AbstractThe soil carbon dynamics of southern hemisphere temperate rainforests have rarely been studied. Here, we report for the first time soil CO2 effluxes and their partitioning for medium‐age and old‐growth Fitzroya cupressoides forests growing under contrasting environmental conditions in the Coastal Range and Andean Cordillera of southern Chile. Fitzroya is a high biomass and one of the longest lived species in the world. We characterized soil respiration patterns over almost 2 yr. Annual soil respiration was slightly higher in younger forests from the Coastal Range compared with Andean forests during the first studied year (6.37–6.66 vs. 5.06–6.14 Mg C·ha−1·yr−1), and significantly higher during the second year mainly due to a warmer and drier summer (8.08–8.64 vs. 4.98–5.35 Mg C·ha−1·yr−1). Therefore, warmer and drier conditions, likely to become more common in this region under future climate change, were associated with significantly higher respiration in the shallow soils of the coast, but not in the Andes. A higher proportion of autotrophic respiration was found in the Coastal Range forest probably due to a much higher fine root biomass in this site. However, fine root productivity, an important contributor of belowground carbon fluxes, was a little lower (not significantly) in the coastal site (0.81 ± 0.60 vs. 1.50 ± 0.42 Mg C·ha−1·yr−1), indicating a longer root residence time in forests from this area. Soil CO2 effluxes from these forests and their root productivity are at the lower end of values recorded for other mature and old‐growth temperate wet forests worldwide. The intrinsic longevity and the particularly poor soils and rainy conditions where these forests grow may influence these facts. Interannual climate variability appears to be especially important for soil respiration in the Coastal Range due to the more Mediterranean climate influence and shallow, poor water retention soils in this area.