Many studies from the northeastern part of Tibetan Plateau have demonstrated that tree growth was sensitive to hydroclimatic variability (Gao et al. , 2013 ). Many tree‐ring‐based precipitation, PDSI and streamflow reconstructions have been performed in this region (e.g. Gou et al. , 2010 ; Chen et al. , 2013 ; Deng et al. , 2013 ). In addition, positive correlations with precipitation and negative correlations with temperature, particularly for the growing season, were also found in previous studies (Liang et al. , 2010 ). As shown in the Anfodillo et al. ( 1998 ) and Fritts ( 1962 ) research, the combination of high temperatures and low precipitation prior to the growing season results in less accumulation of photosynthetic matter for the second year. Also, the climatic conditions in the current early growing season have impacts on the early wood width, which in turn, mostly determines the whole ring width. Based on the above analyses and observations, we affirm that moisture is the main limiting factor for tree growth in the northeastern Tibetan Plateau.

The correlations between the WQF chronology and the climate data indicate that the growth of Qilian Juniper in the western Qilian Mountains was mainly limited by the moisture conditions. This is in agreement with the physiological characteristics of trees living in arid regions and was demonstrated in previous studies in this area. All of the previous dendroclimate studies in the western Qilian Mountains suggest that tree‐rings are sensitive to precipitation or PDSI variability (Tian et al. , 2007 ; Liang et al. , 2009 ; Liu et al. , 2009 ; Yang et al. , 2010 ). In this study, we found that the WQF chronology shows positive correlations with the scPDSI and precipitation for most months from June in the previous growing season to August in the current year as well as negative correlations with temperature. These climate–growth relationships are especially evident during the growing season, namely, the June to August in previous growing season and the May–July in current growing season. A recent observational study from the Qilian Mountains indicates that the radial growth in June accounts for approximately 56% of the annual ring width (Gou et al. , 2013 ). Because our tree‐ring samples were collected from an inland arid area of northwestern China with mean annual precipitation of only 86.7 mm (from the Jiuquan station), the radial growth of trees is mostly dependent on soil moisture recharged from precipitation and thus responds positively to precipitation and scPDSI variability. The temperature should be expected to influence tree growth because it influences soil moisture and the stomatal conductance of trees (McDowell et al. , 2008 ; Adams et al. , 2009 ; Williams et al. , 2013 ). Overall, the WQF tree‐ring chronology responds to hydroclimate variability in the growing season very well.

Results of the comparison illustrate that these tree‐ring records from the northeastern Tibetan Plateau shown similar hydroclimatic fluctuations, particularly from AD 1300 to 1800 during the LIA. The common drought periods recorded by these series are from the AD 1160s to 1170s, 1260s to 1340s, 1430s to 1540s and 1640s to 1740s; the common wet periods are the AD 1180s–1250s, 1350s–1420s, 1550s–1630s and 1750s–1780s. Despite the common drought and wet periods, there are some differences between these records. For example, the DL and HH series indicate dry conditions from the AD 1790s to 1840s, while the other three series did not reflect this drought period. Most of the series indicate that the climatic conditions have been relatively wet since the AD 1860s, while the QF series indicates a drying trend since the AD 1950s. In addition, the duration of dry and wet periods and the starting and ending times are not completely consistent among these tree‐ring records. For example, the dramatic drought period from the AD 1430s to 1530s began earlier and had a longer duration in the QF, WQF and DLH series than in the DL and HH series. The HH, DL and DLH series recorded a significant drought from the AD 1260s to 1340s, while the WQF series reflected weaker dry conditions during this period. These differences might be related to the different locations and microhabitats of these tree‐ring records. Overall, the significant dry and wet periods over the past 850 years agreed very well among these records, particularly for the three mega‐drought periods (AD 1260s–1340s, 1430s–1540s and 1640s–1740s) during the LIA (Figure 8 (f)).

Comparisons of tree‐ring‐based hydroclimate reconstructions ((a)–(e)) from the northeastern Tibetan Plateau and the solar activity curve (f). All series were normalized and smoothed with 100‐year low‐pass filter. The diagonal shading represents the mega‐drought periods during the LIA. The short bold black lines at the bottom indicate the solar activity minimum periods. (a) The annual precipitation reconstruction over the past 620 years (AD 1390–2007) in the Hexi Corridor (Yang); (b) the May–July scPDSI reconstruction from this study; (c) the precipitation reconstruction (AD 566–2002) from the previous July to the current June in the Delingha (Shao); (d) a tree‐ring record representing May–June precipitation over the past 2326 years (326 BC–AD 2000) from Dulan (Zhang); (e) the annual streamflow reconstruction from the prior August to the current July for the upstream Heihe River from AD 1000–2008 (Qin).

