Understanding how trees respond to variation in water availability is critical for predicting forest ecosystem responses to projected increases in drought frequency and severity under climate change. Intrinsic water-use efficiency (iWUE), derived from tree-ring stable carbon isotopes (δ¹³C), provides means to investigate the coupling between carbon assimilation and water use. In this study, we examined how spatial and temporal variations in water availability influence iWUE in temperate deciduous forests of eastern Canada. Annual tree-ring width and δ¹³C data were collected from four deciduous species (Acer saccharum, Betula alleghaniensis, Populus tremuloides and Tilia americana) across three temperate forest sites spanning a precipitation gradient (annual precipitation: 807, 1003 and 1408 mm). Interannual variability in the response to water availability was assessed by contrasting annual tree rings from years characterized by normal and dry summers, identified using the 3-month Standardized Precipitation-Evapotranspiration Index (SPEI) for July, with values close to 0 (normal) and <-1 (dry), respectively. Preliminary results indicate that δ¹³C values are higher (less negative) under drier conditions, both when comparing years with dry versus normal summers and when comparing across sites. These findings suggest that trees adjust their water-use efficiency in response to water limitation, as shown by spatial and temporal δ¹³C patterns. Overall, this work highlights water availability as a key regulator of forest carbon-water dynamics.