically in the TSAP- 2171 m a.s.l.), which are ca. 120 km to the east and win program (Rinn 1996). The computer program southwest, respectively, from the sampling sites. COFECHA (Holmes 1983) was also used to check Jumla station has records of temperature data for the quality of measurement and crossdating. The over 40 years (AD 1969–2013) and precipitation tree-ring series were standardized to remove the data for over 50 years (AD 1957–2013). The next age-related trends and to determine the common station in Mukteshwar, India, has temperature signal between individual series. We used cubic and precipitation for more than 100 years (AD smoothing splines that were 2/3 the length of each 1897–2013). We acquired the temperature and series to detrend the cores and followed that with precipitation from the KNMI Explorer monthly 76 BHANDARI, GAIRE, SHAH, SPEER, BHUJU, and THAPA

Figure 2. Climograph of three different meteorological stations near our study sites. station data, which draws data from the Global correlation coefficient was calculated between the Historical Climate Network (GHCN version 3). ring-width chronologies and monthly climate data Previous studies (Sano et al. 2005; Thapa et al. for 14-month windows starting from previous 2015) have used temperature data from the Muk- September to current October for the period AD teshwar station for temperature reconstructions. 1897–2013. We also correlated the March-May To establish the tree-growth climate relationship average temperature, SPEI01, scPDSI, and to- and to verify our reconstructions we used the tal monthly precipitation with our ring-width longest available climate data available from the chronology. Mukteshwar station. The temperature and precipitation patterns from the Mukteshwar station SPEI reconstruction are similar to the other two stations (Jumla and Darchula), but they vary in magnitude (Figure 2). The reconstruction of average Standard Pre- We calculated both scPDSI and SPEI01 datasets cipitation Evapotranspiration Index (SPEI01) for using temperature and precipitation records of the spring months (March-May) was based on Mukteshwar meteorological station for 1897– significant correlations with these months inour 2017 to determine correlation with our tree-ring growth-climate model. We used a simple linear re- records. gression model to relate tree-ring width to sea- sonal climate and tested the time stability of that model by dividing the period into two equal sub- Tree-Growth and Climate Relationship periods for calibration (AD 1898–1955) and verifi- We carried out tree-growth climate analysis cation (AD 1956–2013, Snee 1997). Correlation co- by examining correlations between our tree-ring efficient (r), reduction of error (RE), and coefficient index chronology of T. dumosa against the climatic of efficiency (CE) were tested to check the signifi- variables (temperature, precipitation, SPEI01, and cance and reliability of the fitted model. Once the scPDSI) from Mukteshwar station. The Pearson model was judged effective and stable, we applied Drought Reconstruction in Western Nepal 77

Figure 3. (a) Ring-width index chronology of Tsuga dumosa; (b) Number of cores used to develop this chronology; (c) EPS threshold greater than 0.85 since 1707; (d) Rbar graph from AD 1657 to 2013.

it to estimate SPEI01 based on tree-ring width and (1657–2013) chronology of T. dumosa from the Api- truncated the reconstruction at the point where the Nampa Conservation Area (Figure 3). The sam- EPS value dropped below 0.85 (Wigley et al. 1984). ples that were not included were rejected because The reconstructed SPEI01 was examined in KNMI of poor quality of the cores. The age of trees in Climate Explorer (Trouet and Oldenborgh 2013) our chronology ranged from 76 to 357 years with and plotted in Panoply (Schmunk 2018) to iden- a mean length of 221.6 years and average annual tify the spatial coverage of our tree-ring chronology. radial growth of 1.005 mm. The chronology statis- We also compared our reconstruction with other tics (interseries correlation = 0.636, mean sensitiv- regional reconstructions produced from previous ity = 0.271) showed the dendroclimatic potential of studies. T. dumosa (Table 1). We found two major tree-growth periods, i.e. an increase in the tree growth in the early 18th Cen- RESULTS AND DISCUSSION tury and second half of 20th Century, although tree growth is suppressed in the recent years. We Tree-Ring Width Chronology did not find the two major anomalies (extensive We collected 89 cores from 49 trees and used growth suppression in the early 1800s and ele- 57 cores from 37 trees to develop a 357-year-long vated growth in recent years) in our chronology as 78 BHANDARI, GAIRE, SHAH, SPEER, BHUJU, and THAPA

Table 1. Chronology statistics of Tsuga dumosa from the Api- from Tibetan plateau (Gou et al. 2007; Liang et al. Nampa Conservations Area. 2010).

