Tropical forest water use is important for maintaining ecosystem function, tree productivity, growth, survival and nutrient cycling. However, explaining such use is complex in the field. Water stable isotope tracing of plant water use can shed light on such plant water sources but to date, species numbers tested at any given site across the globe have been minimal. Additionally, past tree water source studies were mostly based on methods using single isotopes of either (δD or δ 18 O) at a coarse spatiotemporal resolution. This thesis used a combination of a dual stable isotope approach together with sap flux to understand species specific water utilization strategies of co-occurring tropical rainforest species at a fine spatiotemporal scale.A systematic review of global research on woody plant water sources was undertaken to evaluate the spatiotemporal dynamics of research on woody species water sources and to assess the research priorities in the study of woody species water sources (Chapter 2). Most studies were from the USA with various other countries having between one and four studies only and mostly focused on the Pinaceae family. The review indicates there is a clear variation in woody plant water sources in the forest due to season, climate, leaf phenology and method of measurement.Majority of the tree species obtained water from soil, followed by groundwater. Most of the research focus has been on identifying plant water sources using a single isotope. Much less focus was given to the nexus between water source and tree growth, drought, water use efficiency, agroforestry, groundwater interaction and many other topics.A further systematic review was undertaken on research related to tree water use (Chapter 3).This review found a clear bias in research focus relating to geographic area and species group selection. Most of the studies (33.33%) were undertaken in Central America. Only Fabaceae, Myrtaceae, Dipterocarpaceae and Anacardiaceae families were given priority. Tree size and seed mass was positively correlated with water use. In contrast, wood density showed a negative relationship with tree water use. Season is highly significant in explaining the variation of tree water use as was leaf phenology. Tropical trees' water use significantly increases in dry season.There is no significant difference in water use between native and exotic species.A comparison of δD and δ 18 O isotopes between soil water and xylem water was used to investigate niche segregation among wet tropical rainforest tree species at a fine spatial scale (Chapter 4). Tropical forest water use is critical for tree productivity, growth, survival and nutrient iii cycling, but describing such uses is difficult in the field. Stable isotope tracing of plant water use can illuminate plant water sources but to date, the number of species tested at any given site has been minimal. Here, 46 tropical hardwood tree species (49 individual trees) in a 0.32 ha plot with uniform soils were sampled. Soil water was characterized at 6 depths at 0.2 m ...