© 2015, Springer-Verlag Berlin Heidelberg. Key message: Relationship between sap flow and functional traits changes with altitude and changes in water availability can impose a conservative water use in woody species of tropical rainforest. Abstract: Using a trait-based approach, we have identified that tropical trees are vulnerable to decreases in water availability, especially in montane areas, where higher radiation and vapor pressure deficits lead to higher water loss from trees. Changes to functional traits are useful descriptors of the response of species to variation in resource availability and environmental conditions. However, how these trait-environment relationships change with altitude remains unclear. We investigated changes in xylem sap flow along an altitudinal variation and evaluated the contribution of morphological traits to total plant water use. We hypothesize that (1) at the Montane forest, plant species will show a more conservative water use and (2) seasonally, there will be a much greater increase in conservative water use during the dry season at the Lowland site, since the climate conditions in the Montane site impose constraints to water use throughout the year. Remarkably, although water is assumed to be a non-limiting resource for Atlantic rainforest in general, we observed ecophysiological adjustments for more conservative water use in Montane forest. Our findings demonstrate that changes to water supply and demand as determined by rainfall, VPD and soil water storage can impose restrictions to water loss which differ across spatio-temporal scales. We suggest that the next steps for research in Montane forest should focus on traits related to hydraulic failure and carbon starvation to address the question whether the higher conservative water use observed at the Montane Forest translates into a higher or lower susceptibility to intensification of drought which might arise due to climate change.