| The Tibetan Plateau(TP),as one of the most sensitive ecosystems in response to global warming,provides an ideal place to study the relationship between vegetation and climate.High-elevation Rhododendron shrubs are widely distributed in the TP and has high dendrochronological potential.In this study,Rhododendron przewalskii Maxim was selected as the study species in three typical forest regions of eastern TP(Nangqian forest region(NQ),Leiwuqi forest region(LWQ)and Basu forest region(BS)).We collected ring-width data from 131 Rhododendron shrubs in three forest regions at the elevation of 4300 m and 4500 m.Methods of dendrochronology,correlation analysis and linear mixed-effects model(LMM)were combined to explore the growth dynamics and growth responses of Rhododendron shrubs to climate change at 6 sampling sites.In addition,growth pattern of Rhododendron shrubs in different altitude gradients and regions were compared and analyzed,aiming to reveal the change pattern of alpine bushes ecosystem in the eastern TP.The main conclusions of this paper were as follows:(1)The analysis of the chronology of Rhododendron shrub showed no significant difference in the mean ring-width of Rhododendron shrubs in the forest regions of NQ,LWQ and BS.The average ring-width varied between 0.20-0.43 mm,and decreased with the increasement of elevation in all forest areas.The mean sensitivity(MS)was between 0.19 and 0.42.Before the year of 2000,the radial growth of shrubs in all regions showed a significant increasing trend,but the BAI sequence of three samples showed a decreasing trend in the past 10 years.With the increase of elevation gradient,the shrub growth rate decreased in all forest regions except for NQ.(2)Correlation analysis between radial growth of Rhododendron shrub and climate showed a similarity in the growth-limiting climatic factors of Rhododendron shrub at different elevations in the three forest regions of TP,which indicates both a significant limitation of temperature factor,and insignificant influence of humidity factor.Specifically,the growth limiting factor of Rhododendron shrubs at different elevations in NQ forest area was the monthly mean maximum temperature in September of the previous year.The radial growth of shrubs in LWQ forest area was limited by the temperature of the growing season.With the change of elevation gradient,growth limiting factor changed from the monthly minimum temperature in August(LWQ43)to the monthly average temperature in August(LWQ45).In the BS forest region,shrubs in lowaltitude site were primarily limited by the maximum temperature in late winter(February of the current year),whereas shrubs in high-elevation site were mainly limited by the average temperature in September of the current year.In addition to climate factors,random effects with the shrub level condition(e.g.individual differences of shrub)exerted a certain influence on the radial growth of shrubs.(3)The discussion on the change of growth limiting factors of Rhododendron shrub along altitude gradient showed that the radial growth of Rhododendron shrub at different elevation in NQ forest region reflected the same climate signal,which was related to the warmer and drier trend in climate of autumn season.The relationship between the radial growth and climate change along the elevation gradient in the LWQ forest region reflects the water stress effect brought by higher temperature in the growing season.The shrub radial growth of low-elevation site in BS forest region benefited from the increasement of water supply,which is induced by the melting of snow cover due to the high average temperature in February of the current year,whereas that of high-altitude site was limited by the water deficit due to the high average maximum temperature in the late growing season(September of the current year).The comparative study on the growth dynamics of Rhododendron shrubs in other areas of southeastern TP shows regional differences in the growth limiting factors of Rhododendron shrub,which is mainly depend on regional climate but partly influenced by sampling site conditions.In conclusion,the radial growth of Rhododendron shrubs was mainly limited by growingseason temperature factor in the three typical forest regions of eastern TP.The radial growth of Rhododendrons increased significantly with time at all altitude gradients in NQ,LWQ and BS forest regions,but varying degrees of decreasing trend were also found in three sites in recent 10 years,which reflected the adverse effects of regional warming and drying.This study revealed the growth response of Rhododendron to climate changes,and provided a reference for understanding the ecological response pattern and strengthening forestry protection in TP region. |
