Radial Growth Characteristics Of Piuns Pumila And Its Response To Climate Warming In Treeline Of Northeast China

In recent decades,the northeast forest area is the largest forest region and the most significant regions of warming in China.Global change affects the spatial distribution of vegetation.As a biogeographic transition zone,the treeline is an important indicator of environmental change.There are many key factors and abrupt changes in ecological processes within a short distance of the treeline,which is regarded as an ideal "monitor" of global change due to its sensitivity to climate change and instability of its own structure.Under the background of global warming,this thesis utilizes dendrochronology method to analyze the response difference and consistency of Pinus pumila,Larix gmelinii and Pinus sylvestris var.Mongolia to climate change.To study the response of shrubs and trees to climate change under the background of global warming is of great significance for the study of vegetation dynamic change of treeline in northern China,and provides theoretical support and scientific basis for the management and sustainable utilization of forest in northern China.(1)Pinus pumila is a crucial shrub species of subalpine treelines in northeast Daxing’an Mountains,China,but the growth-climate relationship for this eccentric shrub is not well understood.We investigated the growth-climate relationships in four reaction directions(A and B:left and right perpendicular radii;C:opposite radius to compression wood;D:compression wood radius).Results indicated that the shrub-ring width at the D direction was the widest(0.70±0.14 mm/a)of the four directions,while the opposite direction(C)is narrowest(0.43 ±0.07 mm/a).Water availability in June was the major limiting factor for P.pumila growth in the A and B directions.However,the radial growth in the C direction was mainly affected by mean and maximum,temperatures in June,and growth in the D direction positively correlated with mean temperature in the current February and the previous October and minimum temperature in the previous November.In addition,the radial growth at all four directions negatively correlated with the sunshine hours in June of the current year,which could be related to the increase of soil drought and respiration caused by higher temperatures.Radial growth at the A,B,and C directions negatively correlated with the freeze-thaw magnitude and frequency in May,which may cause the frost damage in shrub leaves and roots.Our findings suggested that the eccentric shrub growth at the perpendicular radii may not contain the best climate signals,while the growth at the compression wood direction seems to be of normal use in dendroclimatology.(2)Temperature and precipitation can change at different elevations.Will the climatic factors that limit the growth of shrubs at different elevations change?The relationship between the growth and climate of Pinu pumila at different elevations was analyzed in the Aokelidui Mountain of Along Borest Bureau in the Daxing’an Mountains.The relationship between the radial growth of Pinus pumila and climate at different altitudes in Aokelidui Mountain of the north Daxing’an Mountains of China is different.The growth of P.pumila at low altitude is mainly limited by spring temperature(March-May),and it at medium altitude is mainly positively affected by the precipitation in growing season.High temperature in winter is beneficial to the growth of P.pumila at low and medium-altitude,but detrimental for the growth at high altitude.After the rapid climate warming(1980s),the relationship between the temperature of growing season and the growth of P.pumila at high altitude changed from negative to positive,whereas the relationship between the precipitation of growing season and the growth changed from positive to negative.We also found that the sensitivity of P.pumila growth to SPEI increase significantly at medium altitude since 1980s.Therefore,in recent decades,the water availability is becoming more and more important for the radial growth of P.pumila at medium altitude.However,climate warming is favorable to the growth of P.pumila at low and high altitude.Due to the low-stature of shrub,microtopography and wind speed,more studies on shrub growth in different sites are needed to explain the mechanism of the climate-growth relationships of P.pumila and shrub expansion into the sub-alpine regions.(3)Along the latitudinal gradient,the environmental conditions will also change greatly.Then,under different latitude gradients,will the limiting factors for the growth of line shrubs be different?In northeast China,the radial growth data of Pinus pumila were collected from 7 treelines from north to south,and the relationship between growth and climate was analyzed.There is a significant difference between the ring width of Pinus pumila in the southern sampling sites and that in the northern sampling points.Among them,the difference between DBS and AK is the largest,and there is no difference between AES and HM,but there is no rule in the variation of annual ring width along the latitude gradient.The radial growth of most of the P.pumila at higher latitude(HM,AK,ZL)was positively correlated with temperature,while the radial growth of most of the P.pumila at lower latitude(TS,AES)was negatively correlated with temperature.The results of moving correlation showed that the growth of P.pumila at high latitude(ZL,AK,HM)and high altitude LBS were positively correlated with winter temperature,especially after 1990s.The correlation between the winter temperature and the growth of P.pumila at AES was changed from significant negative correlation to non-significant.The growth of P.pumila at ZL has changed from negative to non-significant correlation with summer temperature.If global warming continues,the southernmost the distribution of P.pumila may shift northward.However,the distribution of northernmost P.pumila can move north is still uncertain and needs further study.(4)Trees and shrubs are the two most important plant life types in forest ecosystem.The relationship between the growth and climate of Pinus sylvestris var.Mongolia,Larix gmelinii and Pinus pumila was compared in the samples collected from Mo’erdaoga National Forest Park(ME),Hanma National Nature Reserve(HM),Yikesama National Forest Park(YK)and Yong’an Forestry Bureau(YA)in the northern of the Daxing’an mountains.Our results are consistent with those in previous studies,i.e.,shrubs are less responsive to air temperature than coexisting trees in mixed forest.However,the radial growth of shrubs is more affected by precipitation in winter and spring than that of trees,which may be due to the shallow root system of the shrubs.The results showed that the temperature threshold at the beginning and end of the shrub(P.pumila)growing season was higher than that of trees because of the differences in their xylem characteristics and wood formation time.This suggests that trees are more vulnerable to global warming than shrubs in cold regions.This not only explains why shrubs exist in environments where trees cannot survive but also supports the idea that shrubs spread and have a "greening" effect in high-latitude or high-altitude areas.Therefore,P.pumila can be used as a credible indicator of potential climate change in the Daxing’an Mountains,and shrubs will play an increasingly important role in dendroecology and dendroclimatology studies.In addition,the different impacts of climate change on the growth of trees and shrubs can be used to further predict the dynamic changes in boreal forest vegetation in the future.