Continuous half-hourly measurements of soil (Rs) and bole respiration (Rb), as well as whole-ecosystem CO2 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 (Re) 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 Rs 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 Rs were highly correlated with variations of soil temperature (Ts) and water content (θ) in the surface soil layers. Both variables explained 96, 95 and 90% of the variance in daily mean Rs from 2001 to 2003. Aspen daily mean Rb 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 Rb 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 (Tb) in 2001 and 2003. Nonetheless, Rb was highly correlated with Tb and this variable explained 77, 87 and 62% of the variance in Rb in the respective years. Partitioning of Rb between its maintenance (Rbm) and growth (Rbg) components using the mature tissue method showed that daily mean Rbg occurred at the same time as aspen radial growth increment during each growing season. This method led, however, to systematic over- and underestimations of Rbm and Rbg, respectively, during each year. Annual totals of Rs, Rb and estimated foliage respiration (Rf) 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 Re from chamber measurements. Scaled Re estimates were 25% higher (1190 g C m-2 year-1) than the annual totals of Re obtained from EC (949 g C m-2 year-1). The independent effects of temperature and drought on annual totals of Re and its components were difficult to separate because the two variables co-varied during the 3 years. However, recalculation of annual totals of Rs to remove the limitations imposed by low θ, suggests that drought played a more important role than temperature in explaining interannual variations of Rs and Re.
Bibliographical noteFunding Information:
This research was funded by the Fonds de Recherche sur la Nature et les Technologies du Québec (FCAR) in the form of a 3-year graduate scholarship to DGG and the Natural Sciences and Engineering Research Council of Canada (NSERC) through an operating grant to TAB. Additional support was provided by the Fluxnet Canada Research Network (through NSERC, the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS) and BIOCAP, Canada) and the Meteorological Service of Canada through a Contribution Agreement to the University of British Columbia. The authors sincerely acknowledge the technical assistance of Andrew Sauter for the development of the automated chamber system, Ted Hogg for providing dendrometer measurements of aspen radial growth increment and Dave Wieder (Parks, Canada) for maintaining the equipment in the field. The authors also wish to thank Michael Novak, Robert Guy, Cindy Prescott and two anonymous reviewers for constructive reviews of the manuscript.
Copyright 2008 Elsevier B.V., All rights reserved.
- Carbon exchange
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- Soil water content
- Temperature sensitivity