model nor the Arrhenius theory has been able to describe the highly variable response of respiration to changes in temperature under field conditions (Tjoelker et al. 2001 , Atkin and Tjoelker 2003 , Atkin et al. 2005) . The response of plant respiration to changes in environmental conditions may be especially complex and far-reaching with respect to highlydiverse tropical ecosystems. In this context, the impacts of global climate change on plant respiratory activity urgently warrant further

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... estigation. Chapter 1 9 rates, which determine net ecosystem production (NEP). Consequently, estimating ecosystem respiration is an important item to identify the carbon source or sink strength of different terrestrial biomes , Malhi and Grace 2000 , Luyssaert et al. 2007) . Measuring forest CO 2 exchange Principally, two technical approaches are considered for determining the amount of CO 2 exchange: the eddy covariance technique and chamber measurements. The recently developed eddy covariance (EC) method allows for a comprehensive measurement of CO 2 exchange between large terrestrial areas and the atmosphere. EC is a high-technology, equipment-intensive method, which facilitates the quantification of net CO 2 fluxes entering and leaving a system over temporally large scales (hourly to annually); highly elaborate, only few studies have been conducted in tropical regions. In addition, for capturing atmospheric CO 2 this technique requires sustained high atmospheric turbulences within the system. An accumulation of CO 2 below the EC measurement height due to low turbulences has often resulted in underestimates of ecosystem respiration especially during night-time, when airstreams cease. This factor particularly constraints night-time measurements in tropical forests (Grace et al. 1995). While the EC technique provides valuable information about net exchange rates of the ecosystem as a whole, chamber measurements on a spatially limited area allows for the investigation of individual respiratory sources and hence, for a more physiological interpretation of carbon fluxes (Amthor 2000). Generally, the impacts of important environmental factors (e.g., temperature, humidity, radiation) can be deduced from net changes in the entire flux, but it remains unknown, which ecosystem component is the responsive one. Underlying mechanisms of flux dynamics and its driving forces can be better understood by breaking the net flux down into flux subcomponents. Within a forest ecosystem such components could comprise the two large ecosystem fluxes (heterotrophic and autotrophic), functional groups (e.g., understory, trees, palms, lianas) or single plant organs (e.g., leaves, branches, stems, roots). Information on the CO 2 exchange patterns of various components is crucial if we want to gain deeper insight into the physiologically meaningful processes within a plant community and its plastic response to changing environmental conditions. At its northern limit, in the valley of the Rio San Francisco, the research station "Reserva San Francisco" (RSF, formerly ECSF) borders the National Park. The Reserva, covering an area of about 1000 ha, is owned by the foundation Nature and Culture International (NCI), Ecuador and is rented by the DFG for the ongoing investigations of the research unit FOR 402. Emerging between the humid Amazon basin and the dry inter-Andean valley, the area encloses various tropical montane forest ecosystems (Richter 2003). The Bendix J, Lauer W (1992) Die Niederschlagsjahreszeiten in Ecuador und ihre klimadynamische Interpretation. Erdkunde 46, 118-134. Bolin B, Fung I (1992) Report: the carbon cycle revisited. In: Modelling the earth system.