The intra-annual rhythm of Pinus sylvestris growth-climate responses under a warming climate at its southern distribution limits

Warming climate has posed a pressing threat on boreal forests and an improved understanding of the intraannual pattern of climatic influences on the tree growth can help interpret the response of boreal forest to climate change. Here, we systematically examined the growth-climate relationship, cambium phenology, and xylem cell dynamics of Pinus sylvestris (PS) to disentangle its intra-annual growth rhythm to external drivers and internal physiological process at its southern distribution limits across Eurasia. We showed the intra-annual rhythm of PS to temperature and precipitation is anti-phase and synchronous at its southern limits. Temperature had both promoting and inhibiting effects on PS growth over growing season where the growth-temperature response gradually changed from positive in winter-early spring to negative in late spring-summer, and then returned to positive in autumn. Precipitation enhanced PS growth in late spring-summer. Temperature dominated the intra-annual growth-climate response rhythm, and the response shifted to negative when temperature reached 13.72 degrees C in spring-summer and shifted to positive when temperature reached 17.41 degrees C in summerautumn in Shenyang, respectively. A high water demand for the rapid earlywood cell formation in springsummer and heat requirement for cambial cell division in summer-autumn caused the shifts in growthtemperature response, respectively. Climatic warming advances and prolongs the time of summer water availability that limits PS growth at its southern distribution limits and thus warming climate may pose a greater threat to the southern population of PS.