Dendroclimatic response of Picea mariana and Pinus banksiana along a latitudinal gradient in the eastern Canadian boreal forest

To decipher spatial and temporal tree-growth responses to climate change we used tree-ring data from Picea mariana (Mill.) BSP and Pinus banksiana Lam b. along a latitudinal transect in western Quebec. The transect encompassed the distinct transition between mixed and coniferous forests at approximat ely 49 degrees N. Correlation analyses and principal component analyses wer e used to identify common spatiotemporal growth patterns, and site- and spe cies-specific patterns since 1825. A moist summer in the year t - 1 and an early start of the current growing season favored growth of both species. A prolongation of the growing season into fall was the most distinguishing f actor between the species. A long and gradual climatic gradient shifted to a short gradient with a clear segregation between the southern and northern parts of the transect. This shift, around 1875, was abrupt and characteriz ed by a turbulent climatic period. The observed pattern was likely related to a large-scale shift in the mean position of the Arctic Front that occurr ed at the end of the 1800s. No discrete climatic setting explained the pres ent switch from mixedwoods to conifers at 49 degrees N. Awareness of such n onequilibrial relations between climate and species distribution is essenti al when assessing vegetation responses to future climate change.