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The treeline,which is defined as the upper limit of altitudinal tree distribution,is one of the most conspicuous vegetation boundaries worldwide.Treeline environments are generally characterized by harsh climatic conditions,which restrict tree growth and metabolic functions.Tree growth at the treeline is very sensitive to environmental changes,and it may be immediately influenced by small changes to limiting factors.Therefore,tree growth at the treeline can be used as an amplifier of external environmental changes,which has important implications for global climate change research.Fine resolution studies of tree radial variation on short temporal scales can be useful for exploring the interplay of the main physical variables that trigger the radial variation.From 19 Oct.2008 to 17 Oct.2009,we continuously monitored the stem radial variation of Larix principis-rupprechtii trees using point dendrometers,at the treeline of the Luya Mountains,in Shanxi,northern China.Our results showed that there were two reverse daily variation patterns in the warm and cold seasons,which were related to the daily transpiration rates and daily stem freeze-thaw cycles,respectively.In the warm seasons,the diurnal stem variation pattern was resolved into three phases:(1) radius contraction,(2) radius expansion,and(3) radius increment.Generally,during daytime,when tree transpiration exceeded water absorption,stem diameters usually decreased,and stem expansion occurred at night because of greater water absorption by the roots compared with water loss due to transpiration.In our study,a wide range of stem radius variations occurred during the stem freeze-thaw cycles during the cold seasons when the air temperature fluctuated around-5℃.The course of the cumulative radial variation during the year showed similarities among individuals representing the characteristic seasonal patterns.The average net stem radial increment of Larix principis-rupprechtii during the growing season was(2014.1±240.5) μm.Annual stem radial variation of Larix principis-rupprechtii was divided into four distinct periods:(1) spring stem rehydration,(2) summer stem rapid growth,(3) autumn stem dehydration contraction,and(4) winter stem stagnation.During different growth periods,the major environmental factors controlling stem radial variation were different.During the first period,the large radial increments were due to stem tissue rehydration by root pressure in early spring,and soil water content became the crucial environmental factor influencing stem radius variation.During the period of rapid stem growth in the summer,soil temperature was the major determining factor.Low soil temperature can inhibit net photosynthesis of Larix principis-rupprechtii,distribution of non-structural carbohydrates in the stem,and root activity and water uptake.The third period was characterized by the cessation of increments accompanied by dehydration of stem cells.These changes accompanied the declines in temperature and soil water content.Changes occurring in the cambia likely allow for over-wintering,which may be a survival strategy to avoid frost damage during cold winter conditions.During the fourth period,Larix principis-rupprechtii entered the dormancy stage and the stem remained relatively stable.It was observed that fluctuations in the radius of Larix principis-rupprechtii were mainly related to the changes in air temperature.