Response of the central Canadian treeline to recent climatic changes

Dendrochronology and the analysis of historical records from eighteenth, nineteenth, and early twentieth-century expeditions are used to reconstruct the response of the central Canadian treeline to recent climatic variations. Cold summer temperatures during the early to mid-nineteenth century contributed to low rates of growth and poor recruitment of white and black spruce at treeline. Despite the harsh conditions, however, individuals of both species were able to persist at sites that define their northern range limits today. After 1880, increases in regional and hemispheric temperature are associated with increased growth rates and recruitment by both white and black spruce. A drop in temperatures during the 1960s and 1970s corresponds with declines in growth rates and recruitment. Growth rates have responded positively to higher temperatures in the 1980s, yet despite this positive response to recent temperature increases, dendrochronological and historical evidence suggest that there has been no significant northward extension of spruce range limits or continuous forest. It is likely that future climate warming, caused by the anthropogenic increases in greenhouse gases, will initially produce increased growth and higher rates of recruitment in treeline stands. The extension of the boreal forest northward, however, could experience a significant lag relative to climate warming.