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

摘要

Continuous half-hourly measurements of soil (R sub(s)) and bole respiration (R sub(b)), as well as whole-ecosystem CO sub(2) 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 sub(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 sub(s) varied from a minimum of 0.1 mu mol m super(-2) s super(-1) during winter to a maximum of 9.2 mu mol m super(-2) s super(-1) in mid-summer. Seasonal variations of R sub(s) were highly correlated with variations of soil temperature (T sub(s)) and water content (胃) in the surface soil layers. Both variables explained 96, 95 and 90% of the variance in daily mean R sub(s) from 2001 to 2003. Aspen daily mean R sub(b) varied from negligible during winter to a maximum of 2.5 mu mol m super(-2) bark s super(-1) (2.2 mu mol m super(-2) ground s super(- 1)) during the growing season. Maximum R sub(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 sub(b)) in 2001 and 2003. Nonetheless, R sub(b) was highly correlated with T sub(b) and this variable explained 77, 87 and 62% of the variance in R sub(b) in the respective years. Partitioning of R sub(b) between its maintenance (R sub(bm)) and growth (R sub(bg)) components using the mature tissue method showed that daily mean R sub(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 sub(bm) and R sub(bg), respectively, during each year. Annual totals of R sub(s), R sub(b) and estimated foliage respiration (R sub(f)) from hazelnut and aspen trees were, on average, 829, 159 and 202 g C m super(-2) year super(-1), respectively, over the 3 years. These totals corresponded to 70, 14 and 16%, respectively, of scaled-up respiration estimates of R sub(e) from chamber measurements. Scaled R sub(e) estimates were 25% higher (1190 g C m super(-2) year super(-1)) than the annual totals of R sub(e) obtained from EC (949 g C m super(-2) year super(-1)). The independent effects of temperature and drought on annual totals of R sub(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 sub(s) to remove the limitations imposed by low 胃, suggests that drought played a more important role than temperature in explaining interannual variations of R sub(s) and R sub(e).