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 (dD or d18O) 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).n 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 treesr water use significantly increases in dry season. There is no significant difference in water use between native and exotic species.A comparison of dD and d18O 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).n Tropical forest water use is critical for tree productivity, growth, survival and nutrient 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 intervals down to 1 m and showed simple and predictable depth patterns, and simple and spatially uniform isotope composition at each depth. But tree xylem water dD and d18O showed remarkable variation covering the full range of soil composition, suggesting strong sorting and niche segregation across the small plot. A multivariable model Principle Component Analysis (PCA) incorporating wood density, tree size and mean basal area increment (MBAI) could explain 54.8% of the variance of xylem water isotope composition. This work suggests that stable isotope tracers may aid a better understanding of hydrological niche segregation among co-occurring tropical species and in turn, help inform better mixed-species plantation designs and predictions about future shifts in the composition and structure of tropical rainforest species under climate change.Water uptake depth variation of 46 rainforest tree species was investigated using a dual isotope approach and a Bayesian mixing model (BMM) (Chapter 5). The null hypothesis is that all the co-located tropical hardwood species show the same xylem water isotope composition and hence the same depths of soil water extraction. By grouping the soil layers into five depths, the BMM showed that sampled trees were either sourcing their water from very shallow or deep soil layers, with very little contribution from the middle portion of the soil layer. The majority (83%) of the observed species relied on shallow soil water (0.0-0.2 m). This layer contributed approximately 62% to xylem water which was significantly higher than the contributions from all other depths. The contribution from shallow soil was highest for high wood density, slow-growing, small-sized trees. However, this explanation was not statistically strong. Therefore, the study infers that soil water uptake patterns are species-specific rather than trait-specific, although all species were exposed to the same environmental conditions. A combination of continuous sap flux measurements and hourly sampling of xylem water stable isotope composition (dD and d18O) were used to observe water use strategies through a 24 h transpiration cycle for co-occurring tree species (Chapter 6). Here, the study quantifies the high frequency changes in water sources and sap flux patterns of two commonly co-occurring tropical rainforest tree species: Dendrocnide photinophylla (Kunth; Chew) and Argyrodendron peralatum (F.M. Bailey; Edlin ex J.H. Boas). Sap flux ranged from 2.82-28.50 L d-1 and was 66.67% higher in A. peralatum compare to D. photinophylla. For both tree species, sap flux increased with tree size and diurnal sap flux increase resulted in more isotopically enriched xylem water. A Bayesian Mixing Model analysis using sampled soil water isotopic composition from five soil depths from of 0 to 1 m showed that D. photinophylla used very shallow or surface layer (0-20 cm) water, while A. peralatum sourced its water mostly from deeper in the soil profile (g20 cm). These contrasting patterns of water use and water sources of co-occurring tree species suggest that to make proper conclusion on species water consumption pattern, plant water storage capacity, quantitative wood anatomical features and xylem isotope composition should be considered together with sap flux measurement and future studies should consider species level water use strategies to build improved process understanding for ecohydrological modeling.Overall, this research work suggests that stable isotope tracers may aid a better understanding of hydrological niche segregation, factors responsible for spatial variation of xylem water and contrasting water use strategies among co-occurring tropical species. The approaches presented in this thesis can be applied in other climatic condition to investigate water uptake pattern of diverse tree species at fine spatiotemporal scale.