Number of dated series 57 Master series (years) 357 Total rings in all series 12633 Tree Growth-Climate Relationships Total dated rings checked 12630 Interseries-correlation 0.636 We calculated the correlation between tree- Average mean sensitivity 0.271 ring index and the climatic data from both Jumla Segments, possible problems 2 and Mukteshwar stations. The correlation between Mean length of series 221.6 ring width and Mukteshwar climatic data was stronger than with the Jumla climatic data. In ad- dition, Mukteshwar data are longer so we used this station for further analysis. Measured ring reported by Thapa et al. (2017) in growth of widths showed significant negative relationships (r conifers in the Nepal Himalaya. This might be be- = -0.486) with the temperature of pre-monsoon cause their analysis is based on multiple records months (March-May), a positive relation was found from diverse environments and different species. In with the precipitation (r = 0.486), scPDSI (r = contrast, tree-ring growth has increased in the late 0.250), and SPEI01 (r = 0.523) of the same pre- 20th Century in western India (Singh and Yadav monsoon months (Figure 4). This indicates that 2000; Borgaonkar et al. 2009) and some conifers moisture in the pre-monsoon months is the most

Figure 4. Top: Histogram showing correlations between the ring-width index chronology of Tsuga dumosa and climate data (AD 1897-2013) from Mukteshwar station, India. Bottom: Histogram showing correlation between ring-width index, scPDSI, and SPEI. Drought Reconstruction in Western Nepal 79

Figure 5. (a) Actual and estimated March-May SPEI01 from AD 1897 to 2013, and (b) Spring reconstructed SPEI along with a 10-year low pass filter for the period AD 1707-2013. limiting factor for the growth of T. dumosa in we decided to develop a linear regression model the western Nepal Himalayas. In spring months, to reconstruct March-May SPEI01 for 307 (1707– the temperature is high while precipitation is low 2013) years (Figure 5). Our reconstruction ex- (Figure 2), which results in more evapotranspira- plains 27.3% variance for the full calibration pe- tion and limits the growth of trees (Dawadi et al. riod and has positive values for both reduction 2013; Gaire et al. 2017; Panthi et al. 2017). The of error (RE) and coefficient of efficiency (CE) growth of conifers in Nepal has been shown to have (Table 2), indicating that the model is robust and a positive relationship with precipitation and a neg- reliable. ative relationship with temperature in other studies The reconstructed SPEI01 contains several as well (Sano et al. 2005; Thapa et al. 2013; Thapa dry periods during AD 1741–1750, 1781–1789, et al. 2015; Tiwari et al. 2017). A similar response of 1950–1959, and 2009–2013 and wet periods during tree-ring growth has been reported from the western AD 1733–1739, 1819–1823, 1850–1853, and 1912– Himalayas in India (Borgaonkar et al. 1996; Yadav 1914 in the past three centuries. A long dry pe- et al. 2014), Pakistan (Ahmed et al. 2011, 2012), and riod was observed from 1921–1960 whereas 1975– the central Hengduan Mountains in China (Fan 1993 was the longest wet period. Within the past et al. 2008). three centuries the driest years were in the 18th Century (AD 1715, 1741, 1782), and 1921 whereas AD 1737, 1738, 1903, and 2001 were the wettest Spring SPEI Reconstruction years. Spring SPEI01 from the Mukteshwar station Adhikari (2018) documented drought-induced has the strongest relationship with tree growth, so agricultural losses since 1972 across Nepal, and 80 BHANDARI, GAIRE, SHAH, SPEER, BHUJU, and THAPA

