3.1 Response of tree growth to hydrological and climatic factors
The tree-ring width of a tree is generally constrained by the genetic characteristics of the tree and the external environment(Fritts 1972). The genetic characteristics of an individual tree are relatively stable, and the growth trend of the tree-ring data is removed, so the growth environment of the tree is the main factor affecting its radial growth. To further understand the relationship between the growth of Korean spruce trees and hydrological and climatic factors, the correlation between the standardized chronology of Korean spruce and the annual, seasonal, and monthly data of hydrological and climatic factors in the study area was studied using SPSS software (Fig. 4).
The figure shows that the standardized chronology of Korean spruce is negatively correlated with the annual, seasonal, and monthly data of the mean temperature, the mean minimum temperature, and the mean minimum temperature and that its correlation with the annual mean temperature is extremely significant. The chronology has a positively correlation with runoff in all periods except for winter and December and a primarily positive correlation with precipitation, which is not significant overall.
In arid and semiarid regions, temperature is a relatively climatic factor that tree growth is sensitive to. The correlation between the standardized chronology of Korean spruce and the annual mean temperature is the highest, with a correlation coefficient of -0.557 (p < 0.001), which passes the significance test at the 99.99% confidence level, followed by the correlation between the standardized chronology and the annual mean minimum temperature (r= -0.507). In terms of seasons, the correlations between the standardized chronology and the three types of temperatures in spring and summer are higher than those in the other two seasons, with the weakest correlation in winter. In spring, the trees are in the early growing season. Since the precipitation is low in spring (Fig. 2), temperature is the most important factor affecting the growth of Korean spruce. The temperature rise causes drought stress to inhibit the radial growth of trees. The study area has a temperate continental monsoon climate. The precipitation is mostly concentrated in July and August. Although the precipitation increases in summer, the temperature also reaches the annual peak in summer. If the increased precipitation is not sufficient to complement the evaporation of soil water caused by high temperature, narrow tree rings are formed due to water shortage. In winter, trees become dormant, so the effect of temperature on trees is weakened. The temperature in May and June has a stronger limiting effect on the growth of the trees than that in other months, and the correlations between the three types of temperatures and the chronology all pass the significance test at the 99% confidence level. During May and June, the precipitation in the study area is relatively low, and the high temperature limits the growth of the tree cambium cells, which results in narrow tree rings. In August, the correlation between the three types of temperatures and the growth of trees in the study area is relatively poor because the effect of temperature on tree growth is not significant due to the high precipitation and runoff and the joint effect of a variety of climatic factors on Korean spruce in August.
The correlation between the standardized chronology and the runoff in spring is the highest (0.543(p < 0.001)), followed that in summer, and the weakest and negative correlation is observed in winter. In spring, the trees are in the early growing period, and the increase in runoff can provide a sufficient water source for tree growth. In summer, the trees are in the vigorous growth period, and runoff promotes the growth of the trees. In winter, the temperature is extremely low in the study area, so excessive moisture causes frost damage to the dormant Korean spruce trees, and soil freezing is not conducive to tree root respiration. The correlation between runoff and chronology from March to June is better than that in other months and passes the significance test at the 99% confidence level. The correlation between the chronology and runoff in December is negative but not significant.
Except for the annual precipitation, the correlations between the precipitation and the chronology of Korean spruce on other time scales are weak and do not pass the significance test. This may be because the precipitation in the semiarid study area is relatively low and mainly concentrated in summer, while the summer temperature in the study area is extremely high, which results in much higher evaporation than precipitation and consequently a limited effect from precipitation on trees and low dependence of tree growth on precipitation.
In summary, air temperature and runoff play an important role in the growth of Korean spruce. Appropriate temperature and high runoff are more conducive to the growth of trees. The seasonal differences in runoff in the study area make tree growth more sensitive to temperature. Hence, temperature is the main limiting factor for the radial growth of Korean spruce.
3.2 Reconstruction and verification of annual mean temperature
Based on the above analysis, the annual mean temperature data and the standardized chronology of the observation period (1951–2016) at the Xilinhot Meteorological Station were used to reconstruct the annual mean temperature variability in the study area during the past period without meteorological records. The reconstructed values were compared with the measured values (Fig. 5). The reconstruction equation of the annual mean temperature in the study area is as follows:
y i = -3.074xi + 7.642
(r = 0.557, N = 66, R2adj = 0.310, F = 28.78, p < 0.001)
where yi is the annual mean temperature in the ith year and xi is the standardized tree-ring chronology series.
