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

Abstract

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.

Contrasting sap flow characteristics between pioneer and late-successional tree species in secondary tropical montane forests of Eastern Himalaya, India

“…Soil moisture was both a limiting variable for pioneer species, as seen by the positive lags and negative autocorrelation coefficients, and a resource depleted by vegetation water use ( Fig. 4C , F ), something that particularly applies for the deep-rooted and large-sized individuals of Fagaceae (late-successional species) ( Kumar et al , 2022 , Preprint). Our results correspond to similar observations from Central Panama where older and large-sized late-successional species continue transpiring unperturbed even under drought conditions, while younger pioneer species reduce their water use ( Bretfeld et al , 2018 ).…”

“…Similar reports from Central Himalayan species belonging to the Fagaceae , including Castanopsis indica , Quercus semicarpifolia , Q. leucotrichophora , and sub-canopy Rhododendron arboreum , have shown that seasonal soil moisture stress can induce a midday slump in leaf water potential ( Zobel et al , 2001 ; Poudyal et al , 2004 ; Tewari et al , 2018 ). The absence of significant midday depression in deep-rooted species such as C. hystrix at the peak of the dry season and strong diel cycles in soil moisture provide evidence of vegetation accessing moisture from deeper soil layers ( Tanaka et al , 2004 ; Allen, 2014 ; Kumar et al , 2022 , Preprint).…”

“…Drier and warmer winters are likely to affect the phenology and negatively affect the productivity of shallow-rooted pioneers such as S. racemosa and E. acuminata . In contrast, deep-rooted species such as C. hystrix and Himalayan oaks might remain unaffected by seasonal droughts ( Kumar et al , 2022 , Preprint). Thus, secondary broadleaved forests, with a dominance of shallow-rooted pioneers, are more prone to the negative impacts of drier and warmer winters than primary forests, which are dominated by deep-rooted species.…”