Table 2. Calibration and verification statistics for our March- et al. (2014) has also shown dry periods in 1740s May SPEI reconstruction. PC = Pearson correlation; RC = Ro- and 1780s along with wetter periods in the early bust correlation; SC = Spearman correlation; RE = Reduction 1910s, late 1970s, and 1980s in Kumaon Himalayas, = of Error; CE Coefficient of Efficiency. India. The dry period in the 1780s and the long- Calibration Verification Calibration Verification term moist period from 1975-1993 are also ob- Test (1956–2013) (1898–1955) (1898–1955) (1956–2013) served in Himachal Pradesh, India (Singh et al. Undifferentiated data 2009). PC 0.505 0.532 The spatial field correlation of our recon- RC 0.492 0.529 structed spring SPEI01 showed that our recon- SC 0.473 0.499 struction captured drought variation in the west- RE 0.249 0.270 CE 0.280 0.252 ern Himalayas (Figure 7). We used multi-taper power spectrum analysis (Mann and Less 1996) 1st differentiated data PC 0.446 0.434 to understand spring SPEI01 in the frequency do- RC 0.444 0.380 main and found significant high-frequency peaks SC 0.430 0.357 2.2-4.9 and 10.2 years (Figure 8). Such high- RE 0.189 0.199 frequency peaks have been seen in spring precipi- CE 0.188 0.199 tation reconstructions from western Nepal (Gaire et al. 2017), spring drought from central Nepal (Panthi et al. 2017), and a rainfall reconstruction from northwest India (Singh et al. 2009). This found that since 2008 Nepal has experienced se- short-term period coincides with the range of El vere drought almost every year, resulting in over Nino- Southern Oscillation (ENSO), which could 1.5 metric tons of crop loss during this period. be one of the factors driving drought or moisture

Adhikari (2018) identifies climate change as a in the Himalayas (Yadav et al. 2014; Gaire et al. major driver for these consistent drought con- 2017). ditions and notices that drought conditions are exacerbated by a vulnerable society because of CONCLUSIONS unequal social structure, gender discrimination, and marginalization of the poor. Our reconstruc- We added a new tree-ring record of T. dumosa tion shows an ongoing drought starting in 2007 spanning more than three centuries from the far and extending to the end of our chronology remote western Himalaya to the Nepal tree-ring in 2013. network. The tree growth-climate analysis reveals We compared our reconstruction with the past that the growth of this species is mainly limited by 289-year spring drought variation in the central spring moisture availability as is evident by having Nepal Himalayas (Panthi et al. 2017) and other strong positive relationships between tree growth regional reconstructions (Figure 6). Our recon- and March-May SPEI01. We have presented a 307- struction does not perfectly resemble the Panthi yr SPEI01 reconstruction for the first time in the et al. (2017) PDSI reconstruction, but some of the Nepal Himalaya, which helps us provide long-term long-term similarities, such as drought in 1740s, context to the recent changes in moisture in west- 1856-1861, 1930s, 1940s-1960, and 2009-2013 and ern Nepal. Within the past three centuries, the mod- moisture in 1977-1981, are present in both stud- ern drought is the deepest of our record based on ies. Some of the drier (1920s, 1950s) and wetter the 10-year running average. Our tree-ring record (early 1850s, early 1910s, mid 1970s-early 1980s) ends in 2013 in suppressed growth, which we doc- periods are also observed in the spring precipi- umented as one of the four most severe drought tation reconstruction from western Nepal (Gaire periods in our 307-year record. Because Nepal et al. 2017). The 2009-2013 event is one of the is experiencing rapid changes in climate, particu- four most extreme droughts in our 307-year record, larly in the higher elevations, tree-ring reconstruc- but not outside the natural range of variability. tions help to put climate change into a longer Pre-monsoon precipitation reconstructed by Yadav perspective. Drought Reconstruction in Western Nepal 81

Figure 6. Comparison of current SPEI01 reconstruction with other regional scPDSI and precipitation reconstructions with grey bars indicating the four most severe droughts in our reconstruction. (a) Spring SPEI reconstruction from the western Nepal Himalayas (this study), (b) Spring scPDSI reconstruction from the central Nepal Himalayas (Panthi et al. 2017), (c) Spring precipitation reconstruction from the western Nepal Himalayas (Gaire et al. 2017), (d) February-May precipitation reconstruction from the Kumaon Himalayas, India (Yadav et al. 2014), (e) March-July precipitation reconstruction from Himachal Pradesh, India (Singh et al. 2009).

ACKNOWLEDGMENTS the fieldwork through the Pro-Nature Fund from Japan. We thank the Department of National Parks The WWF Nepal partially supported the and Wildlife Conservation and the Api-Nampa first author with a research grant to conduct this Conservation Area for giving permission to collect study. Resource Himalaya Foundation supported the samples. The Nepal Academy of Science and 82 BHANDARI, GAIRE, SHAH, SPEER, BHUJU, and THAPA

Figure 7. March-May average reconstruction with March-May average CSIC SPEI01 from Mukteshwar (1907–2013), p < 10%.

Figure 8. Multi-taper power spectrum of the reconstructed spring SPEI01. Drought Reconstruction in Western Nepal 83

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