Spatial distribution and species composition of the boreal forest are expected to change under predicted climate change scenarios. Current research indicates that water limitations control the southern boundary of the central Canadian boreal forest and temperature limitations control the northern boundary. As part of Boreal Ecosystem - Atmosphere Study (BOREAS), we examined this idea by comparing annual variation in tree-ring widths and carbon isotope ratios ( delta13C) of tree-ring cellulose with annual climatic parameters in the northern and southern boreal forest. Contrary to expectations, climate correlations with ring widths at the northern and southern sites were similar in black spruce (Picea mariana(Mill.) BSP). Annual growth was favored by cooler and wetter conditions. For jack pine (Pinus banksianaLamb.), increased temperature and spring precipitation favored annual growth at both sites. In the north, annual growth was negatively correlated with winter precipitation. The delta13C - climate correlations inPinus banksianafollowed current distribution theories. In the south, potential evapotranspiration explained significant annual delta13C variation, whereas in the north, winter and growing season precipitation influenced annual delta13C variations. Our data support the concept that moisture limits the southern range ofPinus banksianaand cold soil temperatures limit the northern extent. However, colder, wetter conditions favored growth ofPicea marianathroughout its range. These observations strengthen the concept that species respond individually to climate change, not as a cohesive biome.