The forest tree mechanism to response to atmospheric drought and soil is poorly known, especially in relation to carbon allocation, how the trees modified their architecture above and below ground in response to variations in climate conditions. Pinus sylvestris var. mongolica is the most extensive conifer as it can be spread throughout Eurasia as on the western Mediterranean to the Russian Far East. Such huge distribution includes a wide array of climates, with the severe and frequent summer drought of southern Iberian Peninsula and extremely cold winters of north eastern Siberia . World climate conditions are placing greater pressure on forests ecosystems from almost all over the globe . In case of Scots pine, growth can be endorsed by increased temperature in northern and upper areas where production is basically cold-limited  . However, there is most evidence that indicates Scots pine growth is also influenced in cold constrained sited due to aggravated soil moisture deficit which linked to increasing demand for evapotranspiration or decreased in precipitation during growing season  . Such contrasting influence of minimum temperature and drought brings uncertainty about productivity and vigor of species throughout the range and specifically in cold-dry environment , besides other dry environments, particularly warmer areas of Mediterranean, where scots pine also subjected to winter and warm climate .
In this consideration, the comparison of low temperature (i.e., Siberian forest-steppe) and warm climate (i.e., Mediterranean mountains) has a simultaneous effect on Scots pine growth which is of particular interest to predict the climate change influences on conifer species . A variety of range of methodological approaches are used to determine tree growth climate relations, but dendroclimatological techniques are valuable in determining long term variations in forest productivity in perspective of climate relationship with variables like precipitations and warmer temperature .
Mongolian Scots pine (Pinus sylvestris var. mongolica), an environmental diversity of Scots pine, is naturally dispersed in the Daxing’anling mountains of China (>50˚10_ - 53˚33_N, 121˚11_ - 127˚10_E), in Honghuaerji of the Hulunbeier sandy plains of China (47˚35_ - 48˚36_N, 118˚58_ - 120˚32_E) and parts of Russia and Mongolia (46˚30_ - 53˚59_N, 118˚00_ - 130˚08_E) (Zhu et al., 2008; Zheng et al., 2012). As of its strong nature and ability to tolerate dry environment, so in 1950 it is first time introduced to condense wind speed and increase sand fixation in the Zhanggutai region, the south east sandy land. So, it becomes the most important specie for reforestation in north east China  . Tree rings are widely used to evaluate the climate variation effects on forests . Most dendrochronological studies focus on radial growth patterns to infer the long-term variations in tree performance. There is a remarkable expansion in desertification across Mongolia due to climate warming at global scale, and it has potential influence on Mongolian environmental health and economy conditions in current years. So Mongolian government has applied reforestation plans. Pinus sylvestris L. mongolica has a significant role in ecology and Mongolian economy  .
The objective of this study was to measure the climate influence on long term changes in tree radial growth, to investigate tree survival in drought conditions, and to check the influence of diurnal temperature and potential vapor pressure effects on tree growth. It is anticipated that the results of this study might be useful for management of forest plantation with respect to growth conditions of Pinus Sylvestris L. mongolica.
2. Material and Methods
2.1. Study Sites
The Daxing’an Mountain is an important natural geographic barrier, in north east inner Mongolia and northwest Heilongjiang province, it is the middle part between semiarid inland area on northwest side and the Pacific Ocean. It is also known to be the transition zone semi-arid region to semi humid . South east side of this area is blocked by mountains so, the summer monsoon cannot enter in this area. The eastern side of this region is humid and western side is more dried. Summer climate is explained by periodic invasions of warm and humid air from low elevation oceans, while winter season is invaded cold air masses on high latitudes. This study was done on high latitude in Daxing’an mountains. The forests are most dominant by Dahurian larch and Scots pines. Soils are mostly brown coniferous and dark brown forest peat . Meteorological data were collected from KNMI 3.01. The annual mean vapor pressure (59.4, 59.8, 64.1) respectively at Mg, Kyh, and Nm sites and annual mean diurnal temperature (16, 15.6, 12.7) respectively at 3-sites. The coldest month is January and maximum hotness in Month of July. Annual rainfall accounts for 68% in June to August. Relative humidity high in growing season. Spring and early summer face severe drought conditions frequently. This area also has an average high burning rate in China .
