Energy budget above a temperate mixed forest in northeastern China

摘要

Components of the energy budget were measured continuously above a 300-year-old temperate mixed forest at the Changbaishan site, northeastern China, from 1 January to 31 December 2003, as a part of the ChinaFlux programme. The albedo values above the canopy were lower than most temperate forests, and the values for snow-covered canopy were over 50% higher than for the snow-free canopy. In winter, net radiation R_n was generally less than 5% of the summer value due to high albedo and low incoming solar radiation. The annual mean latent heat LE was 37-5 W m~(-2), accounting for 52% of R_n. The maximum daily evaporation was about 4-6 mm day~(-1) in summer. Over the year, the accumulated precipitation was 578 mm; this compares with 493 mm of evapotranspiration, which shows that more than 85% of water was returned to the atmosphere through evapotranspiration. The LE was strongly affected by the transpiration activity and increased quickly as the broadleaved trees began to foliate. The sensible heat H dropped at that time, although R_n increased. Consequently, the seasonal variation in the Bowen ratio β was clearly U-shaped, and the minimum value (0-1) occurred on a sunny day just after rain, when most of the available energy was used for evapotranspiration. Negative β values occurred occasionally in the non-growing season as a result of intensive radiative cooling and the presence of water on the surface. The β was very high (up to 13-0) in snow-covered winter, when evapotranspiration was small due to low surface temperature and available soil water. Vegetation phenology and soil moisture were the key variables controlling the available energy partitioning between H and LE. Energy budget closure averaged better than 86% on a half-hourly basis, with slightly greater closure on a daily basis. The degree of closure showed a dependence on friction velocity u~*.