Labrador Sea's influence on black spruce forests: insights from tree-ring stable oxygen isotopes

Black spruce (Picea mariana Mill. B.S.P.), is a dominant species emblematic of eastern Canada's boreal forests. It grows in a vast array of climatic conditions ranging from cold maritime-oceanic climates near the Labrador Sea shore to cold continental conditions in the central portions of Quebec-Labrador. However, along the continentality gradient, timing and provenance of heat and moisture that support growth are uncertain, weakening our capacity to describe and predict the response of boreal ecosystems to climate variability. Here, we measured oxygen isotope composition in tree-ring cellulose of black spruces from three sites, and provide evidence of a direct influence of the Labrador Sea on adjacent continental ecosystems. Our results report a landwards decrease in δ18Otrc, a pattern that is also visible in the simulated oxygen isotope composition of precipitation water (δ18Op). We also reveal a landwards decoupling between δ18Otrc variability (1950-2013) and maximum temperature (Tmax) variations over the Northwest Atlantic. Our results imply that tree-ring oxygen isotopes may be of great help to circumscribe the spatial extent of Labrador Sea's influence on surrounding boreal ecosystems. Moreover, our study reveals that despite of their apparent ecological homogeneity, eastern Canada’s black spruce forests clearly rely on heterogeneous sources of heat and moisture.