INTRODUCTION

In view of a rapidly changing climate system the terrestrial carbon cycle has received an increasing amount of attention over the last two decades, both from scientists and the public. Much progress has been made on characterizing the net exchange of CO2 between terrestrial ecosystems and the atmosphere (NEE) (Baldocchi et al., 2001; Schimel et al., 2001; Chapin et al., 2006; Friend et al., 2007; Luyssaert et al., 2007; Baldoochi et al., 2008). However, to improve our estimates of the carbon sequestration potential of ecosystems and to be able to project current flux observations into the future, it is important to obtain a better understanding of the component fluxes of NEE, and how they are regulated in response to changing environments. Our knowledge of the assimilatory component of the carbon cycle (i.e. photosynthesis) is well advanced both at the leaf and the canopy level (Farquhar et al., 1980; de Pury and Farquhar, 1997). In contrast, there are still substantial gaps in our understanding of the respiratory component, which is a major determinant of ecosystem carbon balance (Valentini et al., 2000; Huxman et al., 2003; Luo and Zhou, 2006; Trumbore, 2006). Even though emissions of CO2 from soils globally constitute the second largest flux of carbon between terrestrial ecosystems and the atmosphere (Raich and Schlesinger, 1992; Schlesinger and Andrews, 2000), their potential response to global change is still largely assessed on the basis of simplistic assumptions and relationships (for recent critical reviews cf. Davidson and Janssens, 2006; Davidson et al., 2006; Högberg and Read, 2006; Trumbore, 2006).