To validate our scPDSI reconstruction, we compared our reconstruction series with four other tree‐ring‐based precipitation or streamflow reconstructions from the northeastern Tibetan Plateau (Figure 8 ). Three of the four series are longer than 1000 years. The fourth is about 600 years but is located close to our tree‐ring sampling sites. These tree‐ring records include the annual precipitation reconstruction over the past 620 years (AD 1390–2007) in the Hexi Corridor from Yang et al. ( 2010 ) (QF, Figure 8 (a)); the precipitation reconstruction (AD 566–2002) in Delingha from the previous July to the current June from Shao et al. ( 2005 ) (DLH, Figure 8 (c)); a tree‐ring record from Zhang et al. ( 2003 ) reflecting May–June precipitation over the past 2326 years (326 BC–AD 2000) in Dulan (DL, Figure 8 (d)); and the annual streamflow reconstruction (from the prior August to the current July) for the upstream Heihe River from AD 1000 to 2008 from Qin et al. ( 2010 ) (HH, Figure 8 (e)). All of the tree‐ring series used ring‐width data from Qilian Juniper trees. To evaluate the coherence among these tree‐ring series, all series were normalized and 100‐year low‐pass smoothed. As the benchmark of the scPDSI reconstruction, all of the records were compared from AD 1161 to 2010 in this study.

4.3 The mega‐drought periods during the LIA and the influence of solar activity

The three mega‐drought periods recorded by the five series above were also indicated by other tree‐ring records in China. However, because most of tree‐ring chronologies in China contain less than 500 years, only the drought period from the AD 1640s to 1740s was widely recorded, while the dry phases from the AD 1430s–1540s and AD 1260s–1340s were reflected by only a few long‐term tree‐ring records. Many tree‐ring chronologies from the Xinjiang region in western China (Zhang et al., 2009; Zhang et al., 2011a, 2011b) to the Xiaolong Mountains and the Kongtong Mountains to the east of Qilian Mountains (Fang et al., 2012a; Fang et al., 2012b) and from northern China and Mongolia (Li et al., 2009) to the southern Qinghai Plateau (Qin et al., 2003) validated the sustained dry period from the late 17th century to early 18th century. In addition, the large‐scale drought and precipitation studies based on the tree‐ring records for monsoonal Asia (Cook et al., 2010) and northwestern China (Fang et al., 2011) also indicated that significant drought events occurred in the AD 1640s and at the end of the 17th century. In addition to the tree‐ring records, the accumulation record from the Dunde ice core on the northeastern margin of the Tibetan Plateau shows a low accumulation phase from the AD 1630s to 1660s (Yao and Thompson, 1992; Thompson et al., 2006). The high‐resolution stalagmite δ18O record from Wangxiang Cave, located between the Qinghai‐Tibetan Plateau and the Chinese Loess Plateau, indicates that the Asian monsoon was distinctly weak and that climate conditions were dry during the AD 1640s, which possibly accelerated the decline of the Ming Dynasty (Zhang et al., 2008). All of the above records indicate that the drought period from the AD 1640s to 1740s occurred in many regions of northwestern China. The drought periods from the AD 1430s to 1540s and 1260s to 1340s were recorded by some long‐term tree‐ring series from the Ayemaqin Mountains (Gou et al., 2010), the Qilian Mountains (Zhang et al., 2011a, 2011b; Sun and Liu, 2012) and the northeastern margin of the Qaidam Basin (Sheppard et al., 2004) in the northeastern Tibetan Plateau. Historical documentary records from the Longxi area at the northeastern margin of the Tibetan Plateau also pointed out that extreme drought years frequently occurred from the AD 1430s to 1540s (Tan et al., 2008). In conclusion, even though there are some differences in time resolution, term length and the definition of dry and wet periods, many records confirm that there were three mega‐drought periods during the LIA, which might be related to large‐scale forcing factors.

