Comparative Transpiration Responses of Chir Pine and Banj Oak Forests in the West-Central Himalaya: Evaluation with Evapotranspiration and Hydrometeorological measurements

The west-central Himalayas are experiencing a major ecological shift with widespread, apparently water-conserving, Banj oak (Quercus leucotricophora) forests increasingly replaced by monoculture stands of Chir pine (Pinus roxburghii). This transition has been associated with declining streamflow and weakened hydrological services, yet the role of forest transpiration—an essential component of watershed hydrology—remains insufficiently understood.

This study reports findings from a two-year measurement campaign quantifying tree-level (T_f) and stand-level (T_s) transpiration in a Chir pine–dominated watershed. We analyze seasonal variability in transpiration, its age-related differences, and sensitivity to hydrometeorological drivers. We also compare this with a few months of transpiration data collected at a nearby Banj oak forest. At the tree scale, T_f increases markedly with tree age and peaks during the post-monsoon autumn season. At the stand scale, mean annual T_s is ~1.1 mm day⁻¹, with a maximum (~1.3 mm day⁻¹) in autumn and minimum (~0.85 mm day⁻¹) in summer, along with suppressed transpiration during the monsoon. T_sin the pine forest is consistently higher than that in the oak forest during the monsoon (~0.3 mm day^-1) and post-monsoon autumn seasons (~0.2 mm day^-1) with similar values in early autumn. Oaks also display a stronger regulation of transpiration post-monsoon with a sharper decline in T_s as compared to nearly stable values in the pine stand. T_s is also strongly controlled by solar insolation in the post-monsoon autumn and winter seasons, signifying energy-limitation, which may not have been earlier reported for these forests. However, pines still opportunistically utilise available water resources during this period with a stable T_s. The forests also seem to be water-limited during the dry summer season, which requires supporting evidence from direct measurements. However, again, pines have an opportunistically high transpiration rate during this period, apparently utilizing watershed storage.

These sap-flux measurements align well with complementary measurements of forest evapotranspiration (ET), made with the Bowen ratio method. T_s percentage contribution in ET varied with seasons, with the lowest in monsoon (24% and 40%) at the oak forest and pine forest, respectively. ET show stronger transipraive regulation in the oak forest, suggesting a conservative impact on regional hydrology. Annually, the pine forest evapotranspires nearly 168 mm more water as compared to the oak forest with a negative impact on watershed storage as shown by complementary discharge measurements. Overall, our study shows that the ongoing land cover change may have significant consequences for regional hydrological resources.