The stability of the equation directly affects the quality of the reconstructed series. The stability, reliability, and accuracy of the reconstruction equation are tested using the one-by-one elimination method used in international tree-ring studies. The tested statistics included the product mean, correlation coefficient, sign test, etc. The reconstruction equation has an error reduction (RE) value of 0.682 and an effective coefficient (CE) of 0.305, which are both positive. This result indicates that the reconstructed annual mean temperature series is accurate and reliable. The correlation coefficient (r) between the reconstructed and measured series us 0.557, and their first-order autocorrelation coefficient passes the significance test at the 99.99% confidence level. Therefore, the correlation between the two is excellent. The high- and low-frequency sign tests both pass the significance test at the 95% confidence level, which indicates that the equation can reflect the changes in high and low frequencies well. All parameters of the equation meet the requirements and pass the reliability test.
Figure 5 shows that the reconstructed and measured series of the annual mean temperature match well and that the overall temperature fluctuation trend remains consistent. However, the reconstructed series and the measured series exhibit some differences in certain years, perhaps because the accuracy of the reconstructed series was affected by a variety of climatic factors that affect the radial growth of trees. In general, all the test parameters of the reconstruction equation pass the significance test. The reconstructed series has high reliability and contains more climatic information. The reconstructed and measured series exhibit the same trend. Hence, the reconstruction equation can be used to reconstruct the annual mean temperature in the study area.
3.3 Variability characteristics of the reconstructed annual mean temperature series
To understand the characteristics of cold and warm cycles in the reconstructed historical climate series, the 11-year moving average method was used to process the standardized reconstructed series. A period with moving average temperature above the mean temperature was defined as a warm period, and a period with moving average temperature below the mean temperature was defined as a cold period.
Since 1845, the study area has experienced five stages of warming-cooling-gradual change-warming-cooling, including five warm periods and five cold periods. The longest cold period started in 1889 and ended in 1923, lasting for 35 years, and the decadal mean temperature of this period was the lowest. The longest warm period started in 1962 and ended in 1991, lasting for 30 years. The mean annual temperature in the study area was the highest in the period of 1996–2012 (2.186°C), and the temperature variability was the most dramatic in this period (CV = 11.3%).
The reconstructed series of annual mean temperature in the study area showed an overall upwards trend (0.0012°C/10a), with relatively large fluctuations at a relatively high frequency. In the past 172 years, there were two abrupt changes, in 1906 and 1981, and a total of 25 cold years (14.5%) and 18 warm years (10.5%). Most (75%) of the past 172 years were normal years. From 1845 to 1853, the temperature rose at 0.0808°C/10 a and reached the highest temperature (5.38°C) in 1853, which was also the highest temperature in the 19th and 20th centuries. In 1856, the temperature abruptly declined at 0.0494°C/10 a. The temperature stopped declining and started to rise in 1873. During this period (1856–1873), the study area experienced 7 cold years, mainly in the 1970s. In the following 30 years, the temperature fluctuated in the normal range. In 1906, the temperature began to decline at 0.0275°C/10 a, resulting in successive cold years from the 1910s to the 1920s, with the occurrence of the lowest decadal mean temperature and the longest cold period (Fig. 6a). A clustering of cold years also occurred from 1950s to 1960s, and the related cold period lasted for 30 years. In 1981, the temperature in the study area abruptly rose at 0.1129°C/10 a. A large number of warm years occurred from the early 2000s to the 2010s, accounting for 61% of all warm years, and the decadal mean temperature in this period reached the historical high of the recent two centuries (Fig. 6b). After 2008, a warming hiatus started, and the temperature decreased linearly at -0.1878°C/10 a. In 2016, the temperature in the study area reached the criterion for cold years.
As shown by the time-frequency distribution diagram of Morlet wavelets (Fig. 6c) and the wavelet variance diagram (Fig. 6d), the reconstructed mean temperature series of the study area show distinct cycles of 3 a, 7 a, 10–12 a, 15–22 a, and 30–40 a. The maximum peak in the wavelet variance diagram corresponds to a cycle of approximately 15–22 a, indicating that the periodic oscillation on the scale of 15–22 a is the strongest, followed by the extreme value of the wavelet variance on the scale of 30–40 a. Large-scale fluctuations in climatic cycles have a regional impact, which indirectly affects the growth conditions of trees. The cycles of 3 a and 7 a in the study area basically coincide with the ENSO cycles. ENSO is one of the major drivers of interannual climate change, which shortens the duration of summer and reduces the precipitation. The cycle of 10–12 a basically coincides with the short cycle (11 a) of sunspots(Guttu et al. 2021). Solar radiation affects the vertical temperature of the Earth, causing changes in temperature, so the activity of sunspots affects the growth of trees. The cycle of 15–22 a might be related to the Atlantic Multidecadal Oscillation (AMO). If the PDO is associated with the positive phase of the AMO, the monsoon would be affected. Since the study area is in the monsoon-continental climate transition zone, the cycle of 15–22 a plays a vital role in tree growth.
