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

Abstract
The interactive role of life-history traits and environmental forcing on plant-water relations is crucial for understanding species response to climate change but remains poorly understood in secondary tropical montane forests (TMFs). Comparing contrasting life-history traits (pioneer vs late-successional species) in a biodiverse Eastern Himalayan secondary TMF, we investigated sap flow responses in co-occurring pioneer species, Symplocos racemosa (n=5) and Eurya acuminata (n=5), and late-successional species, Castanopsis hystrix (n=3), using modified Granier’s Thermal Dissipation probes. The fast-growing pioneers S. racemosa and E. acuminata) had 2.1- and 1.6-times higher sap flux density than the late-successional C. hystrix, respectively, and exhibited characteristics of long-lived pioneer species. Significant radial and azimuthal variability in sap flow (V) between species was observed and attributed to life history traits and the canopy’s access to sunlight. Nocturnal V (1800-0500 hr) was 13.8 % of daily V and is attributed to stem recharge for evening V (1800-2300 hr) and to endogenous stomatal controls for pre-dawn V (0000-0500 hr). Both the shallow-rooted pioneer species exhibited midday depression in V attributed to photosensitivity and diel moisture stress response. In contrast, deep-rooted C. hystrix transpired unaffected across the dry season likely accessing groundwater. Thus, the secondary broadleaved TMFs, with the 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. The study provides an empirical understanding of life-history traits and microclimate modulating plant-water use in widely distributed secondary TMFs in Eastern Himalaya and highlights their vulnerability against warmer winters and reduced snowfall due to climate change.
Methods
Sap flux was measured using custom-built Granier's thermal dissipation probes. Microclimate observations were used to understand the drivers of stand transpiration. Air temperature (°C), relative humidity (Rh, %), wind speed (U, m s-1), and incoming short-wave radiation (Rs, kW m-2) were recorded using an automatic weather station (AWS) (Vantage-pro Davis Net, USA). On-site Precipitation (P, mm hr-1) was recorded using an automated tipping-bucket rain gauge. Additionally, long-term trend analysis was done using daily precipitation (PIMD, mm d-1) extracted from the 0.25⁰ gridded precipitation product by the Indian Meteorological Department (IMD) for the pixel containing the sap flow site (Pai et al., 2014). In-canopy air temperature and relative humidity were recorded using microsensors (iButton hygrochrons, Maxim Int., USA). Forest reference evapotranspiration (EF, mm hr-1) was computed based on FAO’s Penman-Monteith method using meteorological data from the installed AWS and the crop coefficients developed for natural vegetation (for details see Allen et al. 1998). Hourly Vapour Pressure Deficit (D, kPa) was computed from air temperature and relative humidity. Soil water potential was recorded at 10 cm incremental depths from the topsoil to up to 30 cm depth using granular matrix-based (watermark) sensors (Virtual Electronics, Roorkee) and converted to volumetric water content using the site-specific van Genuchten water retention curve. Total soil moisture (S, mm hr-1) was computed for the topsoil (0-30 cm depth) using the trapezoidal method. S was smoothened using a 3-step moving-average window to gap-fill stray missing values. A stilling well fitted with a capacitance water level recorder (Dataflows Systems LTD, New Zealand) was installed on the first-order stream draining the micro-watershed. Streamflow (Q, mm hr-1) was computed from the water-level using a stream-specific rating curve and catchment area.