Seasonal variations in the energy budget and CO2 flux over a temperate deciduous forest

A long-term monitoring of the energy budget and the CO₂ flux has been conducted over a temperate deciduous broadleaf forest at Kawagoe, Japan. The site is one of participants in the FLUXNET. The measurement of the energy budget has been continued since July of 1995, while the CO₂ flux measurement was started in April of 1997. The fluxes of the sensible heat, water vapor and CO₂ are measured mainly using the eddy covariance method and its derivatives, but other indirect measurements are also made concurrently to cross-check. Additional measurements are also made for the radiation components, air temperature, humidity and wind speed above and under the canopy, soil water content, soil temperature, the leaf area index, etc.

The leaf area index estimated by the combined measurements of the light interception and the litter fall showed a clear seasonal cycle that is characterized by a sharp increase in April and a gradual decrease during 4 months from September to December. The maximum of leaf area index was 5.5 observed during 3 months from June to August.

The seasonal variation of the albedo clearly reflected the variation in the amount and color of the leaves. Coinciding with the leaf emergence, the value of albedo jumped from 0.08 to 0.15 during about 20 days, followed by a gradual decrease due to the leaf maturing (change in color) toward the value of 0.09 observed in September. The value of albedo then showed a slight increase during the senescence period and linearly decreased as the solar altitude increased during the leafless season. The minimum of albedo was observed just before the leaf emergence.

The fluxes of the sensible heat, water vapor and CO₂ also strongly correlated with the seasonal variation in the leaf amount. Before the leaf emergence, the sensible heat flux dominated the latent heat flux in the energy budget, and the CO₂ flux above the canopy was positive (release from the forest) at around 6 gCO₂m^-2d^-1. At the same time with the leaf emergence, the ratio between the sensible and latent heat fluxes reversed and the CO₂ flux changed its direction. The water vapor flux reached its maximum of around 5-6 mmd^-1 in the middle of summer, while the CO₂ flux showed the maximum of about -25 gCO₂m^-2d^-1 in May or June followed by a gradual decrease in magnitude throughout the remainder of the growing season. The sensible heat became dominant again and the forest resumed releasing CO₂ in mid-November when the rate of leaf fall was at the maximum.