Abstract:

Aims Although temperate broadleaved deciduous forest accounts for two-thirds of the forest area in northeastern China, its spatio-temporal variations of CO₂ concentration ([CO₂]) have not been quantified. Our objectives were to quantify diurnal and seasonal variations and vertical gradients of [CO₂] and explore controlling factors.

Methods A [CO₂] flux tower with an 8-level [CO₂] profile system (at 0.5, 2.0, 4.0, 8.0, 16.0, 20.0, 28.0 and 36.0 m) was installed at the Maoershan Forest Ecosystem Research Station in Heilongjiang Province (45°24′ N, 127°40′ E) in 2007. [CO₂] at each level was measured with a LI-COR LI-840 infrared gas analyzer (IRGA) by drawing it from each level with a sample pump through tubes of equal length. The IRGA was controlled and data were collected with a datalogger. Automatic calibration was done for the IRGA once a day. A Vaisala GMP343 was installed at 36.0 m to monitor ambient [CO₂] for quality control of the [CO₂] profile data. We simultaneously measured micrometeorological variables, including wind speed, air temperature, relative humidity, photosynthetically active radiation, vapor pressure, soil temperature and water content.

Important findings At a daily scale, maximum [CO₂] occurred at night or sun rise, while the minimum occurred in the afternoon at all levels. This pattern was predominant in the summer. The diurnal course of the [CO₂] was “V”-shaped in winter but “U”-shaped in other seasons. [CO₂] decreased with increasing height, particularly on summer nights. During the daytime of summer, daily mean [CO₂] within the canopy was substantially lower than the ambient [CO₂], suggesting that the vegetation acted as a CO₂ sink due to its photosynthesis. Daily mean [CO₂] above the canopy peaked in spring and autumn and reached a minimum in summer, while that near the forest floor showed a unimodal seasonal pattern with its maximum in summer. The diurnal dynamics of [CO₂] and their vertical gradients during the growing season were jointly controlled by the atmospheric boundary layer (ABL) and forest carbon metabolism, while those during the dormant season were controlled mainly by ABL. The seasonal dynamics of the [CO₂] near the forest floor were determined mainly by soil respiration, while those above the canopy were jointly controlled by canopy photosynthesis and ecosystem respiration.

Key words: atmospheric boundary layer, CO₂ source/sink, vertical gradient, Maoershan