Comparison Of Population Characteristics And Growth-Climate Relationship Of Sabina Przewalskii At The Timberlines With Different Precipitations In The Northeast Qinghai Province,China

To explore the differences of long-term dynamic tendency of Sabina przewalskii population characteristics and the growth-climate relationships of S.przewalskii along a precipitation gradient at timberlines of northeast Qinghai-Xizang Plateau.Three sides were selected to sample the alpine timberline along a precipitation gradient in the northeast Qinghai-Xizang Plateau:Halihatu National Forest Park in Wulan County(HL,annual precipitation 217 mm),Qushigang in Dulan County(QS,281 mm)and Hebei Forest Farm in Tongde County(HB?470 mm).The spatial distribution pattern and the age structure of the dominant tree species(S.przewalskii)population were analyzed,we calculated the long-term variation of population density with the method of compiling the static life table,then we analyzed the relationship between long-term variation of population density and climate change.The correlation and response analysis at seasonal and extreme climate year scales were used to examine the spatial variations of the growth-climate relationship of S.przewalskii at different timberlines.Along the precipitation gradients,the spatial distribution pattern and the location of the S.przewalskii population did not changed in recent years at the low(HL)and middle(QS)precipitation area(HB).However,the location of the S.przewalskii population at high precipitation area went up to the higher elevation.Age structure of the S.przewalskii population at high precipitation area appears like a pot,population density did not change significantly and the population was declining as a whole.The relationship between the long-term S.przewalskii population development and climate along a precipitation gradient shows that,along with the precipitation intensity,the effect of air temperature on the population development was gradually enhanced,and the effect of precipitation on the population development was gradually weakened,contrarily.Our results do not support the hypothesis that water is the main limiting factor of tree growth at the arid alpine timberline.The effect of precipitation on the radial growth of S.przewalskiiwere consistent across all three sampling sites,while the effects of temperature were different across sites.At HL site(low precipitation),the winter and summer minimum temperature were the main limiting factor of S.przewalskii radial growth,and this relationship did not significantly change in different extreme climate years.At QS site(middle precipitation),the radial growth of S.przewalskii was mainly limited by the minimum temperature in spring and summer,but its effect was weaker than that at low precipitation site.At HB site(high precipitation),the spring temperature had a significant negative effect on tree growth,and the positive effect of spring precipitation on tree growth was significantly enhanced in comparison with those at low and middle precipitation sites,especially in extreme high temperature and drought years.Summer precipitation did not significantly affect tree growth at high precipitation site.Our results did not support the hypothesis that the radial growth of trees at alpine timberline in arid/humid area is mainly limited by water/temperature.However,precipitation at timberline will affect the relationship between tree growth and temperature at different seasons.With the warming and humidification of the northeastern Qinghai-Xizang Plateau,the S.przewalskii population at alpine timberline in different regions may present the opposite trends of development,climatic limiting factors of tree growth in different timberline areas may be complicated.