This thesis attempted to determine whether precipitation at CFB Borden, Ontario Canada is derived from recycled or marine source water using δ18O and to explore the enrichment of leaf water in three temperate forest trees. The d-excess values and rainfall intensity were used as indicators of recycled water and were found to suggest that precipitation at Borden is not derived from recycled water. The leaves of ash, aspen and maple showed significant enrichment in δ18O over xylem water. Maple leaves were significantly more enriched over the leaves of ash and aspen (p-value = 0.0019), despite source water not being significantly different (p-value = 0.1782). Modelling leaf water demonstrated the need for the inclusion of the Peclet effect in the steady state model for ash and aspen leaves, however for maple, the Peclet effect is insignificant. The non-steady state model used measured values of δET, which were uncertain and lead to poor model predictions. / NSERC Strategic Project #351040