Relationships between environmental factors and stem radius variation at short temporal scales can provide useful information regarding the sensitivity of tree species’ productivity to climate change. This study used automatic point dendrometers to assess the relationship between environmental variables and stem radius contraction and increment in ten Fitzroya cupressoides trees growing in two sites, the Coastal Range (Alerce Costero National Park) and the Andean Cordillera (Alerce Andino National Park) of southern Chile. The growing season in each site, determined using stem daily cycle patterns for each month, was longer in the Coastal Range site than in the Andes. Warmer and sunnier conditions were positively related with daytime tree radius contraction in both areas, and relationships were stronger in the Coastal Range site where more pronounced shrinking events were associated with prolonged warm and dry conditions compared to the Andes. Stem increment was positively related with precipitation and humidity in both sites, reflecting the positive effect of water on cell turgidity and consequent enlargement. Relationships between stem radius change and environmental variables considering longer temporal scales (7 to 31 days), confirmed a stronger association with humidity/vapor pressure deficit and precipitation, rather than with temperature. Although Fitzroya grows in particularly wet and cool areas, current and projected drier and warmer summer conditions in southern Chile may have a negative effect on Fitzroya stem increment and carbon accumulation in both sites. This effect would be more critical in the Coastal Range compared with the Andes though, due in part to more limiting soil conditions and less summer precipitation in this area. Long-term research is needed to monitor different aspects of the response of these endangered ecosystems to this additional threat imposed by climate change.