[1] High-resolution climate proxy records covering the last two millennia on the Qinghai-Tibetan Plateau are scarce yet essential to evaluation of the patterns, synchroneity and spatial extent of past climatic changes including those in the Medieval Warm Period (MWP) and the Little Ice Age (LIA). Here we present a 2326-year tree-ring chronology of Sabina przewalskii Kom. for Dulan area of northeastern Qinghai-Tibetan Plateau. We find that the annual growth rings mainly reflect variations in regional spring precipitation. The greatest change in spring precipitation during the last two millennia seems to occur in the second half of the 4th century. The North Atlantic MWP was accompanied by notable wet springs in the study region during A.D. 929-1031 with the peak occurring around A.D. 974. Three intervals of dry springs occurred in the period of LIA. Our tree-ring data will facilitate intercontinental comparisons of large-scale synoptic climate variability for the last two millennia.

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To address the central question of how climate change influences tree growth within the context of global warming, we used dendroclimatological analysis to understand the reactions of four major boreal tree species -Populus tremuloides, Betula papyrifera, Picea mariana, and Pinus banksiana -to climatic variations along a broad latitudinal gradient from 46 to 541N in the eastern Canadian boreal forest. Tree-ring chronologies from 34 forested stands distributed at a 11 interval were built, transformed into principal components (PCs), and analyzed through bootstrapped correlation analysis over the period 1950-2003 to identify climate factors limiting the radial growth and the detailed radial growthclimate association along the gradient. All species taken together, previous summer temperature (negative influences), and current January and March-April temperatures (positive influences) showed the most consistent relationships with radial growth across the gradient. Combined with the identified species/site-specific climate factors, our study suggested that moisture conditions during the year before radial growth played a dominant role in positively regulating P. tremuloides growth, whereas January temperature and growing season moisture conditions positively impacted growth of B. papyrifera. Both P. mariana and P. banksiana were positively affected by the current-year winter and spring or whole growing season temperatures over the entire range of our corridor. Owing to the impacts of different climate factors on growth, these boreal species showed inconsistent responsiveness to recent warming at the transition zone, where B. papyrifera, P. mariana, and P. banksiana would be the most responsive species, whereas P. tremuloides might be the least. Under continued warming, B. papyrifera stands located north of 491N, P. tremuloides at northern latitudes, and P. mariana and P. banksiana stands located north of 471N might benefit from warming winter and spring temperatures to enhance their radial growth in the coming decades, whereas other southern stands might be decreasing in radial growth.

40The interaction between xylem phenology and climate assesses forest growth and productivity 41 and carbon storage across biomes under changing environmental conditions. We tested the annual temperature, from 83.7 days at -2 °C to 178.1 days at 12 °C, at a rate of 6.5 days °C -1 . 54April-May temperatures produced the best models predicting the dates of wood formation. 55Our findings demonstrated the uniformity of the process of wood formation and the 56 importance of the environmental conditions occurring at the time of growth resumption. 57Under warming scenarios, the period of wood formation might lengthen synchronously in the 58