The relationship between stem CO2 efflux (ES), cambial activity and xylem production in Pinus cembra L. was determined at the timberline (1950 m a.s.l.) of the Central Austrian Alps, for 1 year. The ES was measured continuously from June 2006 to August 2007 using an infrared gas-analysis system. Cambial activity and xylem production were determined by repeated microcore sampling of the developing tree ring, and radial increment was monitored using automated point dendrometers. Besides temperature, the number of living tracheids and cambial cells was predominantly responsible for ES, and ES normalized to 10 #C(ES10) was significantly correlated to the number of living cells throughout the year (r2= 0.574; P < 0.001). However, elevated ES and missing correlation between ES10 andxylem production were detected during cambial reactivation in April and during transition from active phase to rest, which occurred in August and lasted until early September. Results of this study indicate that (i) during seasonal variations in cambial activity, nonlinearity between ES and xylem production occurs and (ii) elevated metabolic activity during transition stages in the cambial active-dormancy cycle influences the carbon budget of P. cembra. Daily radial stem increment was primarily influenced by the number of enlarging cells and was not correlated to ES
