Responses Of The Subalpine Fir (Abies Fargesii) Forests In The Qinba Mountains To Regional Climatic Variability

It is widely accepted that global warming had occurred with an increase ofapproximate 0.6℃in the global mean air temperature by the end of the 20thcentury. In the Qinba Mountains, meteorological station records show that theregional climate has obviously changed with increases in mean airtemperature and precipitation during the past fifty years. Such changes mayhave affected the regional subalpine fir (Abies fargesii) forests and, in turn,these forests have been responsive to the regional climate changes. Thus,chronological techniques, coupled with methodology of plant community, wereemployed to explore the responses of the subslpine fir forests to regionalclimatic changes from multiple-scale level, including population, communityand landscape, in the Qinba Mountains. The main results are as following:Correlation analyses indicate that the radial growth of Abies fargesii variesalong the elevational gradient and in different aspects in response to differentclimate variables in the Qinling Mountains. At the middle elevations (typicaldistribution zones of the fir forests) and lower limits of the subalpine fir forestson both the south and north aspects of the Qinling Mountains, the radial growthof A. fargesii is significantly affected by the spring and summer temperatures ofthe current year, while precipitation has no significant effects on A. fargesiiradial growth. As to the upper limits of the subalpine forests on both the southand north aspects of the Qinling Mountains, precipitation in current summerimposes noticeable effects on its radial growth, but temperature does not havesignificant effects. In addition, climatic variables of the previous year havesignificant effects on A. fargesii radial growth of the current year on the southaspect but not on the north aspect of the Qinling Mountains.Correlation analyses between ring-width index and climate variables alsoindicate that radial growth of A. fargesii in different elevations is influenced bydifferent climate factors in the Shennongjia Mountain. At the middle elevations (typical distribution zones of the fir forests) and lower limits of the subalpine firforests on both the south and north aspects, radial growth of A. fargesii issignificantly limited by a combination of temperature and precipitation; At upperlimits of both the south and north aspects, however, temperature is the mainfactor limiting its radial growth. So, the influence of precipitation on A. fargesiiradial growth decreases with the increase in elevation, while the influence oftemperature increases with the increase in elevation. Additionally, climatevariables of previous year have significant effects on A. fargesii radial growthon both the south and north aspects of the Shennongjia Mountain.Distribution of A. fargesii shows random patterns at lower limits andmiddle elevations, while clumped patterns at upper limits of the subalpine firforests in the Qinba Mountains. According to the viewpoint "clumpeddistribution conduces to individual survival", we believe the characteristics ofdistribution patterns of A. fargesii along elevation gradient to be one mean ofthe fir forests' responses to the regional climate changes.Age-class distribution along the elevation gradient shows that the numberof fir individuals on small age-class and middle age-class increases with theincrease in elevation, but the number of fir individuals on the large age-classdeclines with the increase in elevation in the Qinba Mountains. In addition, theage structure of fir population exhibits a declined trend in the middle elevationsand lower limits, and a stable trend in the upper limits of the subalpine firforests on both the north and south aspects in the Qinba Mountains.On both the south and north aspects in the Qinba Mountains, the standoriginated later in the higher elevations than that in the lower elevations. At thehigher elevations, especially in the upper limits of the subalpine fir forests inthe Qinba Mountains, a considerably large number of fir trees were recruitedinto the stand before the 1970s in the 20th century, while little recruitment of firtrees occurred in the lower elevations during the 20th century, demonstratingthat climatic warming in the 20th century resulted in recruitment of fir trees aswell as an increase in the amount of fir trees at the higher elevations, especially in the upper limits of the subalpine fir forests in the QinbaMountains.Analysis of the age structure of fir (A. fargesii) population shows that themean age of fir trees declines with the increase in elevation on both the southand north aspects in the Qinba Mountains. The fir tree age in the upper limits issignificantly smaller than that in the lower limits of the subalpine fir forests,suggesting that the distribution range of the subalpine fir forests has shiftedupward during the past century, attributable to the regional climate warming inthe Qinba Mountains. However, as a codominant species in the fir forests,birch age does not exhibit significant difference along altitude gradient,indicating no distinct shift of birch distribution range under the context ofregional climate warming in the Qinba Mountains.