Tropical forests act as a major sink of atmospheric carbon dioxide, and store large amounts of carbon in biomass. India is a tropical country with regions of dense vegetation and high biodiversity. However due to the paucity of observations, the carbon sequestration potential of these forests could not be assessed in detail so far. To address this gap, several flux towers were erected over different ecosystems in India by Indian Institute of Tropical Meteorology as part of the MetFlux India project funded by MoES (Ministry of Earth Sciences, Government of India). A 50 m tall tower was set up over a semi-evergreen moist deciduous forest named Kaziranga National Park in north-eastern part of India which houses a significant stretch of local forest cover. Climatically this region is identified to be humid sub-tropical. Here we report first generation of the in situ meteorological observations and leaf area index (LAI) measurements from this site. LAI obtained from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) is compared with the in situ measured LAI. We use these in situ measurements to calculate the total gross photosynthesis (or gross primary productivity, GPP) of the forest using a calibrated model. LAI and GPP show prominent seasonal variation. LAI ranges between 0.75 in winter to 3.25 in summer. Annual GPP is estimated to be 2.11kg C m⁻²year⁻¹.

The eddy covariance method is a powerful technique for quantification of $\hbox {CO}_{2}$, $\hbox {H}_{2} {\rm O}$ and energy fluxes in natural ecosystems. Leaf area index (LAI) and its changes are significant drivers of $\hbox {CO}_{2}$ and $\hbox {H}_{2}$O exchange in a forest ecosystem due to their role in photosynthesis. The present study reports the seasonal variation of $\hbox {CO}_{2}$ and energy fluxes and their relationship with other meteorological parameters of a semi-evergreen primary forest of Kaziranga National Park, Assam, India during February 2016–January 2017. The diurnal pattern of half hourly average $\hbox {CO}_{2}$ fluxes over the forest was found to be mostly dominated by the incident photosynthetically active radiation. During the period of study, diurnal variations of $\hbox {CO}_{2}$ flux showed a maximum value of $-9.97 \mu mol \hbox {m}^{-2}\hbox {s}^{-1}$ in the month of June during summer which is also the beginning of the monsoon season. The monthly averaged diurnal $\hbox {CO}_{2}$ flux and variation in LAI of the forest canopy closely followed each other. The annual net ecosystem exchange of the forest estimated from the $\hbox {CO}_{2}$ flux data above the canopy is 84.21 g C $\hbox {m}^{-2}\,\hbox {yr}^{-1}$. Further studies are in progress to confirm these findings. The estimated average annual evapotranspiration of the semi-evergreen forest is 2.8 $\pm$ 0.19 mm $\hbox {day}^{-1}$. The study of partitioning of energy fluxes showed the dominance of latent heat fluxes over sensible heat fluxes. The energy balance closure was found to increase with an increase in instability and the highest closure of around 83% was noted under neutral conditions.

This study uses 1 yr of eddy covariance (EC) flux observations to investigate seasonal variations in evapotranspiration (ET) and surface energy budget (SEB) closure at a tropical semi-deciduous forest located in north-east India. The annual cycle is divided into four seasons, namely, pre-monsoon, monsoon, post-monsoon and winter. The highest energy balance closure (76%) is observed during pre-monsoon, whereas the lowest level of closure (62%) is observed during winter. Intermediate closure of 68% and 72% is observed during the monsoon and post-monsoon seasons, respectively. Maximum latent heat flux during winter (150 W m$^{-2}$) is half of the maximum latent heat (300 W m$^{-2}$) flux during the monsoon. ET is a controlling factor of SEB closure, with the highest rates of closure corresponding to the periods of the highest ET. The Bowen ratio ranges from 0.93 in winter to 0.27 during the monsoon. This is the first time the role of ET in the seasonal variation of SEB closure has been reported for any ecosystem in north-east India using EC measurements.