Bet365

Abstract

Biomes in west-central Himalayas are witnessing a large scale transition from biodiverse broadleaved forests to monoculture forests of Chir pine (Pinus roxburghii), reported to adversely impact watershed stream flows and hydrological services. However, the role of forest transpiration, a major factor controlling watershed hydrology, has not been studied well. This study presents the results of a 2-year long measurement campaign of tree (Tf) and stand (Ts) level transpiration and their response to associated hydrometeorological variables in a Chir pine dominated forested watershed. We quantify the contribution of these fluxes to the forest water budget and identify factors limiting transpiration. At the tree scale, Tf is found to significantly increase with age and to peak in the autumn season. At the stand scale, annual averaged Ts is around 1.1 mm day-1, peaking at 1.3 mm day-1 in autumn and dropping to 0.85 mm day-1 in summer with monsoons witnessing low values as well. Complementary measurements of forest evapotranspiration (ET) agree well with Ts but could more than twice as high as the latter at 3.4 mm day-1 in autumn, highlighting the role of transpiration from understory vegetation whose biomass peaks following the monsoons. Both Ts and ET are at least twice as large as stream discharge (Q) in all seasons except the monsoons in which case Ts is lower than both precipitation (P) and Q. Ts and ET also have different seasonalities than P and Q, highlighting the important role that Ts could play in regulating watershed hydrological reserves. Lastly, the forest is found to be energy limited in the monsoon, autumn and winter seasons and potentially water-limited in the summer season. The evolution of such energy-limited forests under climate warming needs further investigation.