2.2. Tree Ring Data
Tree ring cores of Pinus Sylvestris var. mongolica were selected from three-sites which are infrequently distributed across the Daxing’an Mountain. We collected two cores per tree in May-2016. Total of 160 cores was selected from living old trees from breast height at 1.3 m. Each tree which selected for sample collection tried to avoid influence of visible stand disturbances (including windstorm, animal and human disturbances or fire damage). The average distance is more than 100 km between selected sample sites. All cores were air dried, mounted, surfaced and cross-dated following standard techniques of dendrochronology  . Ring widths were measured with a precision of 0.001 mm using a Velmex measuring system (Velmex, Inc., Bloomfield, NY, USA). The cross-dating quality was checked by COFECHA program (Holmes, 1983). Successively, the age-related trends were removed by fitting a cubic smoothing spline with a 50% frequency response cut-off at two-thirds of the series length using the ARSTAN program . Autocorrelation was separated by autoregressive modeling, and each site chronology was measured using a bi-weighted robust mean . In Figure 1 map shows the distribution of Pinus sylvestris v. mongolica along the Daxing’an Mountain. Figure 2 indicates the variation in monthly climate data of vapor
Figure 1. Sampling sites set up across the natural distribution area of Pinus sylvestris woodlands in Daxing’an Mountain north east China.
Figure 2. Variation in diurnal temperature and vapor pressure from 1901-2016. Monthly mean maximum-minimum temperature and vapor pressure along 3 sites Mangui (Mg), kehyihe (Kyh) and Nanmu (NM).
pressure and mean maximum and minimum diurnal temperature variations from period 1901-2016 along 3-sites Mangui (Mg) kheyihe (KYH) and nan (NM). There is higher diurnal temperature from Jan-Apr and maximum vapor pressure during May-Jul. In Figure 3, we make correlation between tree ring index and with the years there is a maximum increase in growth trend at Nm site during 1901-1911 and 1971-1980 the decline in growth trend.
2.3. Climate Data and Statistical Analyses
Large scale Climate data were used from 1901-2016 collected from the National Meteorological Information center e.g., gridded diurnal temperature and Vapor pressure, and soil moisture indices were downloaded from
http://climexp.knmi.nl/. The gridded climate data has much coherency and homogeneity than station data. Pearson correlation was conducted to climate
Figure 3. The growth pattern of tree ring index series in Daxing’an mountain in north east China, comparison of 3 sites red color (Mg) site, blue line (Kyh) and green line indicates (Nm) site growth series from 1901-2016.
growth relationship. And used growth trends of summer and winter diurnal temperature were made. To determine climatic variables that control the radial growth of P. sylvestris mean monthly vapor pressure and mean diurnal temperature were compared with the local chronologies for each sampling sites. The sampled chronologies make correlations with the meteorological chronologies which neare to the analyzed sites. The periods were explored in Summer from June-September and winter season from November-January in Figure 4. Stem dimeter at breast height was measured by using scattered plot with latitude. Table 1 indicates the sampling site information, along with latitude.
When building up a large chronology it is most important to reminisce the fluctuation in growth with the climate conditions in chronology when large area being summarized by different growing conditions. The Daxing’an mountains tree chronologies along 3-elevations have various ranging gradients, its highest peak range 2035-meters. It was formed about 200 - 145 million years ago. The site information is given in Table 1. Figure 2 indicates the spatiotemporal variations and drought patterns in Daxing’an mountains and north inner Mongolia. The relevant chronology and statistic information’s are listed in Table 2. Figure 3 represents the tree growth of Pinus Sylvestris var. mongolica from 1901-2016. We clearly observed variations in growth trends there is a significant increase in growth 1920-1930 and then decline in curve 2nd growth increase interval from 1991-2001. Figure 4 represents the growth trend of Pinus Sylvestrisvar. mongolica. We divide it into 3-periods 1921-1952, 1953-1984, 1985-2016. There is an obvious increase in growth trend during 1925 in Mg site in Figure 4(A) and
Table 1. Site description and statistical characteristic of the Pinus sylvestris var. mongolica dendrochronology in the Daxing’an Mountains.
Figure 4. Mean intra-annual course of the DRI for three periods in the 20th century. At all 3 sites (A) represents 1921-1952, (B) from 1953-1984 and (C) represents the 1985-2016 growth trends.
Figure 5 we explain the scatter plot with diurnal temperature of summer and winter seasons and we use growth trends to indicate the growth rate at 3-sites.
