I was browsing through some old science issues and this title -An Intimate Knowledge of Trees caught my eye. A study by husband and wife team in La Selva, Costa Rica showed that at very warm times of the year, the trees in that forest slowed their growth and pumped out significant amounts of CO2. So they begin to act as sources instead of sinks. It really puts the brakes on the notion, that forests absorb CO2 from human activity, acting as carbon sinks and somehow slowing down green-house warming.
Ref: Joycelyn Kaiser, 25 April 2003. "An Intimate Knowledge of Trees." Science 300: 566-567.
[ed.] see also: Clark et al., 2003. Tropical rain forest tree growth and atmospheric carbon dynamics linked to interannual temperature variation during 1984-2000. PNAS, 100 (10): 5852-5857 (13 May 2003; published online 28 April 2003).