Transpiration drives diurnal and seasonal streamflow in secondary tropical montane forests of Eastern Himalaya

Vegetation studies establishing direct mechanistic linkages between stand transpiration and streamflow are rare from sub-tropical and tropical montane forests (TMFs) like Himalaya. We quantified the impact of diurnal and seasonal transpiration on lean season streamflow in a broad-leaved evergreen secondary TMF in Eastern Himalaya. Whole-tree and stand transpiration were measured using Granier’s thermal dissipiation sap flow probes at one of the wettest (4500 mm yr-1) and highest elevation (2100 m) sites in the world to date. The observed daily and annual transpiration rates were double of the reported values from TMFs in relatively drier Central Himalaya, but at the lower bound of TMFs globally. Solar radiation was the key driver of transpiration in energy-limited winter under hydrated conditions. Vapour pressure deficit (D) controlled transpiration in energy-abundant summer. We also found that moderate precipitation events (10-30 mm) followed by clear skies can induce significant increase (93±110 %) in stand transpiration. In turn, transpiration was the main driver of lean season streamflow in dry winter and to a lesser extent in wet summer. Thus, in winter, the transpiration-driven abstraction induced corresponding diurnal cycles in soil moisture and streamflow with an average lag of 1.3±1.8 hours and 2.9±2.5 hours, respectively, and strong negative correlations (-0.8±0.1). Thus, changes in vegetation cover and precipitation patterns are likely to impact local and regional moisture recycling by vegetation and lean season flow, thereby affecting regional water security in the Eastern Himalaya.