Üllar Rannik¹, Nuria Altimir², Jukka Raittila¹, Tanja Suni¹, Anca Gaman¹, Petri Keronen¹, Frank Berninger², Timo Vesala¹, Pertti Hari² and Markku Kulmala¹

1. Department of Physical Sciences, P.O. Box 64, FIN-00014 University of Helsinki, Finland
2. Department of Forest Ecology, P.O. Box 24, FIN-00014 University of Helsinki, Finland

The carbon dioxide and water vapour fluxes were measured from July to September 2000 at two closely located sites in Southern Finland: over a young pine forest and over a forest clearing. These two sites represented the same forest ecosystem at different stages of life-cycle. The Scots pine (Pinus sylvestris L.) stand at SMEAR II station (61^o51' N, 24^o17'E, 181 m above sea level) was established in 1962. The height of the dominant trees in the stand was 14 m and tree density 2500 per ha. The forest at the clearing site was clear-cut about 5 years ago. Prior to clearing, the area was covered by 90 years old Scots pine forest of medium quality. After clear-cutting the soil was scarified. About 1/3 of the ground area was bare mineral soil, 1/3 was mostly bare humus and logging debris from the scarification and 1/3 was intact. The ground vegetation covered about 40% of the area. Grasses were present (main species Luzula pilosa). Natural regeneration had started on the site, but pine seedlings of less than 20 cm in height did not reach an important percentage of the ground cover.

The fluxes were measured at 23.3 and 7 m heights at pine forest and clearing sites, respectively, with commercial instrumentation. In flux interpretation factors such as wind direction and forest fetch and turbulence conditions were taken into account.

During the summer and early autumn the forest was a net sink of CO₂ and the clearing a source (see Table). The average day-time CO₂ flux above the clearing was close to zero, indicating that CO₂ uptake of ground vegetation balanced the release from the soil. The night-time respiration was of the same magnitude at both sites, whereas at day-time the pine forest was a strong sink. This study suggests that soil decomposition does not decline much during five years after clear-cutting. The productivity on the clearing has been low (e.g. only about half of the area was covered by vegetation). Thus carbon losses after clear-cutting are probably substantial. The evapotranspiration was higher over the forest compared to clearing due to transpiration from the canopy.

Table. Average night-time (elevation of sun < 3° ) soil temperature (Tsoil) and respiration fluxes (R) under turbulent conditions (u_* > 0.2 ms^-1); daily sums of net ecosystem exchange (NEE) and evapotranspiration (ET). Plus-minus standard error is given.