The MTM spectral analysis showed significant spectral peaks at 60‐ to 70‐year and 102‐year intervals in the scPDSI reconstruction over the past 850 years. These cycles agree with the low‐frequency variations of solar activity (Sonett and Finney, 1990; Damon and Sonett, 1991). Therefore, we compared a reconstructed solar activity series (Muscheler et al., 2007) with the tree‐ring records used in this study. It is clear that the solar activity series corresponds well with the tree‐ring records from the northeastern Tibetan Plateau, especially for the LIA. The relatively dry periods correspond to the periods of low solar activity, while the wet periods occurred during solar activity maxima. The three sustained mega‐drought periods correspond to the prominent Maunder minimum (AD 1645–1715), Spörer minimum (AD 1420–1530) and Wolf minimum (AD 1280–1340) of solar activity (Stuiver et al., 1998). However, the correspondence between tree‐ring records and solar activity variability disappeared over the past two centuries. The reasons for the decoupling of this relationship are not yet clear and require future study.

To clarify further the influence of solar activity on regional hydroclimate variability, we employed the wavelet power spectrum (Torrence and Compo, 1998), cross‐wavelet transform and wavelet coherence (Grinsted et al., 2004) analyses to assess the temporal relationships between our scPDSI reconstruction and solar activity. The wavelet power spectrum analysis (Figure 9(a)) showed that the scPDSI reconstruction has cycles similar to the results from the MTM analysis, suggesting that centennial‐scale cycles have always existed over the past 850 years. Significant increases of spectral power on a 70–100‐year scale were recorded since approximately AD 1750, which might explain the divergence between solar activity and tree‐ring records over the last two centuries. Wavelet analyses for the solar activity record indicate a strong and persistent 200‐year periodicity from AD 1400 to 1750 and a 6–20‐year periodicity since AD 1500 (Figure 9(b)). The cross‐wavelet transform (Figure 9(c)) and wavelet coherence results (Figure 9(d)) indicate that the drought reconstruction in this study shows centennial spectral coherence with solar activity from the 12th to the 17th centuries as well as out of phase variability at the 60–100‐year cycle since the 17th century. These results suggest that hydroclimate variations in the western Qilian Mountains might be related to solar activity but that the relationship is unstable over time and has changing coherence periodicities. Furthermore, we compared the 100‐year low‐pass filter and the 60–100‐year band‐pass filter curves from the solar activity record and the scPDSI reconstruction to examine the synchronicity on a centennial scale and the inconsistency on an inter‐decadal scale (Figure 10). Comparisons of the scPDSI reconstruction with the solar activity curve at different frequencies show that the two series correspond closely on the centennial scale from 12th century to 17th century but are out of phase on the 60–100‐year scale with the regional hydroclimate variability lagging behind the solar activity fluctuation by approximately 20 years.

Figure 9 Open in figure viewer PowerPoint Wavelet power spectra for the scPDSI reconstruction (a) and the solar activity series (b), respectively. Cross‐wavelet transform (c) and wavelet coherence (d) of the scPDSI reconstruction and the solar activity series. The cross‐hatched region is the cone of influence. The black contour represents the 95% significance level, using a red‐noise background spectrum.

Figure 10 Open in figure viewer PowerPoint (a) Comparisons of the scPDSI reconstruction series (blue dashed lines) and the solar activity series (red lines) with a 100‐year low‐pass filter from AD 1161 to 2002. (b) Same as (a) but the data were smoothed with a 60–100‐year band‐pass filter. All curves were normalized.

Solar activity might influence the hydroclimate variations in the study area by modulating the temperature differences between the land and the ocean and the strength of the monsoon. Stronger solar radiation will increase the land‐sea thermal contrast and strengthen the Asian monsoon (Sun and Liu, 2012; Wu et al., 2012; Gou et al., 2014). A stronger (weaker) monsoon will bring more (less) moisture to the arid areas of northwestern China and increase (decrease) the amount of precipitation in the study area (Zhang et al., 2008). Solar activity also influences the ocean‐atmosphere system, such as the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO) and El Niño‐Southern Oscillation (ENSO) variability (Velasco and Mendoza, 2008), which could modulate the monsoon and westerlies variations (Chen et al., 2008), and effect the regional precipitation and hydroclimate variance. However, there is still a need for more high‐resolution climate proxy records and research seeking to clarify the role of solar activity forcing on regional climate variability.