Heat storage and energy balance closure across five forests of different ages

Incomplete energy balance closure (EBC) has been reported in most eddy covariance studies, but its causes remain poorly understood. To evaluate the role of time lags associated with turbulent and radiative fluxes, we analyzed the energy balance of five forest stands of different ages. We focused on the non-rainy days during the main growing period (June-August) when the lack of EBC was greatest. We hypothesized that forests with greater complexity would show relatively greater heat storage and, consequently, reduce EBC. We detected significant differences in the magnitudes of net radiation (Rn), sensible heat (Hs), and latent heat (LE) among the five chronological forest stands. Both of the LE: Rn and Hs: Rn ratios showed strong correlations (R²=0.65, p=0.1; R²=0.62, p=0.1) with stand age. The Q_air (storage heat in the air): Rn ratio increased with canopy height (R²=0.99, p<0.001) and stand age (R²=0.96, p<0.001); whereas the Q_soil (storage heat in the soil): Rn ratio decreased with leaf area index (LAI) (R²=0.81, p=0.036).  The total heat storage (Q_s) improved EBC by 1-6% in the young stands and 4-9% in the mature stands. However, the energy balance remained unclosed in all the five stands even after Q_s was considered, with EBC ranging from 0.56 to 0.68.  The low ratio of observed to calculated latent heat (LE: LE₀) (0.52-0.70), and the high correlation between the LE₀-LE difference and energy balance residual (R²=0.55-0.85) suggested errors in the turbulent flux estimation. The effect of cloudiness on EBC was minimal in our study sites.