The changes in temperature and precipitation regimes predicted for boreal regions are expected to profoundly affect the productivity of trees situated on waterlogged soils, which underlie large areas of boreal forest in Canada. The consequences for productivity of climatic variation likely depend on site ecology and differ between species. To investigate potential site-dependent responses to climate, we compared the growth of jack pine (Pinus banksiana) trees growing within the same climate conditions but along a transect extending from a fen margin to a sandy ridge. On the fen-margin site, black spruce trees (Picea mariana) were also sampled, allowing for a comparison of the sensitivity of both species. Growth variations at interannual and decadal frequency were analyzed from tree-rings. Our study revealed that the response to climate varied substantially between sites, but, surprisingly, trees growing on the fen margin proved to be very sensitive to fluctuations of precipitation at both interannual and decadal time scales. Black spruce trees responded more sensitively to climate variation than jack pine sampled at the same site. Our study also showed a modest response to temperature even in the driest site. For both species, decadal signals show precipitation as a common, strong productivity driver regardless of water table constraints. These results suggest that the predicted climate warming is therefore less of a threat to the productivity of peatland forested zones than fluctuations in the precipitation regime. Changes in precipitation are expected to have much stronger consequences at both interannual and decadal time scales than projected temperature fluctuations.