With the occurrence of global warming, the climate characteristics of the northern region have exhibited a shift towards a warm-drying climate. In order to determine variations in the response of larch and Mongolian pine pine distributed in the northern Greater Khingan Range to climate change, four sampling points of these species were selected in the study area. The samples were employed to construct a difference chronology and investigate the stability of the relationship between the radial growth of Dahurian larch and climatic factors.
The established chronologies of conifer species in the four sampling sites in the northwestern part of the Greater Khingan Range region contain a large amount of climate information, facilitating the analysis of the relationship between the tree-ring width indices and climate factors.
Compared with the period before the the abrupt climate change, a significantly increasing trend is observed for extreme high temperature events in the study area, while the extreme low temperature events exhibit a decreasing trend, and the trend in the annual differences in temperature is significantly larger. There is an obvious regional warming phenomenon in the Greater Khingan Range region, which is consistent with the significant increase in temperature in northern China [35]. Changes in temperature and precipitation are the main climatic factors affecting tree growth. Analyzing the relationship between the response of the annual tree-ring width index of typical tree species Mongolian pine and larch to climate factors in the Greater Khingan Range region reveals great differences in the response time and type of Dahurian larch and Mongolian pine to the extreme climate indices.
According to the response of the standardized chronology of the tree whorl width and climate indices in the Greater Khingan Range region, the tree growth is not only affected by the temperature of the current year, but also by the temperature of the previous year, which is known as the "time lag effect" [36]−[37].
At high altitudes, a significant negative correlation was observed between larch and the daily difference in temperature of the previous winter (November–January of the previous year). At low altitudes, larch and the daily difference in temperature in the previous winter exhibited a negative correlation, with a significant negative correlation in the previous winter November–December. Mongolian pine pine was also negatively correlated with the daily difference in temperature in the previous winter (November and December) at lower altitudes. A large daily difference in temperature can be interpreted as a high daily maximum temperature or a low daily minimum temperature, and either high or low temperatures in the previous winter can affect tree growth. In November of the previous year, after a sudden change in temperature, Mongolian pine pine and larch at high altitudes exhibited a significant positive correlation with the minimum air temperature maximum. This may be attributed to the enhanced enzyme activity of the dark reaction during photosynthesis following the increase in the minimum air temperature maximum, which can promote the accumulation of organic matter in the dark reaction process, thus facilitating the growth of trees.
In February of the current year, larch at high elevations was significantly negatively correlated with temperature and significantly positively correlated with the number of cold days. Enhanced spring transpiration was combined with increased air drought, yet frozen soil moisture availability remained limited [38]. When the rate of increase in the maximum air temperature exceeds the maximum surface temperature, both winter and early spring snowpacks melt earlier [38]. Low snowmelt evapotranspiration can lead to a rapid increase in soil moisture, resulting in a loss of effective water from early spring runoff, reducing the available effective water in the soil during the growing season and beyond, thus affecting the subsequent growth [38]. The high-altitude Dahurian larch tree-ring width index in May of the current year exhibited a significant positive correlation with temperature, a significant positive correlation with the minimum temperature minima, maximum temperature minima, and number of warm night days, and a significant negative correlation with the number of cold day days. This indicates that in the early part of the growing season, temperature is the main factor limiting tree radial growth, with elevated temperatures observed as favorable for tree radial growth, and vice versa. This is because temperatures lower than the optimal temperature for tree growth during the growing season will affect plant development, and low nighttime temperatures will cause plant frostbite [39]. The results of this study are in agreement with those of Shang et al. [40], who demonstrated the impact of low temperatures in the growing season. Prior to the abrupt climate change, the Dahurian larch tree-ring width index exhibited a significant positive correlation with the minimum temperature minima and a significant negative correlation with the number of cold night days in August of the current year. After the change, the promotion of temperature on larch growth weakened, indicating that in the growing season of Dahurian larchs, the enhancement of radial growth of larch was weakened by the increase in the temperature of the study area. This may be because in August, the temperature of the study area increased more than the larch optimum growth temperature during the growing season. This is linked to the acceleration of soil evaporation and water loss, as well as the increased vapor pressure difference, due to high temperatures during the growing season [41], resulting in insufficient soil moisture. In addition, the tree root system will not receive sufficient water to limit the radial growth of the tree [34], and extremely high temperatures during the growing season will also lead to high leaf temperatures that exceed the optimal temperature for photosynthesis. This reduces net photosynthesis and drives transpiration in vegetation, resulting in a water deficit in trees [42]. The greater temperature in August compared to the optimal growth temperature for larch may be linked to the increased temperature in the study area, which exceeds the optimal growth temperature for larch [42]. When the temperature surpasses the physiological threshold of trees [43], respiration increases, the photosynthetic rate decreases, and the photosynthetic organic matter in the body decreases, which negatively affects tree growth [44]−[46]. Therefore, high daytime temperatures are also unsuitable for tree radial growth. In October of the current year, the radial growth rate of larch exhibited a significant negative correlation with precipitation, while the temperature in the study area increased, and the extreme temperature indices (TX10p, TN10p, FD, ID) showed a decreasing trend, possibly prolonging the growing season of plants. As the growing season lengthens, the likelihood of plants experiencing frost damage increases [47]−[48]. In particular, compared to trees at low latitudes, high-altitude trees have narrower safety thresholds for frost and are thus at a greater risk [49].