3.4 Comparison with historical events and other reconstructed results
To verify the accuracy of the reconstructed series, we compared the reconstructed temperature series with the historical meteorological disasters recorded in the Dictionary of Meteorological Disasters in China—Inner Mongolia Volume(Shen 2008) and found severe natural disasters corresponding to the cold years and warm years in the reconstructed temperature series. The start year of the chronology coincided with the 25th year of Daoguang of the Qing Dynasty, when high temperature led to locust disasters. In the 5th year of Tongzhi, the extreme high temperature led to different degrees of drought in the study area, and people had to eat bark and wild vegetables due to the frequent famines. Form the 3rd to 5th year of Xianfeng, people had to borrow seeds from government-owned barns for resowing, as crops died due to drought in many places, and the imperial government relieved the famine by allocating government-owned grains to the public and exempted the farmers from farm rent. Severe drought in the spring and summer of the 4th year of Guangxu (1878) greatly reduced the grain yields, which, on top of the poor harvest in the previous fall, resulted in inadequate grain storage. According to modern drought records, the severe drought in Chifeng in 1968 made crops small and weak, negatively affecting agricultural harvests. In 1983, a periodical drought occurred in Chifeng in early summer. Persistent drought occurred in the study area in 1950–1953, 1960–1963, 1970–1975, 1980–1982, and 1986–1989, which precisely coincided with the warm period in the reconstructed series, indicating that the reconstructed warm period is reliable. In the 1st year of Xuantong (1909), the study area suffered continuous severe drought and snow disasters in winter. In the 3rd year of Xuantong, the people became helpless victims of the heavy snowfall events in winter and spring. In 1913–1918, the snow disasters in the study area caused severe great livestock casualties and a fuel shortage. In 1934, snow began in October at Xilingol League, and the accumulated snow did not melt until March of the following year. From 1952 to 1958, snow disasters occurred in both winter and spring in the study area, with the lowest temperature reaching − 42°C, causing the loss of tens of thousands of livestock and road blockages. Natural disasters, including snow disasters, cold waves, and chilling damage, were found in the records of the study area corresponding to the cold years in the reconstructed series, which provided strong support for the accuracy of the reconstruction results.
To further verify the reliability of the reconstruction results, the common time periods of the reconstructed historical annual mean temperature series and other reconstructed results in the monsoon-continental climate transition zone in China were comparatively analysed. Figure 7a shows the temperature variability from 1853 to 2012 recorded by tree rings in a paddock (denoted as WC) in China(Wang et al. 2019). The WC borders the study area and has a moderate-temperate continental monsoon plateau mountain climate. Figure 7b shows the results of tree-ring reconstruction of the mean temperature in June and July from 1880 to 2014 in a region in the northern part of the Daxing'an Mountains (denoted as DXA) (Jiang et al. 2020). Figure 7c shows the annual mean temperature of the study area from 1845 to 2016 reconstructed based on the tree-ring width chronology of Korean spruce in a representative area of the transition zone (denoted as GDD). The figure shows that the reconstruction results of the GDD exhibit warming and cooling trends similar to those of the other two regions (the WC and the DXA) and that the three regions experienced the same warm periods (1980–1988, 1996–2002, 2007–2011) and cold periods (1892–1903 and 1931–1947). The GDD had longest cold and warm periods with most intense temperature fluctuations of the three regions (CVWC = 2.8%, CVDXA = 5.1%, CVGDD = 8.7%). The WC had more frequent alterations between warm and cold periods than the other two regions. The temperature in each region showed relatively synchronous cooling and warming trends. The temperature in the GDD showed a decreasing trend from 1853 to 1872 and from 1885 to 1893, and the cooling duration of the WC (33 years) was longer than that in the GDD (23 years). The temperature in the three regions rose continuously from 1935 to 1940 and from 1958 to 1967, which indicated the transition from a cold period to a warm period. During the period from 1968 to 1975, the temperature declined continuously, and the temperature drop was more rapid in the WC than in the other two regions. The temperature began to rise from 1976 and abruptly dropped in 1985, which indicated the transition from the warm period to the cold period in the GDD, but the temperature fluctuations in the DXA were small. In the 20 years after 1992, the temperature in the GDD increased significantly, and the temperature rose at 0.0325℃/10 a, 0.0819℃/10 a, and 0.1167℃/10 a in the WC, the northern part of the DXA, and the GDD, respectively. The temperature rise of GDD was faster than that of the other two regions.