| As the third pole on the earth,the warming rate of the Tibetan Plateau was much higher than the global average level at the same period,and it has become a hot spot for the study of vegetation-climate relationships.Alpine shrubs are widely distributed in this region,they can grow at higher altitudes in treeless area,which plays a critical role in the expansion of tree-ring network around world.As an endemic deciduous shrub species in China,Salix oritrepha is widely distributed on the eastern Tibetan Plateau.Due to its distinctive characteristics of shrub-rings,Salix oritrepha can be a model species for studying the radial growth-climate responses of alpine deciduous shrubs.To this end,this study focused on the shrub chronologies of Salix oritrepha in three typical forest regions of the eastern Tibetan Plateau(Nangqian forest region(BZ),Leiwuqi forest region(LWQ),Basu forest region(BS)).Here,163 stem discs of Salix oritrepha were randomly selected along the elevational gradients(BZ and LWQ:4200 m,4400 m,4600 m;BS:4600 m,4800 m)in three regions of the eastern Tibetan Plateau.Based on the statistical methods,we aim to illuminate the shrub growth patterns and their growth-climate responses at 8 sampling sites(BZ42,BZ44,BZ46;LWQ42,LWQ44,LWQ46;BS46,BS48).Meanwhile,71 tree-ring samples of Picea likiangensis var.rubescens were collected at the timberline in BZ and LWQ forest regions.Growth dynamics of timberline trees were compared with that of willow shrub at the shrubline in order to reveal the growth-climate consistency of different growth form of vegetation.Therefore,the driving factors of shrub growth can be better understood.The main conclusions were as follows:(1)Shrub chronological characteristics and growth trends analyses showed that the growth rings of Salix oritrepha were regular on the eastern Tibetan Plateau,the mean shrub-ring width ranged from 0.26 to 0.36 mm,whilst the BAI varied from 4.35 to 5.95mm~2,and the value of mean sensitivity was more than 0.2,which indicated the higher sensitivity to climate signals.Along the elevational gradients,the radial growth of Salix oritrepha decreased with increasing elevation.Before 2011,shrub growth showed significant increasing trends at all elevational sites,however,growth decline was detected at six sites in recent decade.(2)The shrub growth-climate correlation analyses indicated that the correlations between BAI chronologies and climate were higher than that of RWI chronologies,which demonstrated more abundant climate information in BAI series.In terms of elevational gradients,all shrubs along altitudinal gradients of Nangqian forest region were limited by July temperature.In Leiwuqi forest region,the shrub growth of low and middle altitudinal sampling sites(LWQ42and LWQ44)was mainly affected by July temperature,while shifted to June temperature limited at the shrubline(LWQ46).In Basu forest region,the shrub growth reflected the restriction of July and August temperature along the altitudinal gradients,specifically,shrub growth was more responsive to August temperature at BS46,while shrub growth was more correlated with July temperature at the shrubline(BS48).However,the shrub growth responses to precipitation signals were relatively low in three forest regions.Along the latitudinal gradients,there were similar shrub growth responses in our study regions,showing the growth limitation of temperature during the growing season.(3)The comparative analyses of shrubline shrub growth-and timberline tree growth-climate responses showed that there were two forest regions(BZ and LWQ forest regions)in which timberline spruce showed the similar climate driving factor to willow shrub,i.e.,growing season temperature.Whereas,timberline tree growth showed positive responses to prior growing season precipitation,which reached a significant level.In BS forest region,shrub growth was mainly limited by July and August temperature at the shrubline,which showed"divergence"with tree growth-climate responses(prior growing season and growing season precipitation limited)at the timberline.In conclusion,Salix oritrepha was highly sensitive to climate change along the elevational gradients on the eastern Tibetan Plateau.Radial growth of willow shrub was mainly limited by the growing season temperature regardless of site elevation,while the relationship between shrub growth and seasonal precipitation were not significant.Before 2011,shrub growth showed significant increasing trends at all elevational sites,while decreasing trends were detected at most sites in the past decade,indicating the negative effect of warming-induced drought climates.Growth responses of shrubline shrub and timberline tree were affected by the hydrothermal condition,showing that"growth divergence"was easily caused by summer drought conditions.This study revealed the growth dynamics of typical deciduous shrub(Salix oritrepha)and their responses to climate change,which provided reference for the climate responses of alpine deciduous shrub ecosystems. |
