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

Abstract

Continuous half-hourly measurements of soil ( R s) and bole respiration ( R b), as well as whole-ecosystem CO 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 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 μmol m −2 s −1 during winter to a maximum of 9.2 μmol m −2 s −1 in mid-summer. Seasonal variations of R s were highly correlated with variations of soil temperature ( T s) and water content ( θ) 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 μmol m −2 bark s −1 (2.2 μmol 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 −1) 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 s to remove the limitations imposed by low θ, suggests that drought played a more important role than temperature in explaining interannual variations of R s and R e.