SEASONAL-VARIATION IN RADIATIVE AND TURBULENT EXCHANGE AT A DECIDUOUSFOREST IN CENTRAL MASSACHUSETTS

Temperate deciduous forests exhibit dramatic seasonal changes in surfa ce exchange properties following on the seasonal changes in leaf area index. Nearly continuous measurements of turbulent and radiative fluxe s above and below the canopy of a red oak forest in central Massachuse tts have been ongoing since the summer of 1991. Several seasonal trend s are obvious. Global solar albedo and photosynthetically active radia tion (PAR) albedo both are good indicators of the spring leaf emergenc e and autumnal defoliation of the canopy. The solar albedo decreases t hroughout the summer, a change attributed to decreasing near-infrared reflectance since the PAR reflectance remains the same. Biweekly satel lite composite images in visible and near-infrared wavelengths confirm these trends. The thermal emissions from the canopy relative to the n et radiation follow a separate trend with a maximum in the midsummer a nd minima in spring and fall. The thermal response number computed fro m the change in radiation temperature relative to the net radiation is directly related to the Bowen ratio or energy partition. The subcanop y space follows a different pattern dictated by the presence of the ca nopy; there the midday sensible heat flux is a maximum in spring and f all when the canopy is leafless, while subcanopy CO2 flux is maximum i n midsummer. Subcanopy evapotranspiration did not have a distinct seas onal peak in spring, summer, or fall. The temperature dependence of th e respiration rate estimated from the eddy correlation subcanopy CO2 f lux is comparable to that found using nocturnal flux measurements. The surface energy balance follows a seasonal pattern in which the ratio of turbulent sensible heat flux to the net radiation (Q(H)/Q()) is a maximum in the spring and fail (0.5-0.6), while the latent heat flux ( Q(E)) peaks in midsummer (Q(E)/Q() = 0.5). This pattern gives rise to a parabolic growing season shape to the Bowen ratio with a minimum in early August. Growing season changes in the canopy resistance (R(C)), related to the trends in me Bowen ratio, are more likely to be predic ted using the thermal channels of remote sensing instruments than the shorter-wavelength bands.