Influence of temperature and drought on seasonal and interannual variations of soil, bole and ecosystem respiration in a boreal aspen stand

Continuous half-hourly measurements of soil (R-s) and bole respiration (R-b), as well as whole-ecosystem CO, exchange, were made with a non steady-state automated chamber system and with the eddy covariance (EC) technique, respectively, in a mature trembling aspen stand between January 2001 and December 2003. Our main objective was to investigate the influence of long-term variations of environmental and biological variables on component-specific and whole-ecosystem respiration (R-e) processes. During the study period, the stand was exposed to severe drought conditions that affected much of the western plains of North America. Over the 3 years, daily mean R-s varied from a minimum of 0.1 mu mol m(-2) s(-1) during winter to a maximum of 9.2 mu mol m(-2) s(-1) in mid-summer. Seasonal variations of R-g were highly correlated with variations of soil temperature (T-g) and water content (theta) in the surface soil layers. Both variables explained 96, 95 and 90% of the variance in daily mean R-s from 2001 to 2003. Aspen daily mean R-b varied from negligible during winter to a maximum of 2.5 mu mol m(-2) bark s(-1) (2.2 mu mol m(-2) ground s(-1)) during the growing season. Maximum R-b occurred at the end of the aspen radial growth increment and leaf emergence period during each year. This was 2 months before the peak in bole temperature (T-b) in 2001 and 2003. Nonetheless, R-b was highly correlated with T-b and this variable explained 77, 87 and 62% of the variance in R-b in the respective years. Partitioning of R-b between its maintenance (R-bm) and growth (R-bg) components using the mature tissue method showed that daily mean R-bg occurred at the same time as aspen radial growth increment during each growing season. This method led, however, to systematic over- and underestimations of R-bm and R-bg, respectively, during each year. Annual totals of R-s, R-b and estimated foliage respiration (R-f) from hazelnut and aspen trees were, on average, 829,159 and 202 g C m(-2) year(-1), respectively, over the 3 years. These totals corresponded to 70, 14 and 16%, respectively, of scaled-up respiration estimates of R-e from chamber measurements. Scaled R-e estimates were 25% higher (1190 g C m(-2) year') than the annual totals of R-e obtained from EC (949 g C m(-2) year(-1)). The independent effects of temperature and drought on annual totals of R-e and its components were difficult to separate because the two variables co-varied during the 3 years. However, recalculation of annual totals of R-e to remove the limitations imposed by low theta, suggests that droughtplayed a more important role than temperature in explaining interannual variations of R-s and R-c. (c) 2006 Published by Elsevier B.V.