Table 2. Internal statistic properties of scots pine along with region differences at 3-sites Mg, Kyh, Nm along Daxing’an Mountain, alphabetic letter indicates the LSD-significant differences by multiple comparison.
Letter a’ and c’ represents the significance level between groups.
Figure 5. Influences of the mean max-mini diurnal temperature during winter and summer season on cambial growth of scots pine.
There is an obvious difference at Kyh site between winter and summer season and growth trends in summer season there is growth trend first remain stable then start to decline but in winter season there is an obvious increase in growth. at Mg site also increase in trend in winter diurnal temperature. at Nm site its growth remains stable in winter season. In Figure 6, we measured diameter at breast height along elevation latitude at 3-sites Mg, Kyh, Nm.
Figure 6. Correlation between latitude and stem diameter at breast (DBH) height in Daxing’an Mountains.
Scots pine is an under the effects of drought stress and considered to be a drought tolerant, so it is thought to be a main climate limiting factors which reduce its growth in semi humid and semi-arid regions such as in Mongolian Plateau Daxing’an Mountains   . In previous studies, it also suggests that radial growth of scots pine is influenced by humidity, precipitation in these regions, and most of dendroclimatic studies based on reconstruction of hydroclimatic history  . In these areas’ radial growth for growing season mostly in July-September as in Figure 1. On the other hand, a significant increase in radial growth of non-growing season due to temperature also observed. It is probable protect of dormant buds in winter high temperature from frost damage . The positive growth response with spring temperature could be due to early snow melting process, which supply the early spring water, and eventual tree growth rate stimulated  . This infrequent drought response relatively due to humid climate and might be northern study site, where positive effect is strongly greater than negative effect . Similar drought response also visible in most of reconstruction studies in Qilian Mountains (Sun and Liu, 2013) and also same case of drought stress in western Tian Shan Mountain, China .
Scots pine usually has usually typical drought response patterns, it shows positive response to precipitation and negative response to temperature increasing  . Actually, this typical response to drought patterns also found in other drought or wetness tree ring analysis in other studies  . In this study correlation between tree ring index and diurnal (Maximum-Minimum) temperature it clearly indicates that summer temperature shows negative or decline in growth response and winter temperature indicates the positive growth response in Figure 4. A significant positive growth response in winter temperature supported moisture as the main limiting factor for Pinus Sylvestris growth. The climate (drought) response also observed in Dahurian larch a most important conifer specie in this area . Late summer temperature influence on nutrient storage capacity which affects the sprouting in next spring. This alternatively affects the growing ratio of sprouting and tree ring dimeter in next year’s . The results of the analysis revealed that Pinus Sylvestris revealed the consistent increase in growth pattern in moist conditions. These relationships recommended that specific drivers which affect variation in dry-wet conditions in Daxing’an mountains. Among these differences the most prominent differences in Daxing’an mountain in central Mongolia. Similar studies were investigated by  which reported that different patterns of dry and wet conditions under global warming using models and observations.  found that increase in moisture related to quick warming. Though the reasons for divergence should be further studied, it might be associated to the diverse response to phase shift  .
Drought severity conditions most obvious during 1920-1930 in Daxing’an Mountains rather than other areas. In addition, most of growth series indicates that Daxing’an Mountain has gotten more warm and wet since the late 1970s. This is not only the case of Daxing’an Mountain especially in all transit zones. Our results may be not accurate due to in-adequate spatiotemporal distribution of tree ring data collection in this area. Overall, drought inconsistency in Daxing’an mountains and its consequential affects on surrounding areas due to climate oscillations of the Atlantic and Pacific oceans (e.g., AMO, NAO, PDO, and SNAO ENSO). These climate oscillations at large scale occur due to variations in Asian monsoon and lead to dry and wet fluctuations in the Daxing’an mountains.
This work was supported by the National Key Research and Development Program of China (2016YFA0600800), the National Natural Science Foundation of China (41877426), the Fundamental Research Funds for the Central Universities (2572019CP15), and the Open Fund of Ecological-Meteorological Innovation Laboratory of China Meteorological Administration (stqx2018zd02). We thank Jinsheng Han, Zhenjing Sun and Zecheng Chen for their help in sampling. We also thank the staff of the sampled Forestry Bureaus for their assistance in the field.
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