The Himalayas, known as the Earth’s third pole, are vital to regional and global climate systems, supporting globally significant biodiversity and livelihoods through ecosystem services such as carbon sequestration and water. However, in the west-central Indian Himalayas, moist, broad-leaved mixed-Oak forests are increasingly being replaced by dry, fire tolerant and fire prone Chir Pine forests, posing ecological concerns. This transition threatens biodiversity, reduces ecosystem functionality, and disrupts water availability, raising significant ecological and societal concerns. While the socio-ecological impacts have been explored, the hydro-climatic consequences remain less understood. To address this knowledge and data gap, we established two research observatories in Uttarakhand’s Chir pine and mixed-oak forests (~1600m elevation, 23° slope) to investigate how these forest transitions affect land -atmosphere energy fluxes, soil moisture, streamflow, and transpiration. Our study integrates field measurements with numerical simulations to provide insights into these changes. Bowen ratio (BR) assemblies were installed at 30m (pine) and 18m (oak) heights, equipped with EE181 and HC2S3 temperature and humidity sensors. Seasonal on-site calibration ensured reliable data collection, resulting in a nearly complete year of high-quality data from these remote locations. During the monsoon season, Pines exhibit higher BR evapotranspiration (ET) compared to Oaks, while during the dry period, their ET is only marginally higher. At the tree level, Pines transpire over a larger sapwood area and exhibit less stringent regulation of sap flow and associated transpiration under varying environmental conditions compared to Oaks. Hydrological analyses indicate that the catchments dominated by Pine have lower baseflow to precipitation percentage compared to Oak, rendering streams in these Pine dominated catchments ephemeral, unlike the more sustained baseflow in Oak-dominated forests. All the measurements corroborate the higher evapotranspiration observed in the Chir pine forest compared to Oak. These observations have been used to parameterize vegetation in the Ocean-Land-Atmosphere Model, enabling high-resolution simulations of regional hydro-climatic conditions under different forest covers  This first ever study of these Himalayan vegetation transitions is likely to provide insights into the future changes in ecohydrology in this biodiversity and water security hotspot.