Changes in wood and foliar δ13C in sugar maple at Gatineau Park,Quebec, Canada

Leaf samples and tree rings formed between the mid‐1960s and mid‐1990s from sugar maple (Acer saccharum Marsh.) at Gatineau Park (45°30′ N, 75°54′ W), Quebec were analysed for δ¹³C. Leaf samples were collected at ground level (1–2 m above ground) at monthly intervals during the summer, whereas tree cores were extracted from the largest trees (d.b.h. > 30 cm) in the young deciduous forest in August 1998. Significant linear decreases in δ¹³C over time were found in foliage and tree rings, but the decrease in δ¹³C was significantly greater in foliage than in the wood. The apparent isotopic discrimination (Δ) of tree rings varied insignificantly around a mean of 18‰, whereas foliar Δ increased significantly from 19‰ in the 1960s to around 23‰ by the mid 1990s, likely as a result of an increasing canopy effect as the forest matured. Using models of carbon discrimination and Δ‐values of the tree rings, we calculate that the intrinsic water use efficiency of mature sugar maple has increased by approximately 4% over the study period.