CARBON-DIOXIDE EXCHANGE BETWEEN AN UNDISTURBED OLD-GROWTH TEMPERATE FOREST AND THE ATMOSPHERE

We used the eddy-correlation technique to investigate the exchange of CO2 between an undisturbed old-growth forest and the atmosphere at a r emote Southern Hemisphere site on 15 d between 1989 and 1990. Our goal was to determine how environmental factors regulate ecosystem CO2 exc hange, and to test whether present knowledge of leaf-level processes w as sufficient to understand ecosystem-level exchange. On clear summer days the maximum rate of net ecosystem CO2 uptake exceeded 15 mu mol m (-2) s(-1), about an order of magnitude greater than the maximum value s observed on sunny days in the winter. Mean nighttime respiration rat es varied between approximate to-2 and -7 mu mol.m(-2).s(-1). Nighttim e CO2 efflux rate roughly doubled with a 10 degrees C increase in temp erature. Daytime variation in net ecosystem CO2 xexchange rate was pri marily associated with changes in total photosynthetically active phot on flux density (PPFD). Air temperature, saturation deficit, and the d iffuse PPFD were of lesser, but still significant, influence. These re sults are in broad agreement with expectations based on the biochemist ry of leaf gas exchange and penetration of radiation through a canopy. However, at night, the short-term exchange of CO2 between the forest and the atmosphere appeared to be regulated principally by atmospheric transport processes. There was a positive linear relationship between nocturnal CO2 exchange rate and downward sensible heat flux density. This new result has implications for the development of models for diu rnal ecosystem CO2 exchange. The daytime light-use efficiency of the e cosystem (CO2 uptake/incident PPFD) was between 1.6 and 7.1 mmol/mol o n clear days in the summer but decreased to 0.8 mmol/ mol after frosts on clear winter days. Ecosystem CO2 uptake was enhanced by diffuse PP FD, a result of potentially global significance given recent increases in Northern Hemisphere haze.