At high altitudes, after a sudden change in temperature, Mongolian pine showed a significant positive correlation with precipitation and a significant positive correlation with the minimum temperature minima in May of the current year. This indicates that in the early part of the growing season, the increase in precipitation and minimum temperature favored the radial growth of trees. The sampling site is located in the northwestern part of the Greater Khingan Range, and the increase in minimum temperature advanced the activity of the formation layer [50] and also promoted xylemogenesis [51]. Thus, the minimum air temperature minima at the beginning of the growing season was significantly positively correlated with tree growth. This is in line with the findings of Li Junxia et al., who revealed that the positive response of trees to temperature has become more pronounced in the northern part of the Greater Khingan Range, with a significant increase in temperature in recent decades [43]. After a sudden change in temperature, the high-elevation area Mongolian pine exhibited a significant negative correlation with the number of cold day days in June of the current year, a significant negative correlation with the number of cold night days in July of the current year, and a significant positive correlation with the minimum temperature minima. This suggests that in the growing season of Mongolian pine at high elevations, cold days and nights are the main environmental factors limiting the growth of the trees. High temperature promotes photosynthesis, and in general, during the growing period of trees, the temperature is positively correlated with the width of the tree whorl within the range suitable for tree growth, and the temperature becomes the main climatic factor affecting tree whorl growth [45]. In September of the current year, Mongolian pine displayed a significant positive correlation with the maximum temperature maximum and the number of warm day days, and a significant negative correlation with the minimum temperature maximum. This indicates that with the prolongation of the growing season in September of the current year, the increase in midday temperature facilitated tree growth. In contrast, the enhanced nighttime temperature did not have this effect, perhaps due to the increase in respiration intensity, which is not conducive to the accumulation of organic matter in the plant.
At low altitudes, following a sudden change in temperature, larch exhibited a significant positive correlation with temperature in May of the current year, as well as with the maximum temperature minima and number of warm night days, and a significant negative correlation with the number of cold day days. This indicates that high temperatures at the early stage of tree growth favored the radial growth of trees, while low-temperature events limited tree growth. The sampling site was located in the northwestern part of the Greater Khingan Range, and the increase in minimum temperature advanced the activity of the formation layer [50] and promoted xylemogenesis [51], and thus the increase in temperature at the beginning of the growing season was significantly and positively correlated with tree growth. High soil temperatures facilitate the absorption of water and inorganic salts by the rootstocks, which favors cell division in the tree's formative layer and leads to wider annual rings. However, it is detrimental to tree growth and narrower annual ring widths [45]. Trees perform numerous activities within a certain temperature range, and under the optimal temperature conditions, these activities are the most vigorous, the photosynthesis efficiency is the highest, and the growth is the fastest [52]. Suitable temperatures enhance photosynthesis in trees, which increases the net accumulation of tree nutrients, leading to rapid tree growth and the formation of wide whorls in the current year [53]. This is in line with the results of Junxia Li et al., who reported that the positive response of trees to temperature in the northern Greater Khingan Range has become more pronounced with the significant increases in temperature in recent decades [43].
For the low elevation area, we observed that in July of the current year, the correlation between the Dahurian larch width index and the minimum temperature extreme value changed from a significantly positive before the temperature mutation to a negative correlation after. Moreover, in September of the current year the larch exhibited a significant negative correlation with the average temperature and minimum temperature extreme value.
In July of the current year, larch was significantly positively correlated with precipitation, and in September of the current year it was significantly negatively correlated with the minimum temperature extreme value. This indicates that under the significant warming of the study area, the increase in temperature led to a slowdown of the radial growth rate of larch in the low altitude area. Summer is the highest period of cell production in coniferous species [54], and is also a critical stage for Dahurian larch to synthesize carbohydrates and undergo rapid growth [55] Moreover, the tree needs to consume a large amount of water to meet its metabolic needs while growing rapidly in the summer [49] However, high temperatures in the summer enhance the evaporation of the soil water content. Increasing soil water loss and raising the vapor pressure difference resulted in insufficient soil moisture, reducing the available water for trees. As a consequence, the root system of trees could not get enough water and limited the radial growth of trees [56]. In addition, in warm regions, high temperatures may accelerate the water evaporation and transpiration of trees, which reduces the available water in the soil. This result was the same as that of Yuan Yujiang et al [57]. In May of the current year, the low altitude area of Mongolian pine exhibited a significant positive correlation with precipitation, and a significant positive correlation with the number of warm night days and minimum temperature minima. This reveals that in the low altitude area, temperature and precipitation are the main factors limiting Mongolian pine's growth during the pre-growing season, and the increase in temperature and precipitation is favorable to the growth of this species. At this time, rising temperatures advance the growing season, and the pine ends its "dormant" state earlier. This is conducive to the tree's absorption of water and inorganic salts and accelerates its growth, which agrees with the results of Jiang Qianqian et al. [45].
In conclusion, global warming has a great impact on the growth of larch and pine in the Greater Khingan Range region, and the drought stress caused by high temperatures is the main factor affecting the growth of larch and pine in the growing season. As the growing season lengthens, the likelihood of larch experiencing frost damage in winter increases. Larch at higher latitudes are at greater risk of frost damage compared trees at lower latitudes. With global warming, the radial growth of Dahurian larch and Mongolian pine in the study area exhibited a differentiated response to extreme temperatures. This suggests that close attention should be paid to the stability of the response of tree growth to extreme climatic elements when carrying out tree-ring climatology studies on the influence of forest ecosystem development trends in the Greater